Electron Density Estimation in the Magnetotail: a Multi-Instrument Approach

Abstract

Electron density is a key physical quantity to characterize any plasma medium. Its measurement is thus essential to understand the physical processes occurring in the environment of a magnetized planet, both macroscopic and microscopic. Since 2000, the four satellites of the European Space Agency (ESA) Cluster mission have been orbiting the Earth from 4 RE to 20 RE and probing the density with several types of instruments. In the magnetotail, this rare combination of experiments is particularly useful since the electron density and the temperature fluctuate over several decades. Two of these experiments, a relaxation sounder and a high-time resolution wide-band receiver, have rarely been flown together in the far tail. Such wave data can be used as a means to estimate the electron density via the identification of triggered resonances or the cutoffs of natural wave emissions, typically with an accuracy of a few percent. For the first time in the magnetotail ( ∼20 RE), the Z-mode is proposed as the theoretical interpretation of the cutoff observed on spectrograms of wave measurements when the plasma frequency is greater than the electron gyrofrequency. We present examples found in the main regions of the magnetotail, comparing simultaneous density estimation from active and passive wave measurements with a particle instrument and calibrated spacecraft-to-probe potential difference data. With these examples, we illustrate the benefit of a multi-instrument approach for the estimation of the electron density in the magnetotail and the care that should be taken when determining the electron density from wave data.