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Polytropic Behavior of Solar Wind Protons Observed by Parker Solar Probe

AuthorNicolaou, Georgios; Livadiotis, George; Wicks, Robert; Verscharen, Daniel; Maruca, Bennett;
KeywordsParker Data Used; parker solar probe; Solar Physics; Solar Probe Plus; Solar wind; Space plasmas

A polytropic process describes the transition of a fluid from one state to another through a specific relationship between the fluid density and temperature. The value of the polytropic index that governs this relationship determines the heat transfer and the effective degrees of freedom during a specific process. In this study, we analyze\ solar\ wind proton plasma measurements, obtained by the Faraday cup instrument on board the\ Parker\ Solar\ Probe. We examine the large-scale variations of the proton plasma density and temperature within the inner heliosphere explored by the spacecraft. We then address the polytropic behavior in the density and temperature fluctuations in short time intervals, which we analyze in order to derive the effective polytropic index of small-scale processes. The large-scale variations of the\ solar\ wind proton density and temperature, which are associated with the plasma expansion into the heliosphere, follow a polytropic model with a polytropic index similar to 5/3. On the other hand, the short-scale fluctuations, which are potentially associated with turbulence, follow a model with a larger polytropic index. We investigate possible correlations between the polytropic index of short-scale fluctuations and the plasma speed, plasma beta, and the magnetic field direction. We discuss candidate mechanisms leading to this behavior including energy transfer and possible mechanisms restricting the effective particle degrees of freedom at smaller scales.

Year of Publication2020
JournalThe Astrophysical Journal
Number of Pages26
Date Published09/2020