***** File GIORPA.TXT NOTE: This file was created by scanning the original hardcopy article and only the Figure captions are included. The Giotto RPA-Copernic Plasma Experiment H. Reme, F. Cotin, A. Cros, J.L. Medale, J.A. Sauvaud & C. d'Uston Centre d'Etude Spatiale des Rayonnements, CNRS-Toulouse University, France A. Korth, A.K. Richter & A. Loidl Max-Planck-Institut fur Aeronomie, Lindau, Germany K.A. Anderson, C.W. Carlson, D.W. Curtis & R.P. Lin Space Sciences Laboratory, University of California, Berkeley, USA D.A. Mendis Department of Electrical Engineering & Computer Sciences and Center for Astrophysics & Space Science, University of California, San Diego, USA Abstract The RPA-Copernic plasma experiment for the Giotto mission consists of two Sen- sors, the RPA1-EESA spectrometer and the RPA2-PICCA electrostatic mass analyzer. RPA 1-EESA measures the three-dimensional distributions of electrons between 10 eV and 30 keV. These electron measurements should contribute to defining the properties of the cometary plasma and its interaction with the solar wind. The RPA-2 PICCA sensor is an electrostatic mass analyzer designed to measure the composition and the distribution of thermal cometary positive ions, including clathrate hydrates in the mass range 10 to 203 amu. This instrument should probe the interaction processes produc- ing these ions, and provide information on the spatial distribution and chemical com- position of the solid, gas and plasma components of the cometary environment. Such information should allow identification of parent molecules and hence give some knowledge of the composition of the cometary nucleus. 1. Introduction The RPA-Copernic experiment aboard Giotto is designed to measure the three- dimensional distributions of electrons between 10 eV and 30 kev, and the composi- tion and distribution, close to the comet, of thermal positive ions, including clathrate hydrates, in the mass range 10 to 203 amu. It should help in achieving several of the major scientific objectives of the Giotto mission, by determining: - the nature of the comet/solar-wind interaction and of comet tails - the chemical and physical nature of the comet's atmosphere and ionosphere - the chemical and physical structure of the comet's nucleus. The electron measurements should contribute to defining the structure of the cometary plasma environment and its interaction with the solar wind (Fig. 1). In addition, these electron measurements may provide detailed information on the magnetic-field struc- ture of the comet nucleus and comet/solar-wind interaction region via electron reflec- tion magnetometry, and information on the role of energetic electrons in ionization of the cometary plasma. Figure 1. Trajectory of the spacecraft during the encounter with comet Halley. Conceptual solar-wind/comet interaction and cometary atmosphere regions are also indicated, as well as the timing of the different operating modes on the right axis Before the comet encounter, the electron spectrometer should provide detailed infor- mation about the plasma in the interplanetary medium. Positive ions are produced via various interactions between the comet nucleus, com- etary dust, micrometeoroids, cometary and solar-wind plasmas, neutral gas, and solar electromagnetic radiations. As in the Earth's atmosphere, these ions are expected to be present not only as free ions, but probably also as clathrate hydrates, i.e. bound to water clusters stemming from dissociated icy grains evaporating from the cometary nucleus. Thus measurements of ions, of their clathrate hydrates, and of their mass distribution close to the comet should provide a sensitive probe of these interaction processes, as well as information on the spatial distribution and composition of the solid, gas and plasma components of the cometary environment, and of the chemical composition of the cometary nucleus (parent molecules). The detailed scientific objectives of the experiment were described in Reme et al. (1981). In order to achieve electron and thermal ion measurements with optimal perfor- mances, the RPA-Copernic experiment