Average plasma sheet polytropic index as observed by THEMIS

Abstract

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.

Transport of Mass and Energy in Mercury's Plasma Sheet

“…Similar feature in the radial profile of γ at Earth was reported by Frühauff et al (2017), who suggested a connection between γ and the flow-braking region in the near-Earth magnetotail region. Similar feature in the radial profile of γ at Earth was reported by Frühauff et al (2017), who suggested a connection between γ and the flow-braking region in the near-Earth magnetotail region.…”

“…Analysis of the polytropic index γ with radial distance shows a minimum in γ at R~1.4 R M . Similar feature in the radial profile of γ at Earth was reported by Frühauff et al (2017), who suggested a connection between γ and the flow-braking region in the near-Earth magnetotail region. At Mercury, this minimum also occurs near the region where Poh et al (2017b) reported observations of Mercury's substorm current wedge.…”

“…The location of the minima in γ coincides with the location of Mercury's substorm current wedge identified by Poh et al (2017b). A similar minimum in γ as a function of R was also observed in the terrestrial study by Frühauff et al (2017). They suggested a connection between the decrease of γ and the flow-braking region typically observed at X~À15 R M (e.g., Shiokawa et al, 1997).…”

“…The 2/3 value was also used by Kivelson and Spence (1988) in their modified magnetotail model while investigating the effects of finite tail width. More recently, Frühauff et al (2017) obtained a γ of~1.72 using Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. More recently, Frühauff et al (2017) obtained a γ of~1.72 using Time History of Events and Macroscale Interactions during Substorms (THEMIS) data.…”

Dependence of Thermodynamic Processes on Upstream Interplanetary Magnetic Field Conditions for Magnetosheath Ions

“…Therefore, the polytropic index can be estimated from either the number density‐thermal pressure ( n ‐ p ) relation or the number density‐temperature ( n ‐ T ) relation. Using the linear fit between the logarithms of number density and those of thermal pressure (or temperature), previous studies examined thermodynamic conditions for plasmas in the various regions near the terrestrial magnetosphere, such as the interplanetary space (Kartalev et al, ; Newbury et al, ; Nicolaou et al, ), the magnetosheath (Pang et al, ; Song et al, ), and the plasma sheet (Baumjohann & Paschmann, ; Frühauff et al, ; Huang et al, ; Pang et al, ; Zhu, ).…”