This chapter contains sections titled: Objectives The Spherical Section “Top Hat” Electrostatic Energy Analyzer Methods of Numerical Analysis Results of the Design Optimization Simulation The Effect of Fabrication Tolerances. Conclusions

We present a new design of a dual retarding potential analyser with a differential energy response, suitable for plasma energies from leV to at least 1.5keV. The energy passband can be electrically controlled so that the analyser's sensitivity and resolution can be varied in flight to adapt to the plasma conditions. This design features low weight and low power consumption, which makes it well suited for the new regime of cheaper, faster missions. The Cold Ion Detector (CID), an instrument based on this design, has been flown aboard the Space Technology Research Vehicle 1A (STRV-1A) and some flight data, as well as laboratory calibration data, are presented.

This chapter contains sections titled: Introduction Triplet Instrument Fonema Instrument Conclusions

This chapter contains sections titled: The Mars96 Mission Design Objectives for the Fonema Instrument Description of Instrument Electrostatic Mirror Focusing Particle Collimator Magnetic Deflection Region Electrostatic Deflection Region The Image Plane Results Instrument Specification Sheet

One of the major aims of the Cassini spacecraft is to characterize in detail the interaction of the solar wind with the Saturnian system. The Cassini Plasma Spectrometer (CAPS) addresses this aim by measuring the plasma particles, ions and electrons, with a suite of three complementary sensors. One of these, the Electron Spectrometer (ELS), will measure the energy and angular distributions of solar wind and magnetospheric electrons at Saturn with unprecedented accuracy.

Low energy (≤100 eV–10 keV) field‐aligned electrons are often observed by the Low Energy Plasma Analyzer (LEPA) on the CRRES satellite. These electrons usually occur in bursts of <10 min duration and are mostly bidirectional, though opposing fluxes are not always equal. The events can be seen from L‐values of 5 outward (to at least L = 7) and from 0800 magnetic local time (MLT) through midnight to 1400 MLT. Larger numbers of events were seen at the outer edge of CRRES' coverage and in the evening and early morning sectors, when the apogee of CRRES was at higher latitudes. The bursts normally occur within 20 min of substorm onset. High‐latitude, low‐altitude spacecraft often observe upgoing field‐aligned electron beams. We suggest that these are the source of the field‐aligned electron beams seen by CRRES near the equator.

The relativistic electron response in the outer radiation belt during magnetic storms has been studied in relation to solar wind and geomagnetic parameters during the first six months of 1995, a period in which there were a number of recurrent fast solar wind streams. The relativistic electron population was measured by instruments on board the two microsatellites, STRV-1a and STRV-1b, which traversed the radiation belt four times per day from L ~ 1 out to L ~ 7 on highly elliptical, near-equatorial orbits. Variations in the E > 750 keV and E > 1 MeV electrons during the main phase and recovery phase of 17 magnetic storms have been compared with the solar wind speed, interplanetary magnetic field z-component, Bz , the solar wind dynamic pressure and Dst *. Three different types of electron responses are identified, with outcomes that strongly depend on the solar wind speed and interplanetary magnetic field orientation during the magnetic storm recovery phase. Observations also confirm that the L-shell, at which the peak enhancement in the electron count rate occurs has a dependence on Dst *.Key words. Magnetospheric physics (energetic particles, trapped; storms and substorms) – Space plasma physics (charged particle motion and accelerations)

Using data from the Low Energy Plasma Analyzer we present detailed pitch angle versus energy electron distributions at the time of field‐aligned electron events. We present three case studies, as well as the findings of a larger survey of 158 events. These distributions show that electrons are apparently scattered in pitch angle out of field‐aligned electron beams observed around 0.1–1 keV. Generally, this pitch angle scattering is accompanied by simultaneous acceleration. As electrons are scattered from the field‐aligned beam to pitch angles of ∼60°, they typically increase to energies of around 10 keV. The electrons are then seen to scatter to 90° pitch angle with no further change in energy. The observations suggest that this scattering and acceleration takes place near the geomagnetic equator and over a limited range of latitudes.