consists of two separate instruments (Reme et al., 1981; 1983): - a symmetric quadrispherical electrostatic analyzer (RPA1-EESA) of novel design (Carlson et al., 1982) which provides 4 pi electron measurements with high sen- sitivity and high energy, angular and time resolution; it was designed jointly by the Space Sciences Laboratory in Berkeley and the Centre d'Etude Spatiale des Rayonnements in Toulouse and was built by CESR; - a light electrostatic mass analyzer (RPA2-PICCA), which is designed to detect thermal positive ions with high sensitivity, low background and good mass resolu- tion in the range 10 to 203 amu, close to the comet. This instrument was built by the Max-Planck-Institut fur Aeronomie in Lindau. These instruments are mounted on the spacecraft's experiment platform in such a way that the field of view includes the ram direction. The RPA-Copernic experiment is designed to: - measure the flux and spectra of electrons from 10 ev to 30 kev, in 39 steps, with a large dynamic range, in the complete 4 pi solid angle, with high time resolution for half a spin period (2 s) - process the data through a microprocessor, which is programmed to provide the electron pitch-angle distribution around a symmetry direction, i.e. the magnetic- field direction - detect thermal positive ions incoming from the ram direction and to measure their spatial distribution and their composition from 10 amu to 203 amu with a good mass resolution (Delta m 80 eV (steps 1 to 23) and steps 24, 26,28 and 30, the calcula- tions are made by the PAD Processor (ODES-B). The calculations for the other steps are made by the main calculator (ODES-A) - the Low Energy Distribution (LED): in order to search for possible azimuthal anisotropies in the distributions of low-energy electrons (E <80 eV), thereby allowing the possible effects of the asymmetries in spacecraft potential or of predominant streaming directions to be seen, snapshots of the azimuthal distribu- tions are regularly calculated and transmitted. A snapshot is performed in 4 s using one half of the MCP. The next snapshot is performed using the second half of the MCP. Thus, a complete spacecraft rotation is necessary to obtain the snap- shot corresponding to all azimuths (16 sectors phi). In the case where there are two snapshots per data packet (Formats 2/1 and 2/2), they are performed with the same half of the MCP: LED (phi(j), E(k)) = Summation(i) CR(theta(i), phi(j), E(k)) (for i = 0-7 or i = 8-15). In Format 2/2, this calculation is also performed for energies E> 80 eV and is then called the ED (Energy Distribution). - 2s snapshots of the Distribution Function (DF) are transmitted frequently. Beyond their scientific interest, they also permit the validity of the preceding calculations (principally PAD) to be verified. However, these snapshots are not the most elementary distribution function: the ram and the 128 angular bins sampled are grouped on only 89 solid angles, following the principle outlined below. As the directions along the spin axis are oversampled, the high-elevation azimuthal sectors are grouped, whereas the low-elevation ones are transmitted individually. The Distribution Function can be transmitted for selected energy steps or summed over several consecutive steps - in several bands (of one or more energy steps), the maximum count rate in a spacecraft half-rotation is searched for. The azimuthal as well as the elevation position of this maximum and the corresponding flux are transmitted. For several energy steps, the step corresponding to this maximum is also identified. - the ram sector in telemetry Formats 2/1 and 2/2 is used to search for an eventual cometary negative ion contribution. Accumulated counts on 32 (Format 2/1) or 8 (Format 2/2) energy spectra are divided into 32 (Format 2/1) or 33 (Format 2/2) energy steps and transmitted. The operation of the different telemetry formats is given in detail in Section 4. These data are compressed from 21 bits to 8 bits, according to a special algorithm, such that the precision is always better than 3.3% if the count rate is between 32 and 131 071 s**-1 and 7.2% if the count rate is between 131 072 and 1 996 080 s**-1. 