PLASMA DENSITIES

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PLASMA DENSITIES FROM SPACECRAFT POTENTIAL
MEASUREMENTS CALIBRATED BY THE ASPOC, EDI, CIS,
PEACE AND WHISPER EXPERIMENTS ON CLUSTER
Arne Pedersen and Bjørn Lybekk
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Electric field probe on Cluster
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The potential near a probe (Vn) 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 (Vn V0) be lifted to be
approximately 18 of (Vs V0), where Vs is the
spacecraft potential and V0 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 Vn and Vn V0. In the presence of an electric
field will two opposite probes and the spacecraft
be near different plasma potentials, V01, V02,
V0s. Vs V0s ½ (Vs Vp1) (Vs Vp2) 1.5
V In a tenuous plasma, with a longer Debye
length, must the influence of the boom tips be
considered. Vs V0s ½ (1 0.18)-1(Vs Vp1)
( Vs Vp2) 1.5 V
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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(Vs
V0s).
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WHISPER electron densities in the solar wind
versus (Vs Vp) ½ (Vs Vp1) (Vs Vp2).
Data are from respectively spring 2002 and spring
2005. 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.
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The spacecraft photoelectron characteristic is
the current of escaping photoelectrons from the
spacecraft and probes and the probe supports
(Iphs) as a function of Vs V0s. Iphs Ies 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 Ies up to (Vs V0s) 12 V. Iphs 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 Vs V0s.
Photoelectrons escaping from the probes to the
plasma will dominate for a very positive
spacecraft.
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With knowledge of Iphs it is possible to
calculate Ne (Vs Vp) curves for different
values of the electron mean energy. Electron
energies in the range 10 100 eV have similar Ne
(Vs Vp) curves.
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Reliable Ne 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 Ne (Vs Vp) or N - (Vs Vp).
Measurements of (Vs Vp), 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. Ne - (Vs Vp) curves, based on
knowledge of the spacecraft photoelectron
characteristic can then be calculated with
electron energy as a parameter.