Electron temperature anisotropy associated to field‐aligned currents in the Earth's magnetosphere inferred from Rosetta MIP‐RPC observations during 2009 flyby

Abstract

AbstractA new approach is proposed for data interpretation of the Mutual Impedance Probe (MIP) instrument from the Rosetta Plasma Consortium (RPC) during the 2009 Earth's flyby gravity assist through the magnetosphere, from dusk to dawn regions. The spacecraft trajectory of ±8 RE (Earth's radius) was crossing several structures of field‐aligned currents (FACs) and radiations belts on both legs of the closest approach (CA, 2.450 km altitude). As routinely revealed by several pioneering space missions, natural and forced electrostatic wave emissions called Fqs were observed over ±3 RE at around CA using a dedicated mode of the MIP instrument. These emissions are lying between consecutive harmonics of the electron‐cyclotron frequency, and their wavelength is perpendicular to the magnetic field lines. Provided that the Fq's wavelengths projected along the MIP antenna might be estimated, it is shown that the local value of the Larmor radius can be deduced; hence, the electron temperature component perpendicular to the magnetic field is subsequently derived. On the other hand, during the time of Fq's observations, the presence of VLF hiss emissions usually observed in these regions gives us the possibility to determine the electron temperature anisotropy associated to the electrostatic electron anisotropy instability according to the theoretical model proposed by Gary and Cairns (JGR, vol. 104, 1999). Significant dynamic constraints revealed by crossing successive series of FACs tubes are shown being controlled by this anisotropy, and the fact that the magnetic pressure is significantly larger than the thermal pressure suggests that the FACs lobes are nonforce free.