2.6 The Pitch Angle Distribution processor The Pitch Angle Distribution (PAD) processor is designed to sort the three- dimensional electron distribution data generated by the RPA1-EESA detector into two- dimensional pitch-angle distributions in real time. The PAD processor hardware consists of an NSC800 CMOS microprocessor together with hardware multiply/divide, ROM, and a pair of dual-ported RAMs. The RPA main processor controls and communicates with the PAD processor via the dual- ported RAMs. The PAD partial processor computes the PAD in three steps: first, it computes two partial-pressure tensors from the input data over two separate energy ranges (Eqn. 1). Next, each of these tensors is diagonalized under the assumption that the distribution has a two-dimensional symmetry, and the nondegenerate eigenvector is computed. This vector corresponds to the symmetry direction of the distribution, which is presumably the magnetic-field direction. Finally, the three-dimensional input data is sorted into a two-dimensional PAD using the computed symmetry direction for each energy range (Eqn.2). Pitch-angle distributions of eight angles, summed into 10 or 16 energy steps, are computed every half spin (2s). The equations used in the computations are: P(i,j) = Summation (theta, phi, E) [(v(i)v(j)/absolute (v))*(absolute cos(theta))*CR(theta, phi, E)] (1) where CR(theta, phi, E) - the count rate sample for energy step E in the theta, phi v(theta, phi, E) - the velocity vector of particles corresponding to data sample CR(theta, phi, E) v(i) - the ith component of velocity vector v absolute cos(theta) - a weighting factor to compensate for the over-sampling at the poles by the analyzer P(i,j) - the i,j element of the pressure tensor =/P (actually =/P is the momen- tum flux tensor; it is approximately equal to the pressure tensor for plasma bulk velocities that are small compared with the thermal velocities). PAD (E, alpha) = Summation(theta, phi) [ OF(alpha,alpha') * (absolute cos (theta)) * CR(theta, phi, E) ] (2) / [16 * Summation(theta, phi) [ OF(alpha, alpha') * (absolute cos(theta)) ] alpha (theta, phi) = cos**-1 (B,S(theta, phi)) where alpha' = the pitch angle of the look direction S(theta,phi) corresponding to data sample CR(theta, phi, E) B = the symmetry direction computed from the pressure tensor =/P OF(alpha, alpha') = the fraction of the data sample at pitch angle alpha' that falls into pitch-angle bin alpha. Typically, each data sample is summed partially into one pitch-angle bin, and partially into the adjacent bin. Note that the denominator factor normalizes the PAD to counts per 22.5deg by 22.5deg bin. 2.7 Position on the spacecraft The configuration of the spectrometer on the experiment platform is shown in Figure 10. The instrument extends beyond the platform in order to get clear of the bumper shield and also of the upper part of the spacecraft body, over the total field of view, which is from -2deg (towards the spacecraft body) to +7deg. It is partly pro- tected by the extension of the rear shield (by a piece of kevlar) and the bumper shield as shown in the figure. The outer hemisphere is made of 0.8 mm-thick aluminium, which is painted on the outside with a white conductive paint. The top cap, which defines the entrance aperture, is similarly made of white-painted aluminium and also grounded to the signal ground. This cap is designed to prevent direct solar illumination at the time of encounter. A portion of the top disc is reduced in the ram direction to minimize the dust-impact problem. 