Electric field measurements on .pdf

Annales Geophysicae, 24, 275–289, 2006 SRef-ID: 1432-0576/ag/2006-24-275 © European Geosciences Union 2006 Annales Geophysicae Electric field measurements on Cluster: comparing the double-probe and electron drift techniques A. I. Eriksson1, M. André1, B. Klecker2, H. Laakso3, P.-A. Lindqvist4, F. Mozer5, G. Paschmann2,6, A. Pedersen7, J. Quinn8, R. Torbert8, K. Torkar9, and H. Vaith2,8 1Swedish Institute of Space Physics, Uppsala, Sweden 2Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany 3Solar System Division, ESA/ESTEC, Noordwijk, Netherlands 4Alfvén laboratory, Royal Institute of Technology, Stockholm, Sweden 5Space Sciences Laboratory, University of California, Berkeley, CA, USA 6International Space Science Institute, Bern, Switzerland 7Department of Physics, Oslo University, Norway 8Space Science Center, University of New Hampshire, Durham, NH, USA 9Space Research Institute, Austrian Academy of Sciences, Graz, Austria Received: 16 June 2004 – Revised: 29 November 2005 – Accepted: 14 December 2005 – Published: 7 March 2006 Abstract. The four Cluster satellites each carry two instru- ments designed for measuring the electric field: a double- probe instrument (EFW) and an electron drift instrument (EDI). We compare data from the two instruments in a rep- resentative sample of plasma regions. The complementary merits and weaknesses of the two techniques are illustrated. EDI operations are confined to regions of magnetic fields above 30 nT and where wave activity and keV electron fluxes are not too high, while EFW can provide data everywhere, and can go far higher in sampling frequency than EDI. On the other hand, the EDI technique is immune to variations in the low energy plasma, while EFW sometimes detects significant nongeophysical electric fields, particularly in re- gions with drifting plasma, with ion energy (in eV) below the spacecraft potential (in volts). We show that the polar cap is a particularly intricate region for the double-probe tech- nique, where large nongeophysical fields