***** File GIOJPA.TXT                                                                                                             
                                                                                                                                  
NOTE: This file was created by scanning the original hardcopy article                                                             
and only the Figure captions are included.                                                                                        
                                                                                                                                  
The Giotto Three-Dimensional                                                                                                      
Positive Ion Analyser                                                                                                             
A.D. Johnstone, J.A. Bowles, A.J. Coates,                                                                                         
A.J. Coker, S.J. Kellock* & J. Raymont                                                                                            
Mullard Space Science Laboratory, Holmbury St. Mary, UK                                                                           
* Now at ESA's Operation Center (ESOC), Darnstadt, West Germany                                                                   
                                                                                                                                  
B.   Wilken, W. Studemann & W. Weiss                                                                                              
Max Planck Institut fur Aeronomie, West Germany                                                                                   
                                                                                                                                  
R.   Cerulli Irelli & V. Formisano                                                                                                
Istituto di Fisica dello Spazio Interplanetario, Frascati, Italy                                                                  
                                                                                                                                  
E.   de Giorgi, P. Perani & M. de Bernardi                                                                                        
Laben, Milan, Italy                                                                                                               
                                                                                                                                  
H.   Borg & S. Olsen                                                                                                              
Kiruna Geophysical Institute, Kiruna, Sweden                                                                                      
                                                                                                                                  
J.D. Winningham                                                                                                                   
South west Research Institute, San Antonio, USA                                                                                   
                                                                                                                                  
D.A. Bryant                                                                                                                       
Rutherford Appleton Laboratory, Chilton, UK                                                                                       
                                                                                                                                  
Abstract                                                                                                                          
  This instrument is designed to measure the three-dimensional energy distribution of                                             
positive ions in order to study the interaction between the solar wind and ionized                                                
cometary particles. The two sensors measure the distribution from 10 eV to 20 keV                                                 
once per spacecraft spin and the distribution from 90 eV to 90 keV, with coarse mass                                              
discrimination, once every 32 spins.                                                                                              
                                                                                                                                  
1. Introduction                                                                                                                   
  The plasma tail of a comet is arguably its most spectacular feature. Its filamentary                                            
structure, with waves, kinks and spirals can be seen stretching for as much as                                                    
10**8 km across the inner solar system for a large comet like Halley near perihelion.                                             
  Following the original proposal of Biermann (1951), it is now known that the plasma                                             
tail is the manifestation of the interaction between plasma from two distinct sources:                                            
ionized particles of cometary origin and the solar wind. Theoretical analyses of this                                             
interaction (Biermann et al., 1967; Wallis, 1973; Ip & Axford, 1982) have provided                                                
a model of the gross features of the plasma flow to be expected near the comet, but                                               
they do not yet give a detailed explanation of the formation of the visible tail. On the                                          
other hand, many of the features predicted by theory cannot be observed from the                                                  
Earth. The gap between theoretical analysis of the solar-wind-comet interaction and                                               
ground-based observations of comet tails can only be filled by in-situ measurements                                               
of the plasma distributions within the visible coma of a comet. Theory says that the                                              
plasma should be divided into three main regimes separated by two surfaces - a                                                    
contact surface and a bow shock. The contact surface encloses the region dominated                                                
by the cold, dense cometary plasma around the nucleus. Some of the neutral cometary                                               
particles are not bound by this contact surface and may travel well upstream into the                                             
solar wind before being ionized. The additional mass they then add to the solar-wind                                              
flow slows it down and eventually creates the second surface, a bow shock.                                                        
The objectives of the Johnstone Plasma Analyser (JPA) instrument can be concisely                                                 
stated as:                                                                                                                        
-   to look for the existence of a bow shock and a contact surface                                                                
-   to observe the mass loading of the solar wind and the resultant deceleration and                                              
    deflection of the flow                                                                                                        
-   to observe the distribution of implanted cometary ions and its stability                                                      
-   to detect the principal ionization mechanisms.                                                                                
                                                                                                                                  
The instrument is designed to achieve these objectives by measuring the three-                                                    
dimensional velocity distribution of positive ions in the energy range from 10 eV to                                              
90 keV. It includes two complementary sensors: the Fast Ion Sensor (FIS) measures                                                 
the energy per charge distribution from 10 eV/q to 20 keV/q in all directions, except                                             
for a cone around the velocity vector, once every rotation (nominally 4 s) of the                                                 
spacecraft; the Implanted Ion Sensor (IIS) measures the energy per charge distribution                                            
from 90 eV/q to 90 keV/q over a similar angular range, with discrimination into five                                              
mass groups, but takes 32 rotations to obtain a complete distribution.                                                            
  The analysis is expected to be undertaken at several levels of processing. The                                                  
existence of a bow shock, or contact surface, should be apparent even in raw telemetry                                            
data as a discontinuity. Studies of the mass loading of the flow require the derivation                                           
of such bulk plasma parameters as density, temperature, velocity, pressure and                                                    
possibly higher order terms. To understand the stability of the cometary ions requires                                            
detailed analysis of the complete distribution.                                                                                   
                                                                                                                                  
2. Scientific Background                                                                                                          
  Neutral molecules and radicals emerge from the collision-dominated region close to                                              
the nucleus with an outward flow velocity V(i) which depends on the energy gained or                                              
lost in the complex photochemical reactions which take place there. The particles are                                             
ionized, it is believed, predominantly by photo-ionization or charge-exchange at a rate                                           
theta(i); which depends on the species, i. The ion production rate A(i) at a distance r from                                      
the nucleus is therefore given by                                                                                                 
                                                                                                                                  
  A(i) = (Q(i)theta(i)/(4 Pi r**2 V(i))) exp (-r theta(i)/V(i))                                                                   
                                                                                                                                  
where Q(i) is the escape rate of the species i from the inner coma. The quantity (V(i)/theta(i))                                  
has the character of a species-dependent scale length which determines the size of the                                            
coma of that particular species. In most theoretical studies a single species is assumed;                                         
in the real comet a number of important species will be encountered with different                                                
values of (V(i)/theta(i)). In Table 1 two species H+ and CO+, with very different scale                                           
lengths, are compared.                                                                                                            
 Once the ion has been implanted in the flow it is picked-up by the electric field in                                             
the solar wind and accelerated into a cycloidal orbit. Using the reference frame shown                                            
in Figure 1 with the magnetic field parallel to the y-axis and the solar-wind velocity                                            
in the yz-plane, the trajectory of the implanted ion is given by                                                                  
                                                                                                                                  