3. The Positive Ion Cluster Composition Analyser (RPA2-PICCA) 3.1 The sensor The RPA2-PICCA experiment (Fig. 11) is designed to measure the spatial distribu- tion and the composition of positive ions, including large-mass ion-water cluster com- pounds, in the cometary coma. In the innermost part of this region, these particles are expected to be singly charged (I+ or I+(H2O)n) and rather cold, i.e. their thermal velocity is negligibly small compared with the relative spacecraft velocity (about 69 km/s). These particles are therefore expected to flow with the spacecraft velocity and to be highly collimated in the direction opposite to the spacecraft ram direction. The kinetic energy of these particles will be given approximately by E(kev) Figure 10. Location of the RPA1-EESA spectrometer on the experiment platform Figure 11. The Positive Ion Cluster Analyser (RPA2-PICCA) ~ 5.1 X 10**-6 N*V**2, where V is the relative spacecraft velocity in km/s and N the ion mass in amu. The Giotto space probe is expected to traverse the inner coma in a rather short time, so that the main observations performed by the RPA2-PICCA sensor will begin 15 min before the closest approach of the spacecraft to Comet Halley. In the inner part of the cometary coma the density, and therefore the flux of positively charged par- ticles is expected to be rather high (~ 10**3 cm**-3), and at the same time to be accom- panied by a very high flux of 'background radiation'. In the case of the RPA2-PICCA experiment, this background could be caused by one or several of the following com- ponents: solar and cometary electromagnetic radiation, incoming dust, neutral gas and sputtered fragments due to collisions of these particles in the analyzer itself, energetic electrons, high fluxes of positive ions interacting with the spacecraft and/or the outer parts of the sensor, and any kind of particles surrounding the spacecraft due to dust- micrometeoroid-neutral gas and cometary plasma interactions with the space probe. To cope with these constraints and at the same time obtain as large an amount of significant information as possible, the RPA2-PICCA sensor has been optimized in the following way. As Figure 12 shows, the main part of this experiment is mounted behind the backup shield or the second bumper shield, but inside the spacecraft. Only its entrance aperture, which is a 50X50X 150 mm**3 tube, is outside of the spacecraft, but it is attached directly to the spacecraft skin and has a large (+/-6deg) unobstructed field of view in the flight direction of the probe. This aperture is completely open, so that light, micrometeoroids, dust and neutral gas can fly through without penetrating the analyzer system. In addition, the field of view of the analyzer itself is chosen in such a way that it never intersects any part of the aperture, nor the spacecraft skin or shields, but at the same time is still large enough (+/-5deg) to accept all particles to be analyzed, even in cases where their trajectories may differ, due to additional ther- mal speed of the ions, slight changes in the speed and orientation of the spacecraft, or electrostatic potentials of the space probe. Thus, any kinds of fragments originating from dust, neutral- or charged-particle interaction with the spacecraft and aperture materials is precluded from entering the analyzer system. As with RPA2-PICCA, a mass analysis of positive ions and their clathrate hydrates will be performed in the mass range 10-203 amu: their kinetic energy will be up to about 5 keV. By applying a steadily increasing deflection voltage (+ HV) between the top and the bottom parts of the aperture, the particles will therefore be deflected from the incoming flow direction into the analyzer system. This analyzer is a 180deg hemispherical electrostatic analyzer, which focusses the particles onto two chan- neltrons with different sensitive areas in order to increase the dynamic range of the instrument. The overall dynamic range covers ion densities from 10**-3 to 10**3 ions/cm**3. Table 2 gives the geometrical characteristics of RPA2-PICCA. The den- sities of positive ions and ion-compounds expected at various distances from the com- etary nucleus, as determined by Huebner & Giguere (1980) by multidimensional computer simulations, lie within the dynamic range of RPA2-PICCA. According to the equation E(keV) ~/= k*N*V**2 mentioned above, any energy/charge (E/Q) measurement of this analyzer will actually be a measurement of the mass of the particle, as Q=1. To obtain a good and constant mass resolution, the ions are decelerated before entering the electrostatic