Transcript Slide 1

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 (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 V n
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
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).
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.
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.
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.
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.