 V(x) = V(s) sin (phi) sin (omega(c)t)                                                                                            
 V(z) = V(s) sin (phi) (1 - cos (omega(c)t))                                                                                      
                                                                                                                                  
Figure 1. The frame of reference used                                                                                             
calculate ion pickup trajectories                                                                                                 
                                                                                                                                  
  The motion consists of a gyration about B with a velocity V(s) sin (phi) and a drift veloci-                                    
ty V(s) sin (phi) perpendicular to B. The maximum velocity reached is 2 V(s) sin (phi); with                                      
typical values for the solar wind, V(s)=400 km/s, phi=45deg and B= 10 nT, one obtains                                             
the values given in Table 2 for ions of the same two sample species as in Table 1.                                                
  High-mass ions are therefore rapidly accelerated in a way that is strongly dependent                                            
on the direction of the magnetic field. In the relatively undisturbed solar wind,                                                 
upstream from the bow shock, the field is normally at a large angle to the flow, and                                              
the ions can be accelerated to high energies. Near the nucleus, where the magnetic                                                
field lines draped around the comet lie nearly parallel to the flow, there will be little                                         
acceleration of the ions by this means. Again the ionic behaviour is strongly species-                                            
dependent. Hydrogen ions, created at large distances (see Table 1), are likely to                                                 
follow cycloidal trajectories. Carbon-monoxide ions, created closer to the nucleus,                                               
have a gyro radius comparable to the size of their coma. Their motion is therefore                                                
going to be different in character from the hydrogen ions. It is worth noting in this                                             
respect that numerical models of the magnetohydrodynamic flow (Biermann et al.,                                                   
1967; Schmidt & Wegmann, 1982) usually assume that the ions are immediately sub-                                                  
sumed into the solar-wind flow, travelling at the solar-wind speed. This extreme case,                                            
which can be called the 'strong coupling case', would result in the average energy <W>                                            
and momentum <p> being given by the equations                                                                                     
                                                                                                                                  
 <W> = 1/2 MV(s)**2                                                                                                               
                                                                                                                                  
 <p>=MV(s)                                                                                                                        
                                                                                                                                  
At the other extreme, fully developed cycloidal trajectories give                                                                 
                                                                                                                                  
  <W> = m(V(s)**2)(sin (phi))**2                                                                                                  
                                                                                                                                  
  <p> = m V(s)(sin (phi))k                                                                                                        
                                                                                                                                  
 where k is a unit vector in the z-direction.                                                                                     
                                                                                                                                  
----------------------------------------------------------------------------                                                      
Table 1. Coma scale lengths                                                                                                       
                                                                                                                                  
                    theta(i)(s**-1)      V(i)(m/s)         V(i)/theta(i)(km)                                                      
----------------------------------------------------------------------------                                                      
H+                   6.7x10**-7          8000              1.2x10**7                                                              
CO+                 11.5x10**-7           700                6x10**5                                                              
----------------------------------------------------------------------------                                                      
----------------------------------------------------------------------------                                                      
Table 2. Cycloidal trajectory parameters                                                                                          
                                                                                                                                  
                    Maximum energy (keV)  Gyro period (s)   Gyro radius (km)                                                      
----------------------------------------------------------------------------                                                      
                                                                                                                                  
                                                                                                                                  
H+                  1.67                  6.5               1850                                                                  
CO+                 50.1                  183              50000                                                                  
----------------------------------------------------------------------------                                                      
                                                                                                                                  
  In the latter case the momentum is now perpendicular to the magnetic field and will                                             
cause a deflection as well as a deceleration of the solar wind. Again the nature of the                                           
interaction is strongly influenced by the direction of the magnetic field in in-                                                  
terplanetary space, which can vary greatly even on short time scales. The strong-                                                 
coupling case could become important if the ring distribution in velocity space created                                           
by the cycloidal trajectories was sufficiently unstable for the distribution to become                                            
rapidly isotropized (Ip & Axford, 1982). Recent calculations (Galeev, 1983) suggest                                               
that this is unlikely to occur rapidly enough to be important.                                                                    
  The solar-wind flow around the comet can be described by the full single fluid equa-                                            
tions expressing conservation of mass, momentum, and energy. These equations have                                                 
been solved numerically by several authors (Schmidt & Wegmann, 1982), albeit with                                                 
many simplifying assumptions. An alternative approach is to use a quasi-one-                                                      
dimensional formulation which can be treated analytically (Wallis, 1973; Wallis &                                                 
Ong, 1975; Galeev et al., 1985):                                                                                                  
                                                                                                                                  
  (d/dx) [rho*u*f(u,mu)] = (Q(i) M(i) theta(i)/(4 pi V(i) r**2)) delta (mu-M(i)u**2/2B)                                           
                                                                                                                                  
  d/dx (rho*u) = Q(i) M(i) theta(i)/(4 pi V(i) r**2)                                                                              
                                                                                                                                  
  d/dx (rho*u**2 + p(perpend) + B**2/2 mu(0) = 0                                                                                  
                                                                                                                                  
From these equations, for the case where B is perpendicular to the flow, it is possible                                           
to derive a solution for the cometary ion distribution function f(u,mu) in the region                                             
upstream from the shock. This is shown in Figure 2. In this distribution each value                                               
of magnetic moment corresponds to ionization occurring at a particular point                                                      
upstream, with the highest magnetic moments being generated in the undisturbed solar                                              
wind. As the flow speed decreases, due to the mass-loading, the magnetic moment of                                                
the implanted ions also decreases. The particles with lowest magnetic moment in this                                              
distribution are created just upstream from the point of observation. Measuring this                                              
distribution therefore gives the possibility of sensing conditions upstream, including                                            
the unperturbed flow speed.                                                                                                       
  From these equations it is also possible to obtain the variation of mass flux as the                                            
flow approaches the nucleus. Continuous flow is only possible up to the point where                                               
(Galeev et al., 1985)                                                                                                             
                                                                                                                                  
 rho*u/rho(infinity)u(infinity) = gamma**2/(gamma**2-1) = 4/3  (gamma=2)                                                          
                                                                                                                                  
where rho*u is the mass flux and the subscript "infinity" indicates the unperturbed upstream                                      
values and gamma is the ratio of specific heats.                                                                                  
  In fact it is found that in numerical simulations the critical value for the formation                                          
of a shock is (rho*u/rho(infinity)u(infinity)) = 1.185.                                                                           
                                                                                                                                  