analyzer: ions with masses 10 to 50 will have energies after this deceleration of 100 eV, and ions with masses 51 to 203, 250 eV. Using an analyzer with Delta E/E ~/= 10% and operating at two different fixed voltages for the two mass ranges, we obtain Delta E = 10 eV and Delta m ~/= 0.4 for the light ions and Delta E = 25 eV and Delta m ~/= 1 for the heavier ions. Figure 12. Cross-sectional view of the RPA2-PICCA sensor ----------------------------------------------------------------------------------- Table 2. Geometrical factor of RPA2-PlCCA in cm**2 (for parallel incidence) G (cm**2) G (cm**2) Step numbers Mass (amu) (large CEM) (small CEM) ----------------------------------------------------------------------------------- 1- 10 10-14 5.3x10**-4 6.2x10**-5 11- 20 14-18 5.2x10**-4 5.2x10**-5 21- 30 18-22 5.1x10**-4 3.0x10**-5 31- 40 22-26 5.1x10**-4 2.0x10**-5 41- 50 26-30 5.0x10**-4 1.5x10**-5 51- 60 30-34 4.9x10**-4 61- 70 34-38 4.7x10**-4 71- 80 38-42 4.3x10**-4 81- 90 42-46 4.0x10**-4 91- 100 46-50 3.7x10**-4 101-110 51-60 4.4x10**-4 111-120 61-70 4.5x10**-4 121-130 71-80 4.7x10**-4 131-140 81-90 4.3x10**-4 141-150 91-100 3.3x10**-4 151-160 101-110 2.5x10**-4 161-170 111-120 1.9x10**-4 171-180 121-130 1.4x10**-4 181-190 131-140 1.1x10**-4 191-200 141-150 9.0x10**-5 201-210 151-160 7.6x10**-5 211-220 161-170 6.5x10**-5 221-230 171-180 5.5x10**-5 231-240 181-190 4.6x10**-5 241-250 191-200 4.0x10**-5 251-256 201-206 3.4x10**-5 ----------------------------------------------------------------------------------- The electronics used within the RPA2-PICCA instrument are shown in Figure 13. All high voltages required for the deflection system, lens system, electrostatic analyzer and the two channeltrons are generated within the sensor, so that there are no high- voltage lines between the two RPA-Copernic instruments. The electronics also include one test generator (100 kHz) and one ultraviolet lamp to test the functions of the amplifiers and the channeltrons. A small negative voltage (-10 V) is applied to the outer part of the sensor in order to repel the secondary electrons of the plasma cloud around the spacecraft, which would otherwise be captured by the positive deflection voltage. 3.2 Associated digital electronics The electronics associated with RPA2-PICCA are included in the RPA1-EESA box, and are organized around a CDP 1802 microprocessor. These electronics are used to control operation of the RPA2-PICCA detector according to telemetry format and the command status: - high-voltage stepping - test generator on/off - UV lamp on/off. Synchronously with this task, this microprocessor acquires data from two 16-bit counters, one for the larger CEM and one for the smaller. These data are compressed from 16 bit to 8 bit, according to a special algorithm, such that the precision is always better than 1.2% if the count rate is lower than 255, and better than 3.3% if the count rate is between 255 and 65535. Every 16 s a packet of data is generated and is sent to the telemetry by using a DMA interface. The high-voltage analyzer is monitored through an 8-bit analogue-to-digital converter. The step number and corresponding control voltage are included in the scientific data stream. Figure 13. Schematic block diagram of RPA2-PICCA 4. Modes of Operation The initial Giotto telemetry was divided into two formats to allow plasma ex- periments to get more data far from the nucleus. A third reduced format (Format 3) was also added in order to have some cruise measurements and as complete as possible coverage during the last days before the closest approach to the comet nucleus. The telemetry allocation for the RPA-Copernic experiment is as follows: 904 bit/s in Format 3, 2530 bit/s in Format 1, 1807 bit/s in Format 2. Format 2 will be used from t(0)-1 hour (t(0) = time of closest approach) until mission end. Format 1 will be used from t(0)-4 h until t(0)-1 h. Prior to that, Formats 1 and 3 will be used in order to obtain complete coverage of the last 24 h before the closest approach to the nucleus and significant coverage during the last five days. An internal