Electron density measurement in space plasma, comparison between two techniques: Sounder by relaxation and mutual impedance probe

Summary form only given. In space plasma, the determination of the electron density is an important parameter to understand the wave propagation and energy transfers between wave and particles. Usually, the plasma density and debye length are much different in solar wind and planetary environment than laboratory plasma and it is needed to sound large volume. Several ways exist to estimate the electron density that can separate into two main categories: the passive and active techniques. In this presentation, we propose to discuss the active techniques and we describe on sounder by relaxation instrument and mutual impedance measurement. The both types consist to emit in the plasma a monochromatic frequency. Then, plasma response is recorded for several frequencies, scanning a frequency range including the characteristic plasma frequencies like plasma frequency Fpe, electron gyro-frequency Fce, gyro-harmonics Fcen and berstein's modes Fqn which are perpendicular electrostatic resonances. However, the way to record and the energy used are completely different following the technique. Sounder by relaxation use a powerful pulse which excites the surrounding plasma and the plasma response is listened after a short delay (few milliseconds). When the emitted frequency fit the plasma resonances, the recorded frequency will be seen with a large amplitude and allow us to identify the plasma resonances whose derive the electron density and magnetic field magnitude. This technique is very efficient but the transmitted energy is above the thermal energy and perturbs strongly the electron distribution function. At the difference, the mutual impedance probes technique the transmitting and receiving frequency is done at the same time and it is the plasma permittivity which is measured. This technique needs to use simultaneously two antennas. The interpretation must take into an account the influence of the conductive structures of the satellite and numerical simulation is needed. But, the emitted energy is less than the thermal plasma energy and perturbations are negligible. We will show examples WHISPER sounder measurement on board CLUSTER/ESA mission and we will illustrate the mutual impedance technique by examples done by Rosetta mission in earth magnetosphere.