Unusual locations of Earth`s bow shock on September 24-25, 1987: Mach number effects
- University of Iowa, IA (United States)
- Goddard Space Flight Center, Greenbelt, MD (United States)
- Massachusetts Institute of Technology, Cambridge, MA (United States)
ISEE 1 and IMP 8 data are used to identify 19 crossings of Earth`s bow shock during a 30-hour period following 0000 UT on September 24, 1987. Apparent standoff distances for the shock are calculated for each crossing using two methods and the spacecraft location; one method assumes the average shock shape, while the other assumes a ram pressure-dependent shock shape. The shock`s apparent standoff distance normally {approximately}14 R{sub E}, is shown to increase from near 10 R{sub E} initially to near 19 R{sub E}. The Alfven M{sub A} and fast magnetosonic M{sub ms} Mach numbers remain above 2 and the number density above 4 cm{sup {minus}3} for almost the entire period. Ram pressure effects produce the initial near-Earth shock location, whereas expansions and contractions of the bow shock due to low Mach number effects account, qualitatively and semiquantitatively, for the timing and existence of almost all the remaining ISEE crossings and both IMP 8 crossings. Ram pressure-induced changes in the shock`s shape are discussed but found to be quantitatively unimportant for the shock crossings analyzed. Approximate estimates of both the deviation of the shock`s standoff distance from the standard model and of the shock`s shape are determined independently (but not consistently) for M{sub ms}{approximately}2.4. The estimates imply substantial changes in standoff distance and/or shock shape at low M{sub A} and M{sub ms}. Mach number effects can therefore be quantitatively important in determining and predicting the shape and location of the bow shock, even when M{sub A} and M{sub ms} remain above 2. This study confirms and generalizes previous studies of Mach number effects on Earth`s bow shock. Statistical studies and simulations of the bow shock`s shape and location should be performed as a function of Mach number, magnetic field orientation, and ram pressure. 25 refs., 12 figs.
- OSTI ID:
- 96411
- Journal Information:
- Journal of Geophysical Research, Journal Name: Journal of Geophysical Research Journal Issue: A1 Vol. 100; ISSN JGREA2; ISSN 0148-0227
- Country of Publication:
- United States
- Language:
- English
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