Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771

Near-simultaneous dawn-side bow shock crossings by WIND and IMP 8 on December 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 crossings took place at sun-Earth-spacecraft angles of 64.7° and 115.3° , respectively. The solar wind speed and interplanetary magnetic field magnitude were near their long-term average values. However, the orientation of the IMF was unusual in that it rotated from an angle of ~50-60° to the sun-Earth line at the beginning of the interval of shock crossings to less than 20° just after the final crossings. The ratio of the downstream to upstream components 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 at both spacecraft. However, at WIND they have a strong compressional component which is much weaker farther downstream at IMP 8. Unexpectedly, the radial distance of the shock at both spacecraft is only ~80-85% of that predicted by recent models. Motivated by this event, we have statistically analyzed a larger data set of bow shock crossings which took place under quasi-field