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The variation of the plasma sheet polytropic index along the midnight meridian in a finite width magnetotail
Author(s) -
Spence Harlan E.,
Kivelson Margaret G.
Publication year - 1990
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/gl017i005p00591
Subject(s) - polytropic process , physics , plasma sheet , adiabatic process , meridian (astronomy) , convection , plasma , polytrope , astrophysics , classical mechanics , mechanics , magnetosphere , astronomy , quantum mechanics
We have obtained the polytropic index as a function of distance along the midnight meridian in the terrestrial magnetotail. As our purpose is to establish the effects of the finite width of the magnetotail, we use a simple theoretical model of plasma sheet convection, i.e., 2‐dimensional (2D) field structure and adiabatic inward convection of a uniform distant tail source. Particle orbits are treated independently for portions of the phase space distribution on shells of constant energy. On the midnight meridian, the moments of the distribution are parameterized by τ, the ratio of half the crosstail potential energy to the characteristic maxwellian energy of a distant downtail plasma source. We infer from the model the plasma pressure, P , and the number density, n , along the midnight meridian as a function of τ. P and n define locally an “effective” polytropic index, γ. We find that γ ranges between5 3and 1, depending on the value of τ and on geocentric distance. We suggest that the qualitative differences between the recent empirical determinations of the polytropic index by Baumjohann et al . [1989] and Huang et al . [1989] may be accounted for in part by this simple model.