Figure 2. The cometary ion distribution                                                                                           
function in the unshocked solar wind as a                                                                                         
function of the magnetic moment mu.  It is                                                                                        
calculated for a position just upstream from an                                                                                   
M=2 shock where the velocity is 0.75 times                                                                                        
the unperturbed solar wind velocity (Galeev et                                                                                    
al., 1985)                                                                                                                        
                                                                                                                                  
 Ions implanted inside the bow shock do not reach high energies because of field-line                                             
draping and because the turbulence is likely to prevent the full development of                                                   
cycloidal trajectories, but those energetic ions created upstream are able to penetrate                                           
the shock, essentially unaffected, and could be detected in the inner region (Galeev                                              
et al., l985).                                                                                                                    
 The objective of the JPA instrument is to obtain an in-situ evaluation of these                                                  
theoretical ideas taking due account of the greater complexity of the real situation                                              
created by the presence of many different species and a variable magnetic-field                                                   
direction.                                                                                                                        
                                                                                                                                  
3.  Instrument Design                                                                                                             
                                                                                                                                  
  This is an exploratory mission and the JPA instrument will be the first to make three-                                          
dimensional ion measurements near a comet, so it is most important that it be able to                                             
cope with a wide range of possible circumstances and that it not be limited by pre-                                               
conceived ideas of what the ion distributions are like near a comet.                                                              
 The instrument must cover as much of velocity space as possible, leaving no gaps                                                 
in coverage for unsuspected distributions to slip through. This has implications for the                                          
ion optics of the analyzer design, as well as for the energy sweep rates and sampling                                             
patterns.                                                                                                                         
 Structures have been observed near the head of a comet with thicknesses of the order                                             
of 1000 km. This upper limit is set by observational techniques and not by cometary                                               
physics. If experience in other space plasmas is any guide, spatial variations on much                                            
smaller scales are also likely. With a spacecraft speed relative to the comet of 68 km/s,                                         
a time resolution of 15 s is essential and a much faster time resolution is desirable.                                            
 The mass distribution as well as the complete angular distribution of the implanted                                              
ions must be measured if the characteristics of the flow are to be understood, because                                            
the behaviour is strongly species-dependent. It is not necessary to have the same mass                                            
resolution as for studies of the chemical constituents of the nucleus.                                                            
 The JPA instrument is one of a group of complementary plasma sensors measuring                                                   
ions on Giotto; the others are the IMS instrument (Balsiger et al., 1986), the PICCA                                              
sensor of the RPA (Reme et al., 1986), and the EPA instrument (McKenna-Lawlor                                                     
et al., 1986). The JPA instrument is directed towards studies on the nature of the solar-                                         
wind interaction with the comet rather than the detailed chemical composition of the                                              
ions.                                                                                                                             
                                                                                                                                  
Figure 3. The flight units of the JPA                                                                                             
instrument. From the left they are the Fast Ion                                                                                   
Sensor, the Data Processing Unit and the                                                                                          
Implanted Ion Sensor                                                                                                              
                                                                                                                                  
  The instrument consists of three separate packages: the Fast Ion Sensor, the Im-                                                
planted Ion Sensor and the Data Processing Unit (Fig. 3).                                                                         
  The purpose of the Fast Ion Sensor is to provide a three-dimensional distribution                                               
over the energy range likely to include most of the ions near the comet as quickly as                                             
possible. It obtains the full azimuthal distribution once per rotation of the spacecraft.                                         
  It can measure the solar-wind distribution at its most anisotropic, giving the flow                                             
speed and direction, temperature and density. It follows the development of the solar                                             
plasma as it is thermalized, slowed down and deflected. It measures the ring distribu-                                            
tions for the low-mass ions in the undisturbed solar wind (complete distributions up                                              
to mass 12), and for all ions once the angle between the flow direction and magnetic-                                             
field direction becomes smaller near the comet. Speed of response is achieved at the                                              
expense of mass discrimination, and by limiting the energy range to 10 eV to 20 kev.                                              
  Its geometric factor is determined by ensuring that the count rate in the most                                                  
anisotropic and dense solar wind to be expected will not exceed the highest allowable                                             
count rate. Its wide dynamic range then ensures that weak secondary populations can                                               
also be detected, with the highest possible statistical significance.                                                             
  It does not cover the distribution of energetic implanted ions (E> 20 keV), nor the                                             
cold cometary ions inside the contact surface. These ions will appear to be moving                                                
antiparallel to the spacecraft velocity vector.                                                                                   
  There are several reasons for not covering this latter population. First, the fluxes                                            
are very much higher than any other fluxes encountered and if the sensitivity of the                                              
Fast Ion Sensor were to be reduced to cope with the cometary ions it would not have                                               
adequate sensitivity for other important populations. Secondly, to detect these ions                                              
would mean exposing the sensor to the flux of cometary dust and neutral particles past                                            
the spacecraft, which would create an undesirable background in the sensor for all of                                             
the measurements.                                                                                                                 
  The task for the Implanted Ion Sensor is to search for massive cometary ions in the                                             
solar wind by extending the energy range of the measurements up to 90 keV, increas-                                               
ing the sensitivity so that very low densities can be measured and providing mass                                                 
discrimination sufficient to separate the ions into the principal mass groups, enabling                                           
the ring distributions to be detected even when diffused by wave-particle interactions.                                           
The technique used to obtain the mass discrimination, namely time-of-flight analysis,                                             
has the additional property of having a very low background because it uses a coin-                                               
cidence technique. This means that extremely low count rates can be measured if suffi-                                            
cient integration times can be allowed. It achieves these properties at the expense of                                            
speed of response because it measures at one energy level each rotation of the                                                    
spacecraft. With its high sensitivity, it is unable to measure the proton flux in the solar                                       
wind because the intense fluxes overload the time-of-flight analyzers.                                                            
  The Data Processing Unit collects the data from the sensors and processes it for                                                
transmission to Earth.                                                                                                            
                                                                                                                                  
