Abstract
We examine the relationships between temperature tensor elements and their connection to the polytropic equation, which describes the relationship between the plasma scalar temperature and density. We investigate the possibility to determine the plasma polytropic index by fitting the fluctuations of temperature either perpendicular or parallel to the magnetic field. Such an application is particularly useful when the full temperature tensor is not available from the observations. We use solar wind proton observations at ~1 au to calculate the correlations between the temperature tensor elements and the scalar temperature. Our analysis also derives the polytropic equation in selected streamlines of solar wind plasma proton observations that exhibit temperature anisotropies related to stream-interaction regions. We compare the polytropic indices derived by fitting fluctuations of the scalar, perpendicular, and parallel temperatures, respectively. We show that the use of the parallel or perpendicular temperature, instead of the scalar temperature, still accurately derives the true, average polytropic index value, but only for a certain level of temperature anisotropy variability within the analyzed streamlines. The use of the perpendicular temperature leads to more accurate calculations, because its correlation with the scalar temperature is less affected by the anisotropy fluctuations.
Highlights
The velocity distribution functions (VDFs) of space plasmas exhibit temperature anisotropies, which are organized by the magnetic field direction (e.g., [1,2,3])
This paper examines the polytropic behavior of solar wind protons in an annual interval, while a future study will expand the analysis in a dataset covering more than 25 years of observations
The scalar temperature increases by an order of magnitude within the interaction regions
Summary
The analysis of plasma particle velocity distribution functions (VDFs) is necessary to understand the kinetic and thermodynamic properties of plasmas. The VDFs of space plasmas exhibit temperature anisotropies, which are organized by the magnetic field direction (e.g., [1,2,3]) In these cases, the complete description of plasmas requires the knowledge of the temperature perpendicular to the magnetic field T⊥ and the temperature parallel to the magnetic field Tk. In order to resolve the full temperature tensor from the data analysis, we need observations of full, three-dimensional (3D) VDFs or observations of 2D pitch-angle distributions of the plasma particles [4,5]
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