ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-19-1483-2001 Four-point high time resolution information on electron densities by the electric field experiments (EFW) on Cluster Pedersen A. 1 Décréau P. 2 Escoubet C.-P. 3 Gustafsson G. 4 Laakso H. 3 Lindqvist P.-A. 5 Lybekk B. 1 Masson A. 3 Mozer F. 6 Vaivads A. 4 University of Oslo, Norway LPCE/CNES, Orleans, France Space Science Department, ESTEC, Noordwijk, The Netherlands University of Uppsala, Sweden Royal Institute of Technology, Stockholm, Sweden SSL, University of California, Berkeley, USA 30 09 2001 19 10/12 1483 1489 Copyright: © 2001 A. Pedersen et al. 2001 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/19/1483/2001/angeo-19-1483-2001.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/19/1483/2001/angeo-19-1483-2001.pdf

For accurate measurements of electric fields, spherical double probes are electronically controlled to be at a positive potential of approximately 1 V relative to the ambient magnetospheric plasma. The spacecraft will acquire a potential which balances the photoelectrons escaping to the plasma and the electron flux collected from the plasma. The probe-to-plasma potential difference can be measured with a time resolution of a fraction of a second, and provides information on the electron density over a wide range of electron densities from the lobes (~ 0.01 cm<sup>-3</sup>) to the magnetosheath (&gt;10 cm<sup>-3</sup>) and the plasmasphere (&gt;100 cm<sup>-3</sup>). This technique has been perfected and calibrated against other density measurements on GEOS, ISEE-1, CRRES, GEOTAIL and POLAR. The Cluster spacecraft potential measurements opens the way for new approaches, particularly near boundaries and gradients where four-point measurements will provide information never obtained before. Another interesting point is that onboard data storage of this simple parameter can be done for complete orbits and thereby will provide background information for the shorter full data collection periods on Cluster. Preliminary calibrations against other density measurements on Cluster will be reported.<br><br><b>Key words. </b>Magnetospheric physics (magnetopause, cusp, and boundary layers) Space plasma physics (spacecraft sheaths, wakes, charging; instruments and techniques)