Unusual locations of Earth's bow shock on September 24–25, 1987: Mach number effects

Abstract

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 ∼ 14 RE, is shown to increase from near 10 RE initially to near 19 RE during an 8‐hour period, followed by an excursion to near 35 RE (where two IMP 8 shock crossings occur) and an eventual return to values smaller than 19 RE. The Alfvén MA and fast magnetosonic Mms Mach numbers remain above 2 and the number density above 4 cm−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. Significant quantitative differences exist between the apparent standoff distances for the shock crossings and those predicted using the observed plasma parameters and the standard model based on Spreiter et al.'s (1966) gasdynamic equation. These differences can be explained in terms of either a different dependence of the standoff distance on Mach number at low MA and Mms, or variations in shock shape with MA and Mms (becoming increasingly “puffed up” with decreasing MA and Mms, as expected theoretically), or by a combination of both effects. Global MHD simulations, 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 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 Mms ∼ 2.4. The estimates imply substantial changes in standoff distance and/or shock shape at low MA and Mms. Mach number effects can therefore be quantitativwely important in determining and predicting the shape and location of the bow shock, even when MA and Mms 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.