These observation: have shown the existence of a hot plasma which exhibits considerable structure in its radial dependence and in the energy spectra at each location. An excellent review of previous observations has been given by Gringauz (1969).The ATS -5 data show a strong local time dependence and even more structure in the energy spectra than previously suspected. Data being presented here shows that this structure is a consequence of the injection of plasma in the vicinity of local midnight during magnetospheric substorms and of the energy dependent dispersion of the plasma in the earth's magnetic and electric fields. The injection and dispersion of the high energy tail of these plasma clouds have been previously reported by Konradi (1967), Arnoldy and Chan L969) . . Pfitzer and Winckler (1969) ) and by . Lezniak and Winckler (1970). r The present paper is also intended to serve as an introduction to the data being obtained from the University of California, San Diego (UCSD) plasma experiment on , ATS -5 and to describe the general characteristics of the particle populations it has encountered. Description of Experi entThe ATS-5 satellite was launched into synchronous orbit (6. Three simultaneous accumulations 0.26 seconds long are taken every 0.32 seconds. Two analyzer outputs are selected by ground command to,feed two of the accumulators while the other two are alternately fed to the third accumulator. Accumulation is stopped for 4 microseconds after each pulse counted so that the small afterpulses which can occur during the first one or two microseconds after a particle is detected are not counted.6Deadtime corrections are applied so that rates up to 10 6 /see are believed to be measured accurately.• The analyzers have an energy resolution of about 13% and a geometric factor such that dividing the counting rate by 4.3 x 10 -5 cm sr yields the differential energy flux in units of ev/cm sec sr ev. The angular response to a uniform energy distribution extends over a rectangular solid angle of about 5 by 8 degrees. rThe inner and outer plates of the analyzers are connected in par-' allel to two high voltage supplies which can be stepped through 62 exponentlally spaced voltages 12% apart (with less than 0.1% accumulative error) thus varying the energy analyzed from 50 ev to 50,000 ev. Two zero voltage steps are also included to provide background measurements (in practice the backgrotuid is usually low and is not subtracted).The system can be commanded to cycle through all 64 steps each 20.48 seconds or in a number of other patterns. One frequently used mode alternates between a 20.48 second scan of all 64 steps and a 61.44 second period in which the analyzed energy is varied such tha ,;t it tracks a peak in the counting rate spectrum of the detector which has been selected for the master channel. All of the data presented in this paper were taken in this track-scan mode with the parallel proton detector in the master channel, the perpendicular proton detector in the second channel (labeled "mate") and the e...

ATS 5 can charge to potentials as high as 10,000 volts during eclipse, and as high as 200 volts in sunlight. This charging has been used to infer the presence of the plasmasphere at synchronous altitude in the midnight region, and its prompt disappearance when hot plasma is injected in association with substorms. Although injected plasma has a density of 1.0/cm³, the remaining cold plasma (<50 ev) has a density of 0.01/cm³. A mathematical model of ATS 5 has been constructed that can be used to predict spacecraft potentials. In addition, parts of ATS 5 can charge to several hundred volts without affecting the over‐all spacecraft potential appreciably.

Particle and magnetic field data from the ATS 6 spacecraft have been combined to study a Pc 4 pulsation event. The event occurred as the satellite passed the dawn meridian during the recovery phase of a moderate magnetic storm, Plasma drifts associated with the wave are determined from the measured modulation of low-energy protons. Using the observed ambient and perturbation magnetic fields in conjunction with the plasma drift velocity, we calculate the Poynting vector of the wave, assuming that the electric field is given by the frozen field approximation. The Poynting vector gives convincing evidence that the wave is a standing hydromagnetic wave along the magnetic field. In addition, it shows that the wave is propagating predominantly azimuthally from noon toward the dawn meridian. The phase relationships between the magnetic and plasma drift velocity oscillations are consistent with the occurrence of an odd harmonic of the standing wave. Comparison of observed particle fluxes with model predictions demonstrates that the fundamental mode is the most reasonable interpretation of the data. Ambient ion number densit.es, predicted by the wave data on the basis of a simple model of standing hydromagnetic waves, are tbund to be somewhat higher than observed values, on the basis of the assumption that the ambient plasma consists of protons and electrons only. Predicted and observed values of the ion number densities can be reconciled if it is assumed that a significant fraction of the ambient plasma consisted of He + and/or O +. By using the fact that the wave is a standing wave the particle flow velocity is predicted from the observed magnetic variation and a model-dependent theory. These predictions are found to agree very well with the flow values determined from the data. [Barfield et al., 1971, 1972; Cummings and Coleman, 1968; Cummings et al., 1969, 1972; Dwarkin et al., 1971; Judge and Coleman, 1962; Patel, 1965; Sugiura and Wilson, 1964; Troitskaya and Gul'elmi, 1967], and these observations have led to several theories and reviews [Chen and Hasegawa, 1974a, b;However, all of these previous observations have been hampered by the lack of simultaneous field and particle measurements. Ideally, one would like to have complete electric and magnetic field measurements along with complete particle spectra. On ATS 6 we approximate the ideal situation with data'from the University of California at Los Angeles (UCLA) magnetometer and the University of'California at San Diego (UCSD) plasma instrument, which makes a complete energy scan every 16 s. This is sufficient resolution to allow the inference of electric fields associated with waves of periods greater than about 100 s. (No other electric field measuring devices are carried on the A TS 6.) This article discusses the observation of a train of hydromagnetic waves with a period of about 150 s seen at synchronous orbit by ATS 6 on June 27, 1974 (day of year 178). The critical observation is a phase shift of 90 ø between east-west oscillations of the particle flow and ...

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