Quasi‐Thermal Noise Spectroscopy in Magnetized Space Plasma: Theory and Model

Abstract The quasi‐thermal noise measured by an electric antenna is routinely used to characterize space plasmas, mainly measuring the electrons' properties. To employ this diagnostic technique, instrumental models are required to turn the instrumental output into physically meaningful measurements. Such models have been developed mainly under the assumption that no magnetic field is present in the plasma. This limit case is not met in planetary magnetospheres, for example, Earth, Mercury. The latter is the objective of the European Space Agency/Japan Aerospace Exploration Agency BepiColombo mission, that has a dedicated quasi‐thermal noise experiment. The aim of this work is to extend the current state‐of‐the‐art in quasi‐thermal noise modeling by taking into account the magnetic field, therefore providing a plasma diagnostic in this magnetized regime. To achieve this goal, we developed a model for the quasi‐thermal noise in a magnetized plasma. We explore four cases: for a Maxwellian and double Maxwellian electron distributions, both in the collisionless limit and in the presence of weak electron‐neutral collisions. Our model is validated against known behaviors of the magnetized quasi‐thermal noise spectrum, including: the characteristic frequency of maxima and minima, the modulation from the antenna spinning around the magnetic field, the electron temperature(s) influence. We explored parameter ranges that were not accessible to previous quasi‐thermal noise models, in particular the high magnetization regime. The model we developed will enable using quasi‐thermal noise experiments for the diagnostic of magnetospheric space plasmas, including but not limited to the Hermean and terrestrial magnetospheres, with foreseen applications to future space missions.