Institut für Geophysik und extraterrestrische Physik, Braunschweig, Germany
Received: 24 Oct 2016 – Revised: 23 Dec 2016 – Accepted: 10 Feb 2017 – Published: 24 Feb 2017
Abstract. Multi-spacecraft data from the years 2008 to 2015 of the THEMIS mission particularly in the near-Earth plasma sheet are used in order to empirically determine the polytropic index in the quiet and active time magnetotail. The results of a number of previous studies in the 1990s can be confirmed. An analysis of the total database, although showing poor correlation, results in an average polytropic index of γ = 1. 72. The active time plasma sheet is well correlated with an average γ = 1. 49. However, the data scattering suggests that the analysis of the data in total is not adequate. In order to reduce the timescales, individual spacecraft orbits are analyzed, giving a broad distribution of polytropic indices throughout the plasma sheet. The major part of the distribution falls in a range between γ = 0. 67 and γ = 2. Our results indicate a variety of thermodynamic processes in the magnetotail and an all-time presence of heat exchange of the plasma. A description of the plasma sheet using an equation of state with a single γ is probably inadequate. This necessitates the application of more sophisticated approaches, such as a parametrization of the heat flux vector in magnetohydrodynamic equations or a superposition of polytropic indices.
Frühauff, D., Mieth, J. Z. D., and Glassmeier, K.-H.: Average plasma sheet polytropic index as observed by THEMIS, Ann. Geophys., 35, 253-262, doi:10.5194/angeo-35-253-2017, 2017.