RPA-Copernic switch in Format 2, 15 min before t(0) will modify bit distribution between the two sensors (Formats 2/1 and 2/2). Tables 3,4,5 and 6 give summaries of the measurements and telemetered data for each format (in HBR for For- mats 1, 2/1 and 2/2). RPA2-PICCA is used mainly in the Format 2/2 (encounter phase). Its cycle of measurement is 3.2 s, i.e. less than one spin period, avoiding the analysis of the same masses always in the same sector. This sensor is not used in Format 3 and is in a survey mode in Formats 1 and 2/1. --------------------------------------------------------------------------------------- Table 3. RPA-Copernic measurements in telemetry Format 3 Detector Data output Parameters Time resolution (s) Bit rate (bit/s) --------------------------------------------------------------------------------------- RPA 1-EESA Pitch-angle 8 alpha x 10 E 2 388 distribution 7E> 80 eV* 3E<80eV Omnidirectional 16E> 80 eV 2 64 energy spectrum low-energy 13e(E<80 eV)** distribution x 16 phi** Snapshot 4 s 104 Distribution every 16 s function 6E x 89 Omega Snapshot 2 s 267 Six maxima and every 16 s positions for six 2 72 integrated energy bands*** RPA2-PICCA No use Housekeeping 9.5 --------------------------------------------------------------------------------------- Total 904.5 --------------------------------------------------------------------------------------- E = Energy alpha = Pitch angle Omega = Solid angle sector phi = Azimuthal angle Remarks * The B-direction is calculated for the 2E bands. In 2 s, the total number of words is (8 alpha x 10E) + (6 pressure tensor elements) x (2E bands) + (B direction (2 words)) x (2E bands) + (1 HK word) = 97 words. ** One snapshot with the first half of the MCP, the following one with the second half of the MCP. *** Giving the energy (1 word) of each maximum (1 word) and its angular position (1 word) for 6 energy bands, i.e. 18 words. --------------------------------------------------------------------------------------- Table 4. RPA-Copernic measurements in high bit rate (HBR)* telemetry Format 1 Detector Data output Parameters Time resolution (s) Bit rate (bit/s) --------------------------------------------------------------------------------------- RPAl-EESA Pitch-angle 8 alpha x 16 E 2 580 distribution 12E>80 eV 4E<80eV Omnidirectional 11E>80 eV 2 92 energy spectrum 12E< 80 eV Low energy 12E(E<80 eV) Snapshot 4 s 96 distribution x16 phi every 16 s Distribution 15E x 89 Omega Snapshot 2 s 1335 tunction every 8 s 39 maxima and 23E>80eV 2 312 positions for l6E<80 eV 39E** RPA2-PICCA l0- 50 amu Delta M=0.4(101 steps) 182 steps 90 51-129 amu Delta M=l (79 steps) in 16 s*** +6**** Housekeeping Includes MCP 17.5 current measurements 16E in 16 s --------------------------------------------------------------------------------------- Total 2528.5 --------------------------------------------------------------------------------------- * The same data are produced in LBR, but from every other spin only. ** Energy information transmission as in Format 3 not needed. *** Spin-segment clock: 16 384 pulses/spin. Integrated measurement each 360 pulses, i.e. Delta t ~/= 88 ms **** 6 bit/s for status. ADC. high-voltage steps, references of RPA2-PICCA where only large CEM used. --------------------------------------------------------------------------------------- Table 5. RPA-Copernic measurenients in HBR* telemetry Format 2 (Format 2/1) Detector Data output Parameters Time resolution (3) Bit rate (bit/s) --------------------------------------------------------------------------------------- RPA1-EESA Pitch-angle 8 alpha x 16 E 2 580 distribution 1 2E > 80 eV 4E<80eV Omnidirectional 11E>80 eV 2 92 encrgy spectrum 12E <80 eV Low energy 12E(E<80 eV) Snapshot 4 s 192 distribution x 16 phi every 8 s Distribution 8Ex89 Omega Snapshot 2 s 7l2 function every 8 s Seven maxima and 2 84 positions for seven integrated energy bands Ram direction 32 energy steps 8 32 10 eV - 5 keV RPA2-PlCCA 10- 50 amu Delta M=0.4 (101 steps) 182 steps 90 51-129 amu Delta M=1 (79 steps) in 16 s +6 Housekeeping Includes MCP 17.5 current measurements 16E in 16 