Earth's bow shock and magnetopause in the case of a field-aligned upstream flow: Observation and model comparison

Abstract

On 5 May 1996 the Interball-1 and IMP 8 spacecraft crossed the bow shock boundary. The upstream conditions were special in two factors: (1) the interplanetary magnetic field was anti-parallel to the solar wind flow within 15° and (2) the conditions were stable for a prolonged period (∼9 hours). At the nose of the magnetosphere, the Interball-1 data revealed that the magnetopause was farther outward by ∼2 RE than model predictions and the subsolar magnetosheath was unusually thin, at most 10% of the magnetopause standoff distance. Both results stand in contrast to predictions of existing magnetopause/bow shock models. Assuming a hyperboloidal (parabolpidal) shook wave, the calculated shock's standoff distance was 13.7 (13.6) R E, and the focus was located on the x axis at 4.5 (4.2) R E. On the basis of the IMP 8 observation, the bow shock flares significantly less than MHD simulations predict for a field-aligned bow shock at the magnetospheric flanks. This study discusses differences between the observations and existing MHD bow shock simulations for field-aligned upstream flow. Furthermore, it is suggested that the flowaligned IMF orientation causes a significant change of the magnetopause shape into a bullet-like obstacle. Copyright 2003 by the American Geophysical Union.