A common observation made by the toroidal imaging mass‐angle spectrograph on the polar orbiting Polar Spacecraft is the acceleration of ionospheric O+ and H+ to similar velocities on open field lines in the low‐latitude boundary layer (LLBL) and cusp regions. A detailed study of nine events made in the midaltitude cusp region is presented here and the acceleration can be split into two regions. On open field lines in the LLBL both H+ and O+ undergo rapid upward acceleration within approximately 0.75° invariant latitude. The acceleration aH+/ao+ ≈ 2 is inconsistent with the proposed electrostatic acceleration. Intense electrostatic waves often associated with the acceleration is consistent with energy transfer from the faster H+ to the slower O+ population via two‐stream instabilities. In the cusp region injected magnetosheath plasma is a possible free energy source for the acceleration of ionospheric ion populations: The component temperature ratio T‖/T⊥. of the upgoing, mirrored injected magnetosheath ions is generally lower than the downgoing ions consistent with a two‐stream instability driven energy transfer from the magnetosheath to the ionospheric ion populations.

We have investigated the physical processes occurring in order to maintain charge neutrality at the equatorward edge of the cusp by examining data from 200 Polar cusp crossings. The significant differences in magnetosheath ion and electron velocities could potentially create a region at the equatorward edge of the cusp where only solar wind electrons have access. Our calculations suggest that Polar should encounter this region at least 3 min before the first solar wind ions are observed. Only six Polar cusp crossings were identified, however, where a clear separate electron edge was observed. We then used state-of-the-art plasma instruments on Polar to examine particle features at the separate electron and ion edges. We find evidence that in the electron only region, electrons have been retarded by an electric potential above the spacecraft. We discuss the validity of examining electron spectra for evidence of retarding potentials, and we conclude that quantitative values cannot be obtained from single spacecraft measurements. We also fmd evidence of intermittent transverse ion acceleration in the electron only region. There are distinct differences in the ion conic distributions observed within the different regions at the equatorward edge of the cusp in terms of energy, pitch angle, and ion species. O+ conics are observed on closed field lines equatorward of the H+ cusp, while H+ and He conics are seen at higher latitudes. In the electron only region, and He+ conics have pitch angles between 90 degrees and 120 degrees. Further into the cusp, after the first solar wind ions are detected, higher-energy H+ and He+ conies with pitch angles extending to 180 degrees are measured. We suggest that processes occurring to maintain charge neutrality at the equatorward edge of the cusp may play a role in the generation of the ion conics observed.

The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere are investigated using data from the CRRES satellite. Equatorial electron distributions and concomitant wave spectra outside the plasmapause on the nightside of the Earth are studied as a function of time since injection determined from the auroral-electrojet index (AE). The electron cyclotron harmonic (ECH) wave amplitudes are shown to be very sensitive to small modeling errors in the location of the magnetic equator. They are best understood at the ECH equator, defined by the local, maximum in the ECH wave activity in the vicinity of the nominal magnetic equator, suggesting that the ECH equator is a better measure of the location of the true equator. Strong ECH and whistler mode wave amplitudes are associated with the injected distributions and at the ECH equator, in the region 6.0 less than or equal to L < 7.0, exponential fits reveal wave amplitude decay time constants of 6.3+/-1.2 and 4.6+/-0.7 hours, respectively. Pancake electron distributions are seen to develop from injected distributions that are nearly isotropic in velocity space and, in this region, are seen to form on a similar timescale of approximately 4 hours suggesting that both wave types are involved in their production. The timescale for pancake production and wave decay is comparable with the average time interval between substorm events so that the wave-particle interactions are almost continually present in this region leading to a continual supply of electrons to power the diffuse aurora. In the region 3.8 less than or equal to L < 6.0 the timescale for wave decay at the ECH equator is 2.3 +/- 0.6 and 1.1 +/- 0.2 hours for ECH waves and whistler mode waves respectively, although the pancakes in this region show no clear evolution as a function of time.

Electron pitch angle distributions sharply peaked at 90° pitch angle were first recorded in the energy range 50 eV < E < 500 eV by the GEOS-1 and GEOS-2 spacecraft in 1977/8, from the plasmapause out to geostationary orbit. At the time they were explained as the remnants of pitch angle diffusion driven solely by Electron Cyclotron Harmonic (ECH) waves. Here we use observations by instruments on board the CRRES spacecraft to study these distributions in more detail. The pancake distributions are now seen to develop from injected distributions that are nearly isotropic in velocity space, on a time scale that is greater than 2 hours. The freshly injected distributions are associated with strong ECH and whistler mode waves suggesting that the pancake distributions are likely to be caused by a combination of both wave types. Our results suggest that whistler mode waves play a dominant role in the formation of pancake distributions outside L = 6.0, whereas inside L = 6.0 and, in particular, in the vicinity of the plasmapause, the ECH waves also play a significant role. Consequently both types of waves should be considered in any attempt to explain the diffuse aurora and the variation with L taken into account.