s --------------------------------------------------------------------------------------- Total 1805.5 --------------------------------------------------------------------------------------- * The same data are produced in LBR, but from every other spin only --------------------------------------------------------------------------------------- Table 6. RPA-Copernic measurements in HBR* telemetry Format 2 (Format 2/2) Detector Data output Parameters Time resolution (s) Bit rate (bit/s) --------------------------------------------------------------------------------------- RPA1-EESA Pitch-angle 8 alpha x 10 E 2 388 distribution Omnidirectional 16E>80 eV 2 116 energy spectrum 13E< 80 eV Energy 12E(E< 8 eV) x 16 phi Snapshot 4 s 304 distribution 7E(E>80 eV) x l6 phi every 8 s Seven maxima and 2 84 positions for Seven integrated energy bands Ram direction 33 adjacent 2 s 132 energy steps 1O eV -6 keV 10- 50 amu Delta M=0.4 (101 steps 262 steps 760 RPA2-P1CCA 51-203 amu with 50 meas. in 3.2 s** with small CEM +6 in parallel) Delta M=1 (153 steps) Housekeeping Including MCP 17.5 current measurements 16E in 16 s --------------------------------------------------------------------------------------- Total 1807.5 --------------------------------------------------------------------------------------- * The same data are produced in LBR, but from every other spin only ** Integrated measurement each 50 pulses of the spin segment clock, i.e. 1520 measurements during four spins (Delta t ~/= 12.2 ms). In four spins we have five measurement sequences; in every sequence we have 262 intervals: 8 are used for resetting the high voltage, 254 with the large channeltron (from 10 to 203 amu) and 50 with the small channeltron (from 10 to 29.6 amu) in parallel. 5. Conclusion The main characteristics of the two sensors of the Giotto RPA-Copernic experiment are summarized in Table 7. The experiment operated successfully for the first time in September 1985. RPA2-PICCA is working well, but is not designed to study solar- wind ions, and is therefore not useful during the cruise phase prior to encounter with Comet Halley. RPA1-EESA is regularly providing fine data on the electrons in the interplanetary medium. Figures 14 and 15 show examples of results obtained on day 289 in 1985. Figure 14 contains 5 h of measurements of the interplanetary B-field direction and of the electron density. Figure 15 is an example of the electron Low Energy Distribution (LED) obtained over one spacecraft spin period (4 s). Hence, these two sensors, specially designed for the Giotto mission, are ready to study the solar wind - Comet Halley interaction and the environment of the comet nucleus on 13 March 1986. The electron and ion measurements should help to define the basic structure of the cometary plasma, and provide a sensitive diagnosis of the interactions between the various components of the cometary environment: solar wind, cometary plasma, micrometeoroids, cometary dust, the nucleus, neutral gas, electrons and ions. In addition, important information on the composition of comets should be obtained which, in combination with that from other experiments, should help us to define the chemical and physical nature of comets. --------------------------------------------------------------------------------------- Table 7. Main characteristics of the two sensors of the RPA-Copernic experiment RPA2-PICAA RPA1-EESA (Positive thermal ion composition Function (Suprathermal electron analyzer) analyzer) --------------------------------------------------------------------------------------- Instrumentation Analyser Symmetrical quadrispherical Hemispherical Detectors Microchannel plates Two channeltrons (one for high count rates) Energy range 10 eV - 30 keV Mass range 10 - 203 amu Energy resolution 10% Mass resolution Delta M=0.4 if M<50 amu Delta M=1 if M>50 amu Geometrical factor 2.5x 10**-3 Between 5.3 x 10**-4 and 3.4 x 10**-5 xE (keV) cm**2 . ster . keV cm**2 for a parallel incidence (Table 2) Field of view 360deg x4deg (FWHM) 60x6deg Best angular resolution 22.5deg x4deg (FWHM) Dynamic range (4x10**3x10**9)/E (keV) 10**-3 - 10**3 cm**-3 (cm**2.sec.ster.keV)**-1 Background rejection Fly-through source - UV rejection Fly-through source - UV rejection Best time resolution 1/2 spin (2 s) for a complete 3D 3.2 s for an ion mass spectrum distribution with 39 energy steps Onboard data processing Pitch-angle sorting --------------------------------------------------------------------------------------- Physical Characteristics Weight 2.3 kg + 0.2 kg (harness from 0.78 kg RPA1 to RPA2) Power 3.IW 0.6W Telemetry (high bit rate) Format 1: 2430 bit/s, Format 1: 98 bic/s, (divided by 2 in low bit Format 2/1: 1709 bit/s Format 2/1: 98 bit/s rate in Format 1, Format 2/2: 1039 bit/s. Format 2/2: 768 bit/s: 2/1 and 2/2) Format 3: 903 bit/s not used in Format 3 --------------------------------------------------------------------------------------- Figure 14. Results of the calculation of the B- field direction on day 289 in 1985 over a 5 h period: B(theta) is given in the lower part and B(phi) in the centre. The electron density is given in the upper sector Figure 15. Example of the electron low-energy distribution obtained during one spin of the spacecraft on day 289 in 1985 at 08.00:01 UT Acknowledgements The design, development, fabrication, testing and calibration of the RPA in- struments would have been impossible without the ingenuity and dedicated efforts of many individuals in the three cooperating institutions. The development and fabrication of the RPA1-EESA instrument were accomplished in the Centre d'Etude Spatiale des Rayonnements. Thanks are due to C. Aoustin, J. Bouyssou, M. Cassignol, J. Coutelier, J. Rouzaud and P. Souleille for their con- tributions. This work was supported by CNES under Grant No. 1212. The development and fabrication of the RPA2-PICCA instrument were accomplish- ed in the Max-Planck-Institut fur Aeronomie. The authors are especially indebted to W. Guttler and H. Schuddekopf for their strong personal involvement during develop- ment, testing and calibration. This work was supported by the Max-Planck- Gesellschaft fur Forderung der Wissenschaften and by the Bundesminister fur Forschung und Technologie under Grant No. 01 OF 052. The research in the United States was supported in part by NASA Contract NASW-3575. Thanks are also due, for their efficient support, to D. Dale, J. Credland and C. Berner and the ESA Giotto Project Team, and to R. Reinhard, Giotto Project Scientist. References Anderson K A, Lin R P, McCoy J E & McGuire R E 1975, Measurement of lunar and planetary magnetic fields by reflection of low energy electrons, Space Sci. Instr., 1,439. Carlson C W, Curtis D W, Paschmann G & Michael W 1982, An instrument for rapidly measuring plasma distribution functions with high resolution, Adv. Space Res., 2, 67. Gosling JT, Asbridge J R, Bame S J & Feldman W C 1978, Effects of a long entrance aperture upon the aximuthal response of spherical section electrostatic analyzers, Rev. Sci. Instrum., 49, 1260. Huebner W T & Giguere R T 1980, Model of comet coma: 2. Effects of solar photodissociative ionization, Astrophys. J., 238, 753. Lin R P, McGuire R E, Howe H C. Anderson K A & McCoy J E 1975, Mapping of lunar surface remanent magnetic fields by electron scattering, Proc. Sixth Lunar Sci. Conf., 2971. Reme H, Cotin F, d'Uston C, Sauvaud J A, Korth A, Richter A K, Anderson K A, Carlson C W, Lin R P, Wekhof A, Mendis D A & Johnstone A D 1981, The Copernic experiment to measure three-dimensional electron distribution and the composition of thermal positive ions including water clusters near Comet Halley, ESA SP-169, 29. Reme H, Cotin F, Cros A, d'Uston C, Sauvaud J A, Bush R, Korth A, Loidl A, Richter A K, Anderson K A, Carlson C W, Curtis D, Lin R P, Mendis D A, Wekhof A & Johnstone A D 1983, An experiment to study the solar wind - Comet Halley interaction and the cometary environment with 3D electron distribution and thermal positive ion measurements onboard the Giotto spacecraft, Cometary Exploration III, 321. d'Uston C & Reme H 1984, Anticipated impact plasma problems for the Copernic- RPA experiment in the cometary environment, ESA SP-224, 81.