4. Fast Ion Sensor                                                                                                                
  The principal design aims of this sensor are: (a(a) high sensitivity, i.e. a large                                              
geometric factor, (b) wide dynamic range, i.e. high maximum count rates and low                                                   
background, (c) complete and continuous coverage of a wide solid angle, in the energy                                             
range from 10 eV to 20 keV for positive ions, and (d) angular and energy resolution                                               
good enough to resolve the supersonic flow in the solar wind. The coverage in solid                                               
angle is achieved by having a wide angle of acceptance (160deg) for the analyzer in a                                             
plane containing the spin axis of the spacecraft. Then, as the spacecraft rotates, the                                            
detectors sweep through the full 4 pi solid angle apart from the 20deg cone around the                                            
velocity vector.                                                                                                                  
  The Fast Ion Sensor (FIS) consists of four principal elements (Fig. 4), a                                                       
hemispherical electrostatic energy analyzer, a quadrispherical angular dispersion sec-                                            
tor, a microchannel plate detector, and a discrete-anode, position-sensitive readout                                              
system (Johnstone et al., 1985).                                                                                                  
  After entering the aperture, ions pass through a hemispherical energy analyzer,                                                 
which selects a narrow band in energy per charge (delta(E)/E=4.7%). After an in-                                                  
termediate aperture, the selected ions enter an 80deg angular dispersion sector, which                                            
disperses them to emerge around a 160deg annular sector according to the angle of in-                                             
cidence at the input aperture. They are then accelerated onto the front face of a                                                 
microchannel plate detector, which produces a cloud of electrons for each ion striking                                            
the input. Finally, the electrons are collected and form a charge pulse on one of a                                               
series of eight metal anodes behind the microchannel plate. Each of the anodes has                                                
a defined angular range (Table 3) and is connected to a charge-sensitive pulse                                                    
amplifier mounted within the sensor which produces a logic level pulse output for each                                            
electron cloud striking the anode. The arrangement provides continuous coverage over                                              
the sensor field of view.                                                                                                         
 The energy of the detected ion is known from the analysis voltage applied to the                                                 
spherical deflection plates. The plate voltages are applied in a fixed ratio                                                      
V(inner)/V(outer) = -1.18, to give the zero potential surface exactly half way between the                                        
spherical plates. The 'gain' of the analyzer (i.e. the ratio of the energy selected to the                                        
plate potential difference) is 3.55. The polar angle is known from the anode which                                                
registers the count. Azimuthal angle is measured by timing with respect to the                                                    
spacecraft Sun pulse.                                                                                                             
 The detector consists of two double-thickness microchannel plates in a chevron con-                                              
figuration, specially cut to cover the 160deg arc of the output aperture of the analyzer.                                         
The combination produces a saturated pulse distribution, with full-width-half-                                                    
maximum of 70% at a gain of 2 X 10**6. The saturated distribution enables reliable                                                
operation in a pulse-counting mode, with little dependence of the overall detection effi-                                         
ciency on the amplifier gain or threshold. Achieving the saturation at a low gain                                                 
enables the plate to operate at high count rates and thus maximizes the dynamic range.                                            
The maximum pulse rate the channel plate can deliver per anode sector is of the order                                             
of 2 X 10**6 pulses/s. In principle, such count rates could occur simultaneously in all                                           
sectors. The discrete anode, with individual pulse counters, is the only type of position                                         
sensitive readout presently capable of handling such count rates.                                                                 
                                                                                                                                  
Figure 4. Diagram of the operation of the Fast                                                                                    
Ion Sensor                                                                                                                        
                                                                                                                                  
__________________________________________________________________________                                                        
Table 3. Fast Ion Sensor analyzer characteristics                                                                                 
                                                                                                                                  
E/q range (keV/q)                                 0.01-20                                                                         
Acceptance angles:                                                                                                                
 azimuthal                                        5deg                                                                            
 polar                                            160deg                                                                          
Outer plate radius r(1) (mm)                      38                                                                              
Inner plate radius r(2) (mm)                      33                                                                              
Centre radius r (mm)                              35.5                                                                            
Analyser gain [=r/2(r(1)-r(2))]                   3.55                                                                            
Aperture diameter (mm)                            2.25                                                                            
Aperture area (mm**2)                             4                                                                               
Plate voltage splitting (V(2)/V(i))               -1.18                                                                           
Energy resolution (delta(E)/E)                    4.7%                                                                            
Geometric factor (mm**2 sr eV)                    4.7 x 10**-3 E (eV)                                                             
 (26deg anode at normal incidence)                                                                                                
__________________________________________________________________________                                                        
                                                                                                                                  
  The energy passband of the analyzer is swept continuously, along an exponential                                                 
decay curve from the maximum energy of 20 keV down to l0 eV in one sixteenth of                                                   
a spin. The sweep is synchronized to the spin by using the spacecraft Spin Segment                                                
Clock Pulse to control the sweep.                                                                                                 
  Since the angle of acceptance in the spin plane is 5 deg, there are gaps in the azimuthal                                       
coverage between successive sweeps which are 22.5deg apart. This is important in the                                              
solar wind where the undisturbed solar wind may be confined within an angular range                                               
of 5 deg. In order to provide contiguous azimuthal coverage, an energy sweep covering                                             
one quarter of the energy range (a factor of 6.7 in energy) is used four times as often                                           
in the 45deg angular sector centred on the solar direction. The solar-wind mode is used                                           
on alternate spins giving a time resolution for solar-wind measurements of 8 s. The                                               
starting energy for the reduced sweep (Table 4) is adjusted automatically on-board                                                
(Section 6) to ensure that the proton and alpha-particle distributions in the solar wind                                          
are always covered.                                                                                                               
  The intrinsic energy passband of the analyzer has delta(E)/E=4.7%. At the sweep rate,                                           
this energy range is covered in 1 ms. If the accumulation time of the counters is in-                                             
creased, the energy passband of the measurement is increased correspondingly. Thus                                                
during solar wind sweeps 2 ms accumulation times will be used, giving delta(E)/E=0.096.                                           
For the High Angular Resolution Distribution mode (Section 6), 8 ms accumulation                                                  
times given delta(E)/E=0.3 and for the Fast Time Resolution with 16 ms accumulation time                                          
delta(E)/E ~ 0.6.                                                                                                                 
  The FIS electronics has two functions: to accumulate the pulses from the anodes,                                                
and to provide the high bias voltages to operate the analyzer and the detector. The out-                                          
puts of the eight amplifiers are routed through mode-switching logic, where they are                                              
combined in two different accumulator modes (wide energy and solar wind) before be-                                               
ing counted by a series of six, 16-bit accumulators. The polar-angle ranges selected                                              
in the six accumulators for the wide energy and solar wind accumulator modes are                                                  
shown in Table 5. The solar-wind mode has been arranged to provide high angular                                                   
resolution measurements around the solar direction and is used simultaneously with                                                
the solar-wind sweeps described above.                                                                                            
                                                                                                                                  
