M. P. Gough

Low energy plasma analyzer

The low energy plasma analyzer (LEPA) that operates in a direct wave-particle mode in which waves (below 30 kHz), measured by separate onboard electric field and wave experiments, and LEPA signals that are directly cross-correlated, is described. LEPA was developed for the Combined Release and Radiation Effects Satellite (CRRES). LEPA is capable of determining the three-dimensional distribution function of electrons and positive ions in this energy range while simultaneously determining the location in the distribution function and the frequency at which coherent wave-particle interactions may be occurring. The LEPA measurement system uses tri-quadrispherical electrostatic analyzers with microchannel plate detectors. >

Banded electron cyclotron harmonic instability-a first comparison of theory and experiment

A study, which is the first of its kind, uses information derived from simultaneously measured wave spectra and particle distributions as the input to a theoretical linear instability model of an electrostatic cyclotron harmonic wave event recorded on GEOS-1. The presence of a hot loss cone component of the particle distribution is established experimentally, and the model accounts reasonably for the observed frequencies and relative strengths of the (n+1/2)fc and upper hybrid emission features.

GEOS-1 observations of electrostatic waves, and their relationship with plasma parameters

In this paper we describe and discuss the occurrence of natural wave emissions detected by GEOS-1 at frequencies above the electron gyrofrequency. The bulk of the data presented comes from the first six months of satellite operation and thus concerns mainly dayside phenomena. The paper is arranged as follows:After some general remarks, a classification of the wave phenomena is developed in Section 2, and experimental evidence and morphological information relevant to this classification are contained in Section 3. Section 4 includes some preliminary comments on nightside observations. The results are discussed in Section 5, where it is argued that they can be understood as manifestations of electron cyclotron harmonic (Bernstein) wave emission in a plasma parameter range which has only very recently received any theoretical examinations. This theme is further developed in a comparison paper (Ronnmark et al., 1978).

The Digital Wave-Processing Experiment on Cluster

The wide variety of geophysical plasmas that will be investigated by the Cluster mission contain waves with a frequency range from DC to over 100 kHz with both magnetic and electric components. The characteristic duration of these waves extends from a few milliseconds to minutes and a dynamic range of over 90 dB is desired. All of these factors make it essential that the on-board control system for the Wave-Experiment Consortium (WEC) instruments be flexible so as to make effective use of the limited spacecraft resources of power and telemetry-information bandwidth. The Digital Wave Processing Experiment, (DWP), will be flown on Cluster satellites as a component of the WEC. DWP will coordinate WEC measurements as well as perform particle correlations in order to permit the direct study of wave/particle interactions. The DWP instrument employs a novel architecture based on the use of transputers with parallel processing and re-allocatable tasks to provide a high-reliability system. Members of the DWP team are also providing sophisticated electrical ground support equipment, for use during development and testing by the WEC. This is described further in Pedersen et al. (this issue).

A fast-adaptive Huffman coding algorithm

The Huffman code in practice suffers from two problems: the prior knowledge of the probability distribution of the data source to be encoded is necessary, and the encoded data propagate errors. The first problem can be solved by adaptive coding, while the second problem can be partly solved by segmenting data into segments. However, the adaptive Huffman code performs badly when segmenting data into relatively small segments because of its relatively slow adaptability. A fast-adaptive coding algorithm which tracks the local data statistics more quickly, thus yielding better compression efficiency, is given. >

The Shuttle Potential and Return Electron Experiment (SPREE)

SummaryThe Shuttle Potential and Return Electron Experiment (SPREE) was designed and fabricated for flight as part of the joint NASA/Agenzia Spaziale Italiana Tethered Satellite System (TSS-1) mission. The SPREE is a complex instrument package designed to measure ion and electron particle flux and wave-particle interactions. The SPREE flight hardware consists of two multiangular electrostatic analyzer units, two rotary tables, a data processing unit, a particle correlator experiment, and two data recording units. The electrostatic analyzers measure both electrons and ions, in an energy range from 10 eV to 10 keV and simultaneously over an angular fan of (100×10) degrees. These units are mounted on the rotary tables to provide a 2π steradian field of view out of the Orbiters payload bay. To assess negative charging of the Orbiter with respect to the ambient plasma, ion data from the analyzers are processed real time by an on-board algorithm operating within the data processing unit. The particle correlator experiment determines wave-particle interactions in the frequency range 0–10 MHz for electrons and from 0–10 kHz for ions. SPREE operated successfully throughout the TSS-1 mission. Examples of the data returned by the SPREE are shown.

