IRF logo Student project work in Space Plasma Physics (2006-2007)
Solar UV flux and Cluster electrostatic potential

Student project (Examensarbete, 20 p)

Student: Erik Winkler
Supervisor: Anders Eriksson
Swedish Institute of Space Physics, Uppsala
Department of Astronomy and Space Physics, Uppsala University

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 DAG-213 (green).
last modified onWednesday, 15-Aug-2007 15:20:45 CEST