Mutual impedance experiments in a magnetized plasma

Abstract

Context. A mutual impedance experiment is an active in situ space plasma diagnostic that is used to determine the electron density and temperature. Such parameters are inferred from the mutual impedance spectrum measured between a pair of electric antennas embedded in the plasma. This state-of-the-art plasma diagnostic technique is limited to unmagnetized plasmas; that is, ones with a plasma frequency much larger than the electron cyclotron frequency. This limit is not expected to be valid in the plasma environment surrounding magnetized planets such as Mercury and Jupiter that will be explored by the ESA JUICE and joint ESA/JAXA Bepi-Colombo missions.Aims. The goal of this work is to extend the mutual impedance diagnostic technique to magnetized plasmas, focusing on measurements of the electron density and temperature, and to extend it to the electron temperature anisotropy.Methods. To achieve this, we developed the first quantitative three-dimensional instrumental model for mutual impedance experiments in a magnetized plasma. This model is valid for arbitrary values of the electron temperature and magnetic field. Our model is based on the linearized Vlasov-Maxwell coupled system of equations. We numerically computed the electric potential generated and simultaneously measured by the mutual impedance experiment, in order to compute the mutual impedance spectrum in a magnetized plasma.Results. First, we identify in the numerical mutual impedance spectra a number of local spectral signatures, associated with characteristic frequencies that can be used for plasma diagnostics. We show how the magnetic field strength and direction modify such spectral signatures. Second, we show that electron-neutral collision smooth out the spectrum, as long as the scattering-to-plasma frequency ratio is greater than 10^{−3 . Below such a value, mutual impedance experiments are insensitive to electron-neutral scattering and the plasma can be considered collisionless. Third, we show that the electron temperature directly controls the amplitude of the mutual impedance spectra, so that such behavior can be used as an electron temperature diagnostic. Fourth, we explore for the first time the possibility of diagnosing electron temperature anisotropies with mutual impedance experiments. We show how an electron temperature anisotropy significantly modifies the mutual impedance spectral signatures, as a result of the modified propagation of the electron Bernstein waves generated by the experiment.Conclusions. The results of our model, in terms of plasma diagnostics, are discussed in terms of the propagation properties in a magnetized plasma of the electrostatic waves generated by the active mutual impedance experiment. The results of our model will significantly extend the plasma diagnostic capabilities of the current and future mutual impedance experiment such as the PWI/AM2P experiment on board BepiColombo and the RPWI/MIME experiment on board JUICE.}