Investigations of spacecraft charging at geostationary orbit reveal close relations between magnetospheric disturbances and electrostatic fields on spacecraft. These links provide a basis for forecasting spacecraft charging and the risk of spacecraft anomalies. In order to improve such forecasts the links between solar wind parameters, radiation conditions at geostationary orbit, and spacecraft charging have been studied. To reveal the relationships between the solar wind and the geostationary radiation environment, 37 magnetic storms between July 1976 and December 1978 were chosen for analysis. Data on electron fluxes in the energy range from 30 keV to 1360 keV from LANL geostationary spacecraft were also used. The analysis methods included moving averages, superposed epoch and correlation. It was found that the smoothed electron fluxes at geostationary orbit are controlled primarily by the solar wind velocity. The time delay between bursts of energetic electron fluxes and solar wind velocity disturbance depends on electron energy and may be up to tens of hours. This time lag makes it possible to separate surface and bulk charging disturbances during magnetic storms. An example of such a separation is shown.

Electron pitch angle distributions sharply peaked at 90° pitch angle were first recorded in the energy range 50 eV < E < 500 eV by the GEOSl and GEOS2 spacecraft in 1977/1978, from the plasmapause out to geostationary orbit. At the time they were explained as the remnants of pitch angle diffusion driven solely by electron cyclotron harmonic (ECH) waves. Here we report new observations by the Low Energy Plasma Analyser on board the Combined Release and Radiation Effects Satellite, which measured the complete pitch angle distribution over the energy range 100 eV < E < 30 keV. The pancake distributions are seen to develop from injected distributions that are nearly isotropic in velocity space, on a timescale that is greater than 2 hours. The freshly injected distributions are associated with strong ECH and whistler mode waves suggesting that the pancake distributions are likely to be caused by a combination of both wave types. Outside L = 6.0 the fitting analysis at energies in the range 100 eV < E < 1 keV shows that in the marginally stable state the phase space density contours lie approximately along the characteristic curves for diffusion by whistler mode waves determined independently from the plasma wave data. However, inside L = 6.0, significant departures are observed. Our results suggest that whistler mode waves play a dominant role in the formation of pancake distributions outside L = 6.0, whereas inside L = 6.0 and, in particular, in the vicinity of the plasmapause, the ECH waves also play a significant role. Consequently, both types of waves should be considered in any attempt to explain the diffuse aurora and the variation with L taken into account.

Our knowledge of space plasmas in general and the Earth's magnetosphere in particular depends largely on in-situ measurements from spacecraft. Such measurements transformed the science in the 1960's by revealing that the behavior of plasma in space was more complex than it was possible to deduce from ground-based observations. Since then, although the quality and coverage of the measurements has improved dramatically, and the advances in scientific understanding have been commensurate, awareness of the two main shortcomings of the method has also increased. The first, the ambiguity between temporal and spatial changes as observed from the spacecraft frame of reference has long been appreciated but the second, the impossibility of obtaining a global picture of events and the need for such a perspective has become forced on us over the past decade. There have been several technical advances which bring us to the point where it is feasible to obtain a global picture for the first time. Those advances include; the development of techniques for magnetospheric imaging; the deployment of large constellations of identical satellites for communications purposes; the miniaturization of instrumentation; and the development of smaller and smaller spacecraft. This is leading to a return to the satellite sizes of the very first space missions hut with substantially enhanced capability.

The medium electron A (MEA) instrument aboard the CRRES spacecraft provided data on terrestrial radiation belt electrons in the energy range from 153 to 1582 keV, during 1990–91. These data have previously been used to produce an empirical model of the radiation belts from L=1.1 to 8.9, ordered according to 17 energy bands, 18 pitch angle bins, and 5 Kp ranges. Empirical models such as this are very valuable, but are prone to statistical fluctuations and gaps in coverage. In this study, in order to smooth the data and make it more easy to interpolate within data gaps, the pitch angle distribution at each energy in the model was fitted with a Bessel function. This provided a way to characterize the pitch angle in terms of only two parameters for each energy. It was not possible to model fluxes reliably within the loss cone because of poor statistics. The fitted distributions give an indication of the way in which pitch angle diffusion varies in the outer radiation belts. The two parameters of the Bessel function were found to vary systematically with L value, energy and Kp. Through the fitting of a simple function to these systematic variations, the number of parameters required to describe the model could be reduced drastically.