FOUR-POINT HIGH TIME RESOLUTION INFORMATION ON

ELECTRON DENSITIES ON CLUSTER

A.Pedersen(1),P.Decreau(2),C-P.Escoubet(3),G.Gustafsson(4),H.Laakso(3),P-A.

Lindqvist(5),B.Lybekk(1),F.Mozer(6),A.Vaivads

(1)Un.of

Oslo,Norway,(2)LPCE/CNRS,Orleans,France,(3)SSD,ESTEC,Noordwijk,The

Netherlands,(4)Un.of Uppsala,Sweden,(5)RIT,S

Spherical double probes for measurements of electric fields are electronically

controlled to be 1-2 volts positive relative to the ambient plasma.The spacecraft will

acquire a potential which balances emitted photoelectrons and collected electrons

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 electron

density over a wide range of electron densities from the lobes to the magnetosheath.

This technique has been perfected and calibrated against density measurements on

other missions. The Cluster spacecraft potential measurements open for new

approaches, particularly near boundaries and gradients where four-point

measurements will provide information never obtained before.Onboard data storage

of this simple parameter(undersampled) can be done for full orbits and will thereby

provide background information for the shorter full data collection periods on

Cluster. Preliminary calibrations against other measurements on Cluster will be

reported.