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

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 (paraboloidal) shock wave, the calculated shock's standoff distance was 13.7 (13.6) RE, and the focus was located on the x axis at 4.5 (4.2) RE. 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.