COMPARISON BETWEEN BULK ELECTRON DENSITIES AND POTENTIAL

DIFFERENCE MEASUREMENTS ON THE CLUSTER SATELLITES

H. LAAKSO, R. GRARD, A. MASSON, P. ESCOUBET, J.-P. LEBRETON, M.

ANDRE, et al.

ESTEC Space Science Department, Noordwijk, The Netherlands

Numerous in-situ measurements have shown that the potential difference between a

biased electrode and the satellite body ($\Delta V = V_p - V_s$) is well correlated

with the bulk electron density ($N_e$) in tenuous plasmas. When $N_e$ is less than

a few hundred electrons

per cm$^{3}$, the potential of a biased electric field sensor is quite constant and

close to that of the ambient plasma potential ($\Delta V$ is nearly equal to $-V_s$).

On the other hand, the satellite potential is very sensitive to the variation of the

electron density and increases when the density decreases. Therefore, measuring

$\Delta V$ provides a sensitive diagnostic method for observing variations of the

bulk electron density in tenuous plasma environments.

We investigate the relationship between $\Delta V$ and $N_e$ on the Cluster

satellites, comparing $\Delta V$ measurements of the EFW experiment to

simultaneous bulk density measurements of the WHISPER experiment, under

various conditions. In particularly we attempt to study the accuracy with which the

electron density can be determined from $\Delta V$ when the electron temperature

($T_e$) is not well known.