__________________________________________________________________________                                                        
                                                                                                                                  
Table 4. Start and stop energy levels for FIS solar-wind sweeps                                                                   
                                                                                                                                  
Solar wind sweep  Start energy  Proton velocity  Stop energy  Proton velocity                                                     
preset            (eV/q)        (km/s)           (eV/q)       (km/s)                                                              
__________________________________________________________________________                                                        
                                                                                                                                  
7                    19963         1962            4161        895                                                                
6                    15956         1754            2494        693                                                                
5                     9562         1358            1494        536                                                                
4                     5730         1051             896        4l5                                                                
3                     3434          813             537        321                                                                
2                     2058          630             322        249                                                                
1                     1233          487             193        192                                                                
0                      739          377             115        149                                                                
__________________________________________________________________________                                                        
__________________________________________________________________________                                                        
Table 5. Fast Ion Sensor Polar angles sampled by the accumulators                                                                 
                         Polar angles sampled     measured from z axis*                                                           
Accumulator              in wide-energy mode      in solar-wind mode                                                              
__________________________________________________________________________                                                        
                                                                                                                                  
1                     98-124                 98-150                                                                               
2                    124-150                 46-98                                                                                
3                     46-72                  46-59                                                                                
4                     72-98                  59-72                                                                                
5                     20-6                   72-85                                                                                
6                    150-180                 85-98                                                                                
__________________________________________________________________________                                                        
* Directed along comet-spacecraft relative velocity vector.                                                                       
                                                                                                                                  
  The high-voltage unit produces two types of high-voltage output. The first is a                                                 
negative high-voltage bias for the microchannel plate, which has four possible settings                                           
selectable by ground command. The three operating voltages are provided in case of                                                
gain degradation in the microchannel plate during the mission. The second type of out-                                            
put is the programmable positive and negative deflection voltages for the outer and                                               
inner electrostatic deflection plates. The output of these units has maximum values of                                            
2600 and -3060 V, respectively, corresponding to selecting ions with energy                                                       
20 keV, Each of the three high-voltage outputs is monitored by spacecraft analogue                                                
housekeeping telemetry.                                                                                                           
                                                                                                                                  
Figure 5. Results of the calibration of the Fast                                                                                  
Ion Sensor. The azimuth degrees correspond to                                                                                     
polar angle in the spacecraft relative to the                                                                                     
spin axis i.e. -80deg azimuth is 0deg polar angle;                                                                                
+80deg azimuth is 160deg polar angle. The                                                                                         
numbers are the discrete anodes of the                                                                                            
position-sensitive detector                                                                                                       
                                                                                                                                  
                                                                                                                                  
 The Fast Ion Sensor was calibrated using an ion beam with low angular and energy                                                 
spreads at various fixed energies, at Southwest Research Institute (Johnstone et al.,                                             
1985). In all, approximately 60000 individual data points were collected per run in                                               
(energy, angle) space, covering the complete angular and energy response of the                                                   
analyzer. The individual measurements were integrated to give the overall response                                                
of the sensor. The results are shown in Figures 5 and 6. Although the aim was to                                                  
achieve a polar angle coverage of 160deg, the response (as expected) falls off at large                                           
                                                                                                                                  
Figure 6. Results of the calibration of the Fast                                                                                  
Ion Sensor. The plot shows the energy angles of                                                                                   
response of one anode integrated over all                                                                                         
angles                                                                                                                            
                                                                                                                                  
incidence such that the practical limit of a measurable response is a range                                                       
of 150deg. This leaves a small hole (~5deg cone) in the coverage of the sensor in the direc-                                      
tion of the spin axis, as well as around the velocity vector (~25deg cone).                                                       
                                                                                                                                  
5. Implanted Ion Sensor                                                                                                           
The Implanted Ion Sensor (IIS) is an ion spectrometer (Fig. 7) which combines elec-                                               
trostatic analysis with a time-of-flight measurement. An electrostatic analyzer selects                                           
positive ions of a given energy per charge, E/Q. The ions are then accelerated by a                                               
potential difference, V, before the time T to travel a path length D is determined. The                                           
measured quantities E/Q and the time-of-flight T can be combined to yield the mass-to-                                            
charge ratio, M/Q, according to the following equation:                                                                           
                                                                                                                                  
  M/Q = 2WT**2/QD**2                                                                                                              
                                                                                                                                  
where W, the total energy after post-acceleration, is given by                                                                    
                                                                                                                                  
  W = Q[V + (E/Q)]                                                                                                                
                                                                                                                                  
Since the cometary particles are ionized by photons or charge-exchange, their charge                                              
state is predominately Q=1 and the ion mass can then be easily determined. In the                                                 
solar wind there are ions with higher charge states, such as alpha particles (Q=2) and                                            
high charge states of oxygen (O6+).                                                                                               
  The instrument contains five sensors, each consisting of a spherical electrostatic                                              
energy analyzer and a time-of-flight (TOF) analyzer (Fig. 7). The five sensors are ar-                                            
ranged as an angular array to cover the range l5 deg to 165 deg, in five equally spaced sec-                                      
tors 10deg wide relative to the spin axis of the spacecraft. As the spacecraft rotates, the                                       
angular distribution of the ions is obtained as with the Fast Ion Sensor.                                                         
  The spherical-plate electrostatic analyzer has a mean radius of 50 mm and a plate                                               
spacing of 3 mm, giving an analyzer gain factor (energy measured/voltage applied)                                                 
of 8.3. A voltage V(0) of up to = -11 kV is applied to the inner plate of the analyzer,                                           
while the outer plate is kept at 0 V. Thus the ions are effectively accelerated by                                                
(V(0)/2) on entering the analyzer, and the effective gain factor is 7.8. The energy                                               
bandwidth, defined as the full width at half maximum, is delta(E)/E= 10%. The energy                                              
                                                                                                                                  