Correlator measurements of megahertz wave‐particle interactions during electron beam operations on STS

We report on the analysis of megahertz modulation of electrons as measured by the Shuttle Potential and Return Electron Experiment (SPREE) during dc firing of the shuttle electrodynamic tether system (SETS) fast pulsed electron generator (FPEG). The SPREE and FPEG were flown aboard the space shuttle Atlantis flight STS 46 as part of the Tethered Satellite System (TSS 1) mission. The principal data reported here are from the SPREE multiangular electrostatic analyzers (ESAs) and Space Particle Correlator Experiment (SPACE). The ESAs, mounted on rotary tables, measured electrons and ions in the energy range from 10 eV to 10 keV over a solid angle of 2π sr. The SPACE is a signal processing system that analyzes the pulse stream from the SPREE ESAs to identify bunching of the electrons and ions produced by coherent wave-particle interactions (WPIs). The SPACE detects modulations in the electron fluxes in frequency range 0- to 10-MHz. This paper concerns 2- to 4-MHz modulations of the electron flux detected by the SPACE when the FPEG was firing in a dc mode at pitch angles close to 90°. During such operations, FPEG emitted a current of 100 mA at an energy of 1 keV. For these times, electrons with energies from 10 to 1850 eV were measured by the SPREE. For energies between ∼10 and 100 eV the electron flux is basically isotropic. At higher energies the flux increases for pitch angles near 90°. The electron distribution functions generally decrease monotonically with increasing energy up to 100 eV. At energies >100 eV the distributions either monotonically decrease or exhibit a peak or plateau at energies near the beam emission energy. Megahertz modulations were observed for electrons with energies from 10 to 1180 eV, on both positive and negative slopes in the distribution function and throughout the 2π sr sampled by the ESAs. The occurrence and strength of the modulations exhibit no clear dependence on the pitch angle at which the electrons are measured. However, they appear to be limited to low parallel velocities (<3×10 6 m s −1 ) where beam-generated waves are in resonance with suprathermal electrons.

AMPTE-UKS observations of current sheets in the solar wind

Abstract Active current sheets or diamagnetic cavities in the solar wind have been observed both by the AMPTE-UKS spacecraft on a number of occasions, and independently from ISEE by Thomsen et al /1/. Preliminary results from one of these UKS observations have been discussed recently by Schwartz et al /2/. In this paper we consider these phenomena in more detail. Results are presented for the position of five events. High resolution plasma data and plasma wave activity associated with these current sheets are examined.

Electrostatic emissions studied in high resolution

Specific examples are presented for two types of electrostatic emission which have been studied on GEOS 1 and GEOS 2 with high spectral and fast temporal resolution. 1. (A) In the mid-morning hours (6 – 9 LT) the strongest electrostatic emissions are often observed as bursts lasting from less than a second to several hours. Simultaneously rapid changes are observed in the warm electron distribution and at ELF (15 – 450 Hz) only on the large electric antenna. High spectral resolution (Δf ∼ 11 Hz) shows that these emissions near the plasma frequency (∼ 30 KHz) sometimes display spectral features whose separation in frequency is equal to the frequency of the electrostatic ELF emission in the region of the lower hybrid frequency. 2. (B)Around local midnight following a substrom rapid step like changes occur in the intensity of electromagnetic ELF emission. Simultaneously the normal structured electrostatic (n + 12 fce emission features are replaced by an intense broad spectrum. Relations between these variations and auroral pulsations Observed at the foot of the field line are presented.

Positive spacecraft charging as measured by the Shuttle potential and Return Electron experiment

The authors report on obsevations of positive charging of the Orbiter during the deployed phase of the TSS-1 (Tethered Satellite System 1). The charging was observed to occur when the Orbiter was in darkness, during periods of low ionospheric density and while the SETS FPEG (Shuttle Electrodynamic Tether System Fast Pulsed Electron Gun) was emitting a 1-keV, 100-mA electron beam. The charging occurred when the ambient plasma density was too low to provide a current to match the FPEG emission. In the cases where the ambient plasma temperature was approximately 0.1 eV, and assuming a conducting area of the Orbiter of approximately 25 m/sup 2/, the positive charging occurred for densities below 6*10/sup 5/ electrons/cm/sup 3/. The charging is seen as an acceleration of the relatively hot electron population in the energy range above 10 eV that had already been greatly enhanced by the operation of the FPEG. Intense fluxes of electrons were observed at low energy for both the charged and uncharged Orbiter. This low energy component tended to be isotropic with densities as high as 4.6*10/sup 3/ electrons/cm/sup 3/. The return flux at the charging peak was anisotropic, with the anisotropy varying with the level of charging and the pitch angle. >