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Multi-spacecraft data from the years 2008 to 2015 of the THEMIS\udmission particularly in the near-Earth plasma sheet are used in order to\udempirically determine the polytropic index in the quiet and active time\udmagnetotail. The results of a number of previous studies in the 1990s can be\udconfirmed. An analysis of the total database, although showing poor\udcorrelation, results in an average polytropic index of γ = 1. 72. The\udactive time plasma sheet is well correlated with an average γ = 1. 49.\udHowever, the data scattering suggests that the analysis of the data in total\udis not adequate. In order to reduce the timescales, individual spacecraft\udorbits are analyzed, giving a broad distribution of polytropic indices\udthroughout the plasma sheet. The major part of the distribution falls in a\udrange between γ = 0. 67 and γ = 2. Our results indicate a variety of\udthermodynamic processes in the magnetotail and an all-time presence of heat\udexchange of the plasma. A description of the plasma sheet using an equation\udof state with a single γ is probably inadequate. This necessitates the\udapplication of more sophisticated approaches, such as a parametrization of\udthe heat flux vector in magnetohydrodynamic equations or a superposition of\udpolytropic indices

Average plasma sheet polytropic index as observed by THEMIS