NEAR-SIMULTANEOUS BOW SHOCK CROSSINGS BY WIND AND IMP-8 ON DECEMBER 1, 1994

Near-simultaneous dawn-side bow shock crossings by WIND and IMP 8 on D ecember 1, 1994 are analyzed to determine shock location and shape and to examine the changes in shock structure and the foreshock MHD wave properties with increasing downstream distance. The WIND and IMP 8 cro ssings took place at sun-Earth-spacecraft angles of 64.7 degrees and 1 15.3 degrees, respectively. The solar wind speed and interplanetary ma gnetic field magnitude were near their long-term average values. Howev er, the orientation of the IMF was unusual in that it rotated from an angle of similar to 50-60 degrees to the sun-Earth line at the beginni ng of the interval of shock crossings to less than 20 degrees just aft er the final crossings. The ratio of the downstream to upstream compon ents of the magnetic field tangential to the shock decreases from 4.1 at WIND to 3.1 at IMP 8 in general agreement with theory. In addition, the overshoot in the shock magnetic ramp observed at WIND is greatly diminished by the downstream distance of IMP 8. In the foreshock, MHD waves with periods of 10-20 s and amplitudes of 3-6 nT were observed a t both spacecraft. However, at WIND they have a strong compressional c omponent which is much weaker farther downstream at IMP 8. Unexpectedl y, the radial distance of the shock at both spacecraft is only similar to 80-85% of that predicted by recent models. Motivated by this event , we have statistically analyzed a larger data set of bow shock crossi ngs which took place under quasi-field-aligned flow conditions. On thi s basis it is suggested that magnetosheath thickness may decrease by s imilar to 10% as the IMF becomes increasingly flow aligned.