Cluster PEACE observations of electrons of spacecraft origin

S. Szita,G. Watson,P. Trávnícek,K. Torkar,A. N. Fazakerley,A. Eriksson,A. M. James,M. André,P. J. Carter

doi:10.5194/angeo-19-1721-2001

Abstract

Abstract. The two PEACE (Plasma Electron And Current Experiment) sensors on board each Cluster spacecraft sample the electron velocity distribution across the full 4<pi> solid angle and the energy range 0.7 eV to 26 keV with a time resolution of 4 s. We present high energy and angular resolution 3D observations of electrons of spacecraft origin in the various environments encountered by the Cluster constellation, including a lunar eclipse interval where the spacecraft potential was reduced but remained positive, and periods of ASPOC (Active Spacecraft POtential Control) operation which reduced the spacecraft potential. We demonstrate how the spacecraft potential may be found from a gradient change in the PEACE low energy spectrum, and show how the observed spacecraft electrons are confined by the spacecraft potential. We identify an intense component of the spacecraft electrons with energies equivalent to the spacecraft potential, the arrival direction of which is seen to change when ASPOC is switched on. Another spacecraft electron component, observed in the sunward direction, is reduced in the eclipse but unaffected by ASPOC, and we believe this component is produced in the analyser by solar UV. We find that PEACE anodes with a look direction along the spacecraft surfaces are more susceptible to spacecraft electron contamination than those which look perpendicular to the surface, which justifies the decision to mount PEACE with its field-of-view radially outward rather than tangentially.Key words. Magnetosheric physics (general or miscellaneous) Space plasma physics (spacecraft sheaths, wakes, charging)

Highlights

Measurements of the low energy plasma surrounding a spacecraft are complicated by the charging of the spacecraft surface
We demonstrate how the spacecraft potential may be found from a gradient change in the PEACE low energy spectrum, and show how the observed spacecraft electrons are confined by the spacecraft potential
High resolution PEACE data reveals the complex structure of the photoelectron population and secondary electrons from the surface, and the variability of the spacecraft electrons in response to variations in the spacecraft potential caused by changes in the plasma environment

Summary

Introduction

Measurements of the low energy plasma surrounding a spacecraft are complicated by the charging of the spacecraft surface. At low energies (of the same order as the spacecraft potential) electrons are accelerated from a wide field-of-view into a narrower one at the detector. Detectors which have a fieldof-view tangential to the spacecraft surface are least likely to receive low energy ambient plasma electrons and may detect electrons released from the spacecraft surface instead This leads to the electron density being overestimated and other directional moments being modified. It is further noted that plasma gradients are misinterpreted where the electron trajectories have been modified by the spacecraft potential: the example is given by Scime et al (1994) of a satellite moving into a region of increased plasma density at constant temperature. These examples point to the inadequacy of simple spherical models of the spacecraft potential for satellites, such as Cluster, which have booms which may distort the satellite sheath potential

The PEACE instrument

Spacecraft electrons observed along the Cluster orbit

Effect of ASPOC

PEACE in eclipse

Findings

Concluding remarks