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Double‐polytropic closure in the magnetosheath
Author(s) -
Hau L. N.,
Phan T. D.,
Sonnerup B. U. Ö,
Paschmann G.
Publication year - 1993
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/93gl02491
Subject(s) - magnetosheath , polytropic process , physics , adiabatic process , magnetopause , instability , plasma , isotropy , astrophysics , magnetosphere , magnetohydrodynamics , geophysics , computational physics , mechanics , nuclear physics , thermodynamics , quantum mechanics
The magnetosheath plasma is usually neither isotropic nor adiabatic. This paper contains an attempt to decribe its thermodynamic properties in terms of two polytropic laws, p⟂/ρB γ⟂−1 = C ⟂ and p ∥ B γ∥−1 / ρ γ∥ = C ∥ , such that for γ ⟂ = 2, γ ∥ = 3 the usual Chew‐Goldberger‐Low double‐adiabatic expressions are recovered and for γ ⟂ = 1, γ ∥ = 1 double‐isothermal conditions are obtained. Using data from the AMPTE/IRM spacecraft, we 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 γ ⟂ = 0.94 ± 0.10 and γ ∥ =1.14 ± 0.13 for the 29 cases we have examined.