The anisotropy of precipitating auroral electrons: A FAST case study

A key component in the magnetosphere-ionosphere coupling chain is the auroral acceleration region (AAR), where electromagnetic energy is converted into kinetic energy of precipitating electrons. We examine the accelerated electrons measured by FAST during an auroral overpass when the spacecraft crosses the AAR several times, as evident from the observation of ion beams. We assume that above the AAR the electron population is bi-Maxwellian and fit accelerated bi-Maxwellian functions to the measured distributions. When the electron motion between the plasma sheet source and the satellite can be considered as adiabatic, two of the fit parameters can be associated with the source properties, while the third one provides the electron anisotropy profile along the satellite track; the fit quality parameter, χr2, gives an indication about the importance of non-adiabatic processes. We explore the possibility of using the anisotropy to determine the altitude of the AAR top boundary (AARTB) and find that when the AAR bottom boundary (AARBB) is located closer to the Earth, as proved by the detection of ion beams, the AARTB appears to be pushed at higher altitudes. This fact is discussed with regard to recent theoretical and experimental results.