Three-dimensional position and shape of the bow shock and their variation with Alfvenic, sonic and magnetosonic Mach numbers and interplanetary magnetic field orientation
- NASA Goddard Space Flight Center, Greenbelt, MD (United States)
A large set of bow shock crossings (i.e., 1392) observed by 17 spacecraft has been used to explore the three-dimensional shape and location of the Earth`s bow shock and its dependence on solar wind and interplanetary magnetic field (IMF) conditions. This study investigates deviations from gas dynamic flow models associated with the magnetic terms in the magnetohydrodynamic (MHD) equations. Empirical models predicting the statistical position and shape of the bow shock for arbitrary values of the solar wind pressure, IMF, and Alfvenic Mach number (M{sub A}) have been derived. Individual crossings have been taken into consideration by normalizing the observed crossings to the average value = 3.1 nPa. The resulting data set has been used to fit three-dimensional bow shock surfaces and to explore the variations in these surfaces with sonic (M{sub S}), Alfvenic (M{sub A}) and magnetosonic (M{sub MS}) Mach numbers. Analysis reveals that among the three Mach numbers, M{sub A} provides the best ordering of the least square bow shock curves. The subsolar shock is observed to move Earthward while the flanks flare outward in response to decreasing M{sub A}; the net change represents a 6-10% effect. Variations due to changes in the IMF orientation were investigated by rotating the crossings into geocentric interplanetary medium coordinates. This study confirms a north-south versus east-west asymmetry and quantifies its variation with M{sub S}, M{sub A}, M{sub MS}, and IMF orientation. A 2-7% effect is measured, with the asymmetry being more pronounced at low Mach numbers. Combining the bow shock models with the magnetopause model of Roelof and Sibeck, variations in the magnetopause size at the subpolar point is found to be 1.46; at dawn and dusk, the ratios are found to be 1.89 and 1.93, respectively. The subsolar magnetosheath thickness is used to derive the polytropic index {gamma} according to the empirical relation of Spreiter. 55 refs., 6 figs., 3 tabs.
- OSTI ID:
- 207234
- Journal Information:
- Journal of Geophysical Research, Vol. 100, Issue A5; Other Information: PBD: 1 May 1995
- Country of Publication:
- United States
- Language:
- English
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