PLASMA DENSITIES FROM SPACECRAFT POTENTIAL MEASUREMENTS CALIBRATED BY THE ASPOC, EDI, CIS, PEACE AND WHISPER EXPERIMENTS ON CLUSTER Arne Pedersen and Bjørn Lybekk

Electric field probe on Cluster

The potential near a probe (V n ) is influenced by the potential of the long radial wire booms that are at spacecraft potential. According to modelling, for Debye lengths longer than the probe system, can (V n – V 0 ) be lifted to be approximately 18 % of (V s – V 0 ), where V s is the spacecraft potential and V 0 the plasma potential (Cully and Ergun, private communication). The upper part of the Figure presents a situation with a small Debye length and good shielding of the probes from the boom tips. Each probe is electronically controlled to be ~1.5 V relative to V n and V n = V 0. In the presence of an electric field will two opposite probes and the spacecraft be near different plasma potentials, V 01, V 02, V 0s. V s – V 0s = ½ [(V s – V p1 ) + (V s – V p2 )] V In a tenuous plasma, with a longer Debye length, must the influence of the boom tips be considered. V s – V 0s = ½ (1 – 0.18)-1[(V s – V p1 ) + ( V s – V p2 )] V

PEACE observations in a lobe plasma of cold electrons, accelerated to the spacecraft. These electrons can be photoelectrons from the probes and/or cold ionospheric electrons. The energy of these electrons show fair agreement with e(V s – V 0s ).

WHISPER electron densities in the solar wind versus (V s – V p ) = ½ [(V s – V p1 ) + (V s – V p2 )]. Data are from respectively spring 2002 and spring Are the differences between the two periods due to a reduction of solar EUV between 2002, near solar max, and 2005? The open circles are CIS ion measurements in 2002 in the cusp. The mean electron energy from PEACE are given in brackets.

The spacecraft photoelectron characteristic is the current of escaping photoelectrons from the spacecraft and probes and the probe supports (I phs ) as a function of V s – V 0s. I phs = I es the current of ambient electrons to the spacecraft. With the knowledge of electron density and mean energy in the solar wind it is possible to determine I es up to (V s – V 0s ) ≈ 12 V. I phs at higher spacecraft potentials have been obtained by probe bias current diagnostics, and using periods with high electron current emissions on EDI to observe changes to V s – V 0s. Photoelectrons escaping from the probes to the plasma will dominate for a very positive spacecraft.

With knowledge of I phs it is possible to calculate N e – (V s – V p ) curves for different values of the electron mean energy. Electron energies in the range 10 – 100 eV have similar N e – (V s – V p ) curves.

Reliable N e and N + data are available from WHISPER and CIS in the solar wind, magnetosheath and the plasmasheet. An analytic function can the be fitted to N e – (V s – V p ) or N + - (V s – V p ). Measurements of (V s – V p ), together with the analytic function, can then give plasma density with high time resolution, 4 s/s. In a tenuous plasma, e.g in the lobes, may the very positive spacecraft reject part of the ion population, and WHISPER may be disturbed by orbiting photoelectrons. N e - (V s – V p ) curves, based on knowledge of the spacecraft photoelectron characteristic can then be calculated with electron energy as a parameter.