Spatial Distribution and Low-frequency Disturbance Modulation of Magnetosheath Ion Polytropic Index

Abstract

We, using the Cluster data from 2001 to 2010, studied spatial distribution of the ion polytropic index in the magnetosheath, and the modulation of polytropic process by the low-frequency disturbances (4–18 mHz). The total of 30,3283 samples is divided into two sorts: quasi-perpendicular and quasi-parallel propagating ones. The median polytropic index increases with spreads narrowing from the bow shock to the magnetopause. The median polytropic indices are basically between isothermal and adiabatic in the inner magnetosheath, and between isothermal and isobaric in the outer magnetosheath. The spatial distributions of the correlation coefficient (CC) between the perturbed ion number density and the parallel magnetic field CC (δn, δB ∥) have a good correlation with those of polytropic index. The quasi-perpendicular disturbances are mostly mirror-like modes (D r ≪ 1) except for some slow-mode disturbances (D r ≥ 1) in the regions near the Sun–Earth line and the inner magnetosheath. The polytropic indices in the inner and middle magnetosheath modulated by mirror-like-mode disturbances are between 0.9 and 1.3. The quasi-parallel propagating low-frequency disturbances are predominantly slow modes in the inner and middle magnetosheath, and Alfvén modes in the outer magnetosheath. For the samples with quasi-parallel propagating disturbances, the polytropic processes are basically between isothermal and isobaric except near the magnetopause. The good correlation between the spatial distributions of polytropic index and low-frequency disturbances indicates that the distribution of the polytropic index in the magnetosheath is modulated by low-frequency disturbances.

On the Influence of the Ionization–Recombination Processes on the Hydrogen Plasma Polytropic Index

“…The knowledge of its value is of high physical interest and it's been researched for over 50 years (Petrie et al 2007). This research actively continues as observed data of the solar wind from Parker Solar Probe (Nicolaou et al 2023), Ulyssess (Nicolaou et al 2020), Wind (Dayeh & Livadiotis 2022), ACE (Jacobs & Poedts 2011), Cluster (Pang et al 2022) spacecrafts are used to determine the polytropic index. Knowing the nature of the deviations of the polytropic index and the physical processes causing it is a significant step forward in the study of the thermodynamics and energy balance of not only the solar and stellar atmospheric plasma, but also for cold proto-stellar molecular clouds (Donkov et al 2021).…”

Influence of Ionization on the Polytropic Index of the Solar Atmosphere within Local Thermodynamic Equilibrium Approximation

“…The knowledge of its value is of high physical interest, and it has been researched for over 50 yr (Petrie et al 2007). This research actively continues as observed data of the solar wind from the Parker Solar Probe (Nicolaou et al 2023), Ulyssess (Nicolaou et al 2020), Wind (Dayeh & Livadiotis 2022), ACE (Jacobs & Poedts 2011), and Cluster (Pang et al 2022) spacecraft are used to determine the polytropic index. Knowing the nature of the deviations of the polytropic index and the physical processes causing it is a significant step forward in the study of the thermodynamics and energy balance of not only the solar and stellar atmospheric plasma but also cold protostellar molecular clouds (Donkov et al 2021).…”

Modeling of the Polytropic Index and Temperature Anisotropy in the Solar Wind

“…The polytopic index is a fundamental physical parameter for investigating the thermodynamic processes present in space and astrophysical plasma, and the formulation of the theoretical relation between the polytropic index and the anisotropic temperature is currently of considerable recent interest in space plasma (see, e.g., Pang et al 2022;Ghag et al 2024), as the polytropic index has broad applications in the field of space and astrophysical science (see Horedt 2004). For the quasi-static process, Ledoux (1958) describes the relationship between density, temperature, and thermal pressure.…”