The effects of pressure anisotropy on Birkeland currents in dipole and stretched magnetospheres

Journal Article · Wed Apr 01 04:00:00 EST 1992 · Journal of Geophysical Research; (United States)

DOI:https://doi.org/10.1029/91JA03030· OSTI ID:5595115

Birmingham, T J ^[1]

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1. NASA Goddard Space Flight Center, Greenbelt, MD (United States)

The expression for the Birkeland current density in an anisotropic P{sub {perpendicular}}, P{sub {parallel}} plasma derived in a companion paper is simplified by linearization: for purposes of computing j {sub {parallel}} the magnetic configuration is assumed to be axisymmetric, either a dipole for a dipole stretched by the field arising from an azimuthal current sheet. The anisotropy is that of a bi-Maxwellian and characterized by equatorial values P{sub {perpendicular}}(s{sub 0}) and r, the equatorial temperature (pressure) ratio. Parameter j {sub {parallel}} is proportional to the azimuthal derivatives of P{sub {perpendicular}} (s{sub 0}) and r, with coefficients that are integral functions along the background B of the pressure and field strength from the equator s{sub 0}, where there is (by assumption) no Birkeland current, to the point of reference. Values of j{sub {parallel}} at the ionosphere are compared between the two magnetic models for 0.1 < r < 10., a range that assuredly spans anisotropy ratios likely to be encountered in planetary magnetospheres. The effect of the anisotropy is modest, amounting to a factor of roughly 10 (larger or small depending on whether r < 1 or vice versa) for a stretched field line crossing the equator at r{sub 0} = 20, less than half this amount for the comparable dipole line. Parameter j{sub {parallel}} grows, owing to the diversion of perpendicular currents, rapidly away from the equator, significantly more so for a pressure anisotropy favoring P{sub {perpendicular}} (r > 1) than for one favoring P{sub {parallel}} (r < 1).

• OSTI ID: 5595115
• Journal Information: Journal of Geophysical Research; (United States), Journal Name: Journal of Geophysical Research; (United States) Vol. 97:A4; ISSN 0148-0227; ISSN JGREA
• Country of Publication: United States
• Language: English

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