Misestimation of plasma temperature when applying a Maxwellian distribution to space plasmas described by kappa distributions

Abstract:

A common method to calculate the space plasma parameters is by fitting observed phase space densities using known distributions. In some cases the phase space density of the plasma is directly derived from the observations, while in other cases a forward modeling of the instrument’s response is used to fit the measured energy spectrograms. In both scenarios, the modeled plasma distribution fits the measurements to estimate the plasma parameters. The distribution function is often assumed to be Maxwellian even though the plasma is better described by a kappa distribution. In this work we show that if the plasma is described by a kappa distribution, the derived temperature assuming Maxwell distribution can be significantly off. More specifically, we derive plasma temperature by fitting Maxwell distribution to pseudo-data produced by kappa distribution and we examine the difference of the derived temperature as a function of the kappa index. We further consider the concept of using a forward model of a typical plasma instrument to fit its observations. We find that the error of the derived temperature is shown to be depended on the instrument’s field of view, the kappa index, and the temperature itself.