  Figure 7. Diagram showing the layout of the                                                                                     
Implanted Ion Sensor. The five electrostatic                                                                                      
analyzers with time-of-flight analyzers are                                                                                       
shown as an array viewing through the single                                                                                      
aperture. Electronic boards for signal                                                                                            
processing are mounted behind, with the high-                                                                                     
voltage power supply underneath them                                                                                              
                                                                                                                                  
range from 90 eV to 90 keV is covered in 32 steps, equally spaced logarithmically by                                              
a factor 1.25. The level is changed once per spin and steps up on the odd-numbered                                                
steps (1, 3, 5, etc.) and down on the even steps (30, 28, 26, etc.).                                                              
 As the ions leave the electrostatic analyzer (Fig. 8) they are accelerated by 10 kV                                              
before striking a thin (5 micro g/cm**2), grid-supported carbon foil at the entrance to the                                       
time-of-flight analyzer.                                                                                                          
 Ions passing through the carbon foil transfer a small fraction of their energy to                                                
secondary electrons. Those secondaries that escape from the foil are accelerated by                                               
0.7 kV and deflected towards the microchannel plates. The fast output pulses of the                                               
microchannel plate (typical rise time of ~0.9 ns) result in an accurate timing pulse                                              
for the 'START' signal. Essentially the same principle is used in the 'STOP' detector,                                            
except that the secondary electrons are generated in the surface layer of an aluminium                                            
absorber. Although the ions enter the time-of-flight system on approximately parallel                                             
trajectories, Coulomb interaction with the atoms in the carbon foil will result in strong                                         
angular scattering. The resulting variations in the flight path are limited to +/-5% by                                           
using a spherical concave converter surface for the 'STOP' detector.                                                              
                                                                                                                                  
Figure 8. Cross-section of one of the five                                                                                        
individual sensors in the Implanted Ion Sensor                                                                                    
                                                                                                                                  
  The output signal from the five 'START' microchannel plates are added together                                                  
by one fast summing amplifier, and the outputs from the five 'STOP' microchannel                                                  
plates in another. A time-to-amplitude converter converts the time interval between                                               
the pulses into a proportional pulse amplitude. The pulses are stretched in a sample-                                             
and-hold circuit and then digitized to give an 8 bit value proportional to the time in-                                           
terval.                                                                                                                           
  The maximum time interval is set at 80 ns. Unless a 'STOP' signal is received within                                            
the 80 ns following a 'START' signal, the event is not converted.                                                                 
  The time required to process the signals from a single event is 25 micro s. Within this                                         
processing period, further 'START' pulses and valid 'START-STOP' combinations                                                     
are recorded but cannot be processed.                                                                                             
  Two separate count rates are monitored:                                                                                         
                                                                                                                                  
(a) the number of 'START' pulses                                                                                                  
(b) the number of valid 'START-STOP' combinations (TAC pulse).                                                                    
                                                                                                                                  
The requirement of a valid 'START-STOP' combination gives a high rejection of                                                     
background signals from penetrating radiation and detector noise and enables very low                                             
counting rates to be reliably measured.                                                                                           
  Monitoring the number of 'START' pulses enables the amount of dead time in the                                                  
instrument to be estimated. For example, it cannot record accurately (and was not in-                                             
tended to do so) the high flux of protons in the solar wind. The 'START' count in-                                                
dicates the number of events that could not be processed.                                                                         
  The TOF value is used in two ways. Together with the step number of the high-                                                   
voltage sweep, it addresses a look-up table where the mass of the ion responsible for                                             
the event is assigned.                                                                                                            
  The events are separated into five contiguous mass groups based on the time-of-                                                 
flight and the energy level of the analyzer. The mass groups correspond to the atomic                                             
mass number unit given in Table 6 below.                                                                                          
  The angular and energy distribution of each of the five mass groups is recorded.                                                
Secondly, a TOF spectrum is accumulated for a complete spin while the energy is con-                                              
stant at one level, by combining the outputs from all five sensors.                                                               
  Angular information is derived from two sources. The azimuthal angle comes from                                                 
the timing in the spin relative to the Sun reference pulse; the polar angle comes from                                            
the identification of the sensor responsible for the event. The identification can itself                                         
be achieved in two ways: in the 'FIND' mode, each event is associated with the 3 bit                                              
number of the sensor; in the 'SCAN' mode, each sensor is enabled alone for one-fifth                                              
of the time in each azimuthal angle sector. The usual mode used is the 'FIND' mode,                                               
but if the intensity in one sector overwhelms that in the other sectors, the 'SCAN'                                               
mode can be used to allow the other sectors to register. It reduces the overall sensitivi-                                        
ty by a factor of five. Alternatively, each sensor can be enabled or disabled individual-                                         
ly by command.                                                                                                                    
  Three types of distribution are telemetered from the sensor. The 256-level TOF                                                  
spectrum is integrated over all angles for each spin; the 4D distribution comprising                                              
five mass groups, five polar angle zones, eight or l6 azimuthal sectors and 32 energy                                             
levels, and the 'START' and 'TAC' totals in 16 sectors each spin. The complete                                                    
distribution requires 32 spins, or approximately 128 s to accumulate. The detector                                                
characteristics are summarized in Table 7.                                                                                        
  Three high-voltage units are required to operate the sensor. The microchannel plate                                             
supply has eight commandable levels between 1000 V and 3100 V to allow the bias                                                   
to be set at the correct level for the gain required. The variable voltage supply provides                                        
the voltage for the electrostatic analyzer and may be set to step through the sequence                                            
described above or to remain fixed on any one of the 32 levels. The acceleration                                                  
voltage can be set to 5 kV or 10 kV.                                                                                              
  During ground testing and the launch phase the aperture is closed by a spring-loaded                                            
cover held in place by a small pellet of biphenol (C12H10). Once in the vacuum of                                                 
space, the biphenol sublimates, the cover is released and the aperture opened. This                                               
should occur within 50 h of launch. The cover's status could not be checked in orbit                                              
for two months, but it was then found to be open.                                                                                 
                                                                                                                                  
_________________________________                                                                                                 
Table 6. IIS mass groups                                                                                                          
                                                                                                                                  
Group  Mass/charge                                                                                                                
_________________________________                                                                                                 
1      1                                                                                                                          
2      2-11                                                                                                                       
3     12-22                                                                                                                       
4     23-33                                                                                                                       
5     34-45                                                                                                                       
_________________________________                                                                                                 
                                                                                                                                  
---------------------------------------------------------------------                                                             
Table 7. Implanted Ion Sensor characteristics                                                                                     
                                                                                                                                  
E/q range (keV/q)                                 0.090-90                                                                        
Acceptance angles (each sensor)                                                                                                   
 azimuthal                                         6deg                                                                           
 polar                                            10deg                                                                           
Outer plate radius (mm)                           51.5                                                                            
Inner plate radius (mm)                           48.5                                                                            
Analyser gain                                      8.3                                                                            
Plate voltage splitting                                                                                                           
 inner                                            -11 V to -11 kV                                                                 
 outer                                              0                                                                             
Aperture area (mm**2)                              42                                                                             
Energy resolution (delta(E)/E)                     10%                                                                            
Time-of-flight path length (mm)                    22                                                                             
Geometric factor (mm**2 sr eV)                     7.6x10**-2 E (eV)                                                              
---------------------------------------------------------------------                                                             
                                                                                                                                  
6. Data Processing Unit                                                                                                           
  The Data Processing Unit (DPU) performs the following functions:                                                                
(a) It is the interface between the sensors and the spacecraft for power, telemetry                                               
    and commands.                                                                                                                 
(b) It controls the measurement sequence of the instrument and synchronizes it to                                                 
    the spacecraft rotation. Two standard signals from the spacecraft are used to                                                 
    achieve the synchronization: the Sun Reference Pulse (SRP) and the Spin-                                                      
    Segment Clock Pulse (SSCP). The latter divides the period between successive                                                  
    SRPs by 16384. The Fast Ion Sensor sequence has a duration of two spins begin-                                                
    ning 22.5deg before the sensor's field-of-view fan crosses the solar direction. Each                                          
    spin is divided into eight sectors of 45 deg. During the first sector the sensor                                              
    operates in the solar-wind mode, making eight short energy sweeps. In the re-                                                 
    maining seven sectors of the first spin, and for all eight sectors of the second                                              
    spin, the sensor operates in the wide-energy mode, making two full energy                                                     
    sweeps in each sector. The Implanted Ion Sensor sequence has a duration of 32                                                 
    spins, holding each of its energy levels for one complete spin. All the                                                       
    measurements are synchronized to the rotation, so that each value received at                                                 
    the ground is already associated with its direction without needing reference to                                              
    the spacecraft attitude solution.                                                                                             
(c) The DPU collects the accumulated counts from registers in the sensors at the                                                  
    conclusion of each sampling period timed with reference to the SSCP. For the                                                  
    Fast Ion Sensor the sampling period is eight cycles (~2 ms) in solar-wind mode                                                
    and 32 cycles (~8 ms) in wide-energy mode. For the Implanted Ion Sensor, it                                                   
    is 1024 cycles (~250 ms).                                                                                                     
    The data are acquired from the sensors each spin in the form of an array of up                                                
    to third order counts [energy (or mass), polar angle, azimuthal angle]. The array                                             
    is produced with the intrinsic resolution of the sensor. For the FIS in wide-angle                                            
    mode, this is 30 energies, six polar angle zones (the detector anodes) and 16                                                 
    azimuthal sectors (corresponding to energy sweeps).                                                                           
    The size of this array is greater than can be accommodated by the telemetry                                                   
    allocation and so the array is compressed by combining adjacent elements. This                                                
    is done in more than one way for a particular array so that different aspects of                                              
    the data are covered. The transmitted arrays are listed in Tables 8 and 9.                                                    
                                                                                                                                  
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
Table 8. Fast Ion Sensor transmitted Distribution                                                                                 
                                                                                                                                  
                    Polar** zones (deg)  Azimuthal res. (deg)   Energy spectrum  Time resolution  Mass information  When used     
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
Wide-energy mode      20-72              45                      15 contiguous   One spin         None               All          
                      72-124             45                      bands                                                            
                     124-180             45                                                                                       
PTR distribution                                                 delta(E)/E=0.6                                                   
                                                                 10 eV to 20 keV                                                  
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
Wide-energy mode      20-46              45                      30 contiguous   Three spins      None               Format 1     
                      46-72              45                      bands                                                            
                      72-98              22.5                                                                                     
HAR distribution      98-124             22.5                    delta(E)/E=0.3                                                   
                     124-150             45                      10 eV to 20 keV                                                  
                     150-180             45                                                                                       
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
Solar-wind mode       46-98               5.6                     30 contiguous  Two spins        None               Format 1     
                                                                  bands                                              Format 3     
SWA distribution                                                  delta(E)/E=0.096                                                
                                                                  E* to 6.7E*                                                     
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
Solar-wind mode      46-59               45                       30 contiguous  Two spins        None               Format 1     
                     59-72               45                       bands                                              Format 3     
SWP distribution     72-85               45                       delta(E)/E=0.096                                                
                     85-98               45                       E* to 6.7 E*                                                    
                    98-150               45                       E* set by                                                       
                                                                  command or                                                      
                                                                  on board                                                        
                                                                  processing                                                      
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
FTR = Fast Time Resolution; HAR = High Angular Resolution; SWA = Solar-Wind Azimuthal; SWP = Solar-Wind Polar.                    
** Measured from z-axis                                                                                                           
                                                                                                                                  
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
Table 9. Implanted Ion Sensor transmitted distribution                                                                            
                                                                                                                                  
                    Polar** zones (deg)  Azimuthal res. (deg)  Energy spectrum  Time resolution  Mass information  When used      
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
4DF distribution      15-25              22.5                   32 levels       32 spins         5 mass groups     Format 1       
                      50-60              22.5                   delta(E)/E=0.1                                                    
                      85-95              22.5                   90 eV to                         1                                
                     120-130             22.5                   90 keV                           2-11                             
                     155-165             22.5                                                    12-22                            
                                                                                                 23-33                            
                                                                                                 34-45                            
----------------------------------------------------------------------------------------------------------------------------------
4DH distribution      15-25              45                     32 levels       32 spins         5 mass groups  Format 2          
                      50-60              45                     delta(E)/E=0.1                   1              Format 3          
                      85-95              45                     90 eV to                         2-11                             
                    120-130              45                     90keV                            12-22                            
                    155-165              45                                                      23-33                            
                                                                                                 34-45                            
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
TOF distribution    15-165               360                    32 levels       32 spins         256 time of    Format 1          
                    (all polar                                  delta(E)/E=0.1                   flight groups                    
                     zones combined)                            90 eV to 90 keV                                                   
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
START                15-165              22.5                   32 levels       32 spins         None           All               
----------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                  
TAC                  15-165              22.5                   32 levels       32 spins         None           All               
                                                                                                                                  
4DF = 4-Dimensional Full Resolution; 4DH = 4-Dimensional Half Resolution; TOF = Time-of-Flight Spectrum; START = start pulse rate;
TAC = Timing processed pulses.                                                                                                    
** Measured from z-axis                                                                                                           
                                                                                                                                  
(d) The accumulation is carried out in 16 bit registers which the DPU compresses                                                  
    to 8 bits in a quasi-logarithmic way. The first four bits, effectively the exponent,                                          
    denote the position of the first non-zero bit in the number, and the remaining                                                
    four transmitted bits contain the next four bits of the original number, the man-                                             
    tissa. The maximum error from the truncation is therefore 3.1%. This scheme                                                   
    is capable of compressing from 19 bits to 8 bits so there are some exponents (the                                             
    hexadecimal values D, E and F) which do not occur in real data and can be used                                                
    for other purposes.                                                                                                           
(e) The instrument is spin-synchronized and produces a fixed number of data words                                                 
    each rotation. The spin period can vary with respect to the telemetry-format                                                  
    duration, so that there must be a means of allowing for a variation in the number                                             
    of words transmitted. This has been achieved by devising an instrument                                                        
    telemetry format that is spin-synchronized and has a basic length corresponding                                               
    to a rotation of 45deg. The JPA format 'floats' within the spacecraft telemetry for-                                          
    mat and is identified by three format sync. words. These words begin with the                                                 
    three hexadecimal numbers not obtained from the data compression, i.e. D, E                                                   
    and F. The second half of each sync. word contains further information about                                                  
    the sequence and the instrument status. The telemetry output is double-buffered.                                              
    1n one side, a table of values is compiled from the data currently being acquired,                                            
    while values acquired during the previous sector are read out to the telemetry                                                
    from the other side. If the table of values has not been completely read out by                                               
    the telemetry by the end of a 45deg sector then the remaining values are lost when                                            
    the buffers are interchanged. The order in which the data are compiled in the                                                 
    table is obviously important and has been prioritized. Figure 9 shows how the                                                 
    table is constructed in each telemetry format and the possible variation in the                                               
    number of values transmitted in each sector. Each type of distribution, e.g. 4DF                                              
    or FTR (see Tables 8 and 9), is transmitted completely before the next is started.                                            
    Where a distribution may be only partially transmitted, the order in which the                                                
    values are listed is also designed to minimize the loss of information. For exam-                                             
    pie, alternative energy levels through the spectrum are transmitted first and then                                            
    the intermediate values are sent. This ensures that a coarse spectrum over the                                                
    full range is transmitted first.                                                                                              
    The main cause of the variation in samples per sector is not the possible change                                              
    in the spin period, but the nonuniform spacing of the JPA words in the telemetry                                              
    format. This effect is most severe in Format 3 where, on some occasions, no                                                   
    data are transmitted in a sector. This is not a disadvantage because more                                                     
    distributions can be transmitted than if the sampling were uniform. The time                                                  
    resolution is reduced because data from two spins must sometimes be combined                                                  
    to obtain one complete distribution.                                                                                          
(f) The final function of the DPU is to set the energy range for the solar-wind mode.                                             
    There are eight possible starting points for the sweep in the upper half of the                                               
    full sweep range. The range can be set by ground control or can be automatically                                              
    tracked. The DPU scans through the solar-wind spectrum and searches for the                                                   
    peak count. If that peak count does not exceed 64, then the range is stepped to                                               
    the next lower one. If no count greater than 64 has been recorded by the time                                                 
    it reaches the bottom level, then it recycles to the top and continues. If a peak                                             
    with more than 64 counts is found, the starting point of the sweep is changed                                                 
    until its energy level lies between two preset limits in the 30-level spectrum.                                               
    These limits are calculated to ensure that the alpha-particle peak is included as                                             
    well as the proton peak.                                                                                                      
                                                                                                                                  
                                                                                                                                  
                                                                                                                                  
7. In-flight Performance                                                                                                          
  The instrument was first turned on on 8 September 1985. The performance                                                         
throughout all testing was nominal. At the time the spacecraft was more than 10                                                   
million kilometres from the Earth in the solar wind. The only population that could                                               
be identified so far is the solar wind. The Fast Ion Sensor has been able to follow the                                           
solar-wind variations with its auto-ranging capability. The Implanted Ion Sensor has                                              
been able to identify higher mass ions such as O6+ and O5+ in the solar wind. The in-                                             
strument has been operated since then whenever spacecraft operations allow.                                                       
                                                                                                                                  
Acknowledgements                                                                                                                  
  The production of this instrument required the dedicated efforts of a large number                                              
of people. In particular thanks are due to: Messrs. F.N. Little, P.H. Sheather and J.                                             
Ootes at the Mullard Space Science Laboratory; H. Wirbs, K.H. Otto, A. Loidl and                                                  
H. Sommer at the Max Planck Institut fur Aeronomie; T. Booker, R. Black and A.                                                    
Lozano at Southwest Research Institute; R. Field at Mullard Limited; J. Coles and P.                                              
Howarth at Cambridge Consultants Limited; M. Jacopini at Laben; and K. Rembach                                                    
at Dornier System.                                                                                                                
                                                                                                                                  
                                                                                                                                  
Figure 9. The structure of the JPA spin-                                                                                          
synchronized floating format in the three                                                                                         
spacecraft formats. The cross-hatched bar                                                                                         
beside each column shows the range of                                                                                             
variation in the number of samples transmitted                                                                                    
in each sector. The distributions (FTR,4D,                                                                                        
etc.) are defined in Tables 8 and 9.                                                                                              
                                                                                                                                  
  The work at Mullard Space Science Laboratory and the Rutherford Appleton                                                        
Laboratory was supported by the UK Science and Engineering Research Council. The                                                  
work at the Max Planck Institut fur Aeronomie was supported by the Max-Planck-                                                    
Gesellschaft zur Forderung der Wissenschaften and by the Bundesminister fur                                                       
Forschung und Technologie under Grant 01-OF-112-4. The work at the Southwest                                                      
Research Institute was supported by NASA Contract Number NASS-27442. The work                                                     
at the Kiruna Geophysical Institute was supported by the Swedish Board for Space Ac-                                              
tivities. The work at the Istituto di Fisica dello Spazio Interplanetario was supported                                           
by the Piano Spaziale Nazionale of the Consiglio Nazionale delle Ricerche.                                                        
                                                                                                                                  
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