UNUSUAL LOCATIONS OF EARTHS BOW SHOCK ON SEPTEMBER 24-25, 1987 - MACHNUMBER EFFECTS

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 cr ossing using two methods and the spacecraft location; one method assum es the average shock shape, while the other assumes a ram pressure-dep endent shock shape. The shock's apparent standoff distance, normally s imilar to 14 R(E), is shown to increase from near 10 R(E) initially to near 19 R(E) during an 8-hour period, followed by an excursion to nea r 35 R(E) (where two IMP 8 shock crossings occur) and an eventual retu rn to values smaller than 19 R(E). The Alfven M(A) and fast magnetoson ic M(ms) Mach numbers remain above 2 and the humber density above 4 cm (-3) for almost the entire period. Ram pressure effects produce the in itial near-Earth shock location, whereas expansions and contractions o f 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. Significant quanti tative differences exist between the apparent standoff distances for t he shock crossings and those predicted using the observed plasma param eters and the standard model based on Spreiter et al.'s (1966) gasdyna mic equation. These differences can be explained in terms of either a different dependence of the standoff distance on Mach number at low M( A) and M(ms), or variations in shock shape with M(A) and M(ms) (becomi ng increasingly ''puffed up'' with decreasing M(A) and M(ms), as expec ted theoretically), or by a combination of both effects. Global MHD si mulations, to be presented elsewhere, confirm that both effects occur and are significant. Ram pressure-induced changes in the shock's shape are discussed but found to be quantitatively unimportant for the shoc k crossings analyzed. Approximate estimates of both the deviation of t he shock's standoff distance from the standard model and of the shock' s shape are determined independently (but not consistently) for M(ms)s imilar to 2.4. The estimates imply substantial changes in standoff dis tance and/or shock shape at low M(A) and M(ms). Mach number effects ca n therefore be quantitatively important in determining and predicting the shape and location of the bow shock, even when M(A) and M(ms) rema in above 2. This study confirms and generalizes previous studies of Ma ch number effects on Earth's bow shock. Statistical studies and simula tions of the bow shock's shape and location should be performed as a f unction of Mach number, magnetic field orientation, and ram pressure.