***** 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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