Earth's bow shock and magnetopause in the case of a field-aligned upstream flow:: Observation and model comparison -: art. no. 1269

[1] 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 15degrees and (2) the conditions were stable for a prolonged period (similar to9 hours). At the nose of the magnetosphere, the Interball-1 data revealed that the magnetopause was farther outward by similar to2 R-E 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 ( paraboloidal) shock 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 flow-aligned IMF orientation causes a significant change of the magnetopause shape into a bullet-like obstacle.