In this project,
the photoemssion current and spacecraft potential of ESA's Cluster
satellites were compared to solar UV data from NASA's satellite TIMED.
The results were presented in the project report and at the 10th
International Spacecraft Charging Technology Conference.
Background. "Empty space" is never
really empty: essentially everywhere, there is a tenous ionized gas, a
plasma. The electrons and ions in a plasma will charge a spacecraft to
some equilibrium potential, where as much positive as negative charge
flows to the spacecraft. In the tenuous plasmas where ESA's
four Cluster satellites move around the Earth, the dominating
currents to and from the spacecraft are due to emission of
photoelectrons and collection of plasma electrons. The fewer electrons
there is around to collect for the spacecraft, the higher its potential
must become in order for it to attract sufficiently many electrons to
compensate for the phtotemission current. This is the basis for a
method to estimate the plasma density, n, by measureing the satellite
potential, Vsc: the higher Vsc, the lower is n. The method is described in
detail by Pedersen (1995), and has seen a lot of use (e.g. Escoubet et
al, 1997; Pedersen et al., 2001). However, Cluster has operated for so
long time, from its launch in 2000 (near solar maximum) to the present
day (near solar minimum) that the solar UV flux, and hence the
satellite photoemission, has changed significantly during the mission.
There is therefore a need to explicitly take the UV flux, probably as
quantified by the F10.7 index, into account when relating Vsc to n.
Project. Use data from the Cluster EFW instrument to
investigate how Vsc correlates with solar UV flux. Investigate if F10.7
or other suitable indices can be used to refine a relation between n and Vsc.
Photoemission current measured by Cluster (blue) and predicted from UV
measurements by TIMED using a modified yield function for Al (red) and
last modified onWednesday, 15-Aug-2007 15:20:45 CEST