Thermodynamic Properties of Mirror Structures in the Magnetosheath: MMS Observations and Double-polytropic MHD Simulations

Abstract

Mirror structures are manifested as a wave train of magnetic peaks and/or dips, which are anticorrelated with plasma density. The evolution behavior of the ion temperatures perpendicular and parallel to the local magnetic field, T ⊥ and T ∥ , in the magnetic peaks and dips is rare to be theoretically studied. In this paper, the thermodynamic properties of mirror structures are investigated using two-dimensional magnetohydrodynamic simulations with double-polytropic laws. Two polytropic exponents, γ ∥ and γ ⊥ , are used as parameters to describe various thermodynamic conditions in the anisotropic plasma—for example, γ ∥  = 1, γ ⊥  = 1 for double-isothermal and γ ∥  = 3, γ ⊥  = 2 for double-adiabatic. Using empirical values of γ ∥  = 1.14 and γ ⊥  = 0.94 for magnetosheath plasma, the variations of T ∥ and T ⊥ in the magnetic dips and peaks observed by the Magnetospheric Multiscale Mission in the Earth’s magnetosheath can be reproduced. This consistent result cannot be achieved by use of γ ∥  = 3, γ ⊥  = 2 and γ ∥  = 0.5, γ ⊥  = 2, where the latter exponents can lead to the same mirror instability threshold as kinetic theory. In magnetic dips, T ∥ is found to be increased for γ ∥ > 1 but decreased for γ ∥ < 1, while T ⊥ is decreased for γ ⊥ > 1 but increased for γ ⊥ < 1. For magnetic peaks, the variation features of T ∥ and T ⊥ are opposite to that of the magnetic dips. It is concluded that the thermodynamic properties of mirror structures are further from double-adiabatic but closer to double-isothermal conditions.