Electron temperature profiles in the magnetosphere of Neptune

Abstract

The electron temperature in the magnetosphere of Neptune decreases with decreasing radial distance over the same distance range over which a similar decrease of the proton temperature has been explained as resulting from interaction of protons with a cloud of neutral hydrogen. The effect on the electron temperature of the interaction with this cloud is studied, taking into account photoionization, electron impact ionization, radiation, and electron-ion Coulomb collisions. It is found that the electron temperature should decrease with decreasing distance, in agreement with observations, but that at the outer boundary the theoretically expected electron temperatures are considerably lower than what is observed, implying a heating process or a source of hot electrons which have not yet been unambiguously identified. In the inner region, where the adiabatic increase of the proton temperature has been taken as implying the absence of any significant density of neutral atoms, the electron temperature remains essentially constant, implying a powerful cooling process that acts only on electrons. This process is also at present an object of speculation; as a possibility, thermal coupling to the ionosphere via strong pitch angle diffusion of electrons is suggested.