Double-polytropic closure in the magnetosheath

The magnetosheath plasma is usually neither isotropic nor adiabatic. This paper contains an attempt to describe its thermodynamic properties in terms of two polytropic laws, p perpendicular/ρBγperpendicular-1 = Cperpendicular and pparallelBγperpendicular-1/ρperpendicular= Cparallel, such that for γperpendicular=2, γparallel = 3 the usual Chew-Goldberger-Low double-adiabatic expressions are recovered and for γperpendicular = 1, γparallel = 1 double-isothermal conditions are obtained. Using data from the AMPTE/IRM spacecraft, the authors show that the subsolar magnetosheath plasma may be better described by the double-polytropic laws than by the mirror instability threshold, in particular in the low beta region near the magnetopause. The inferred polytropic exponents vary from event to event but are typically in the ranges of γperpendicular = 0.94 ± 0.10 and γparallel = 1.14. ± 0.13 for the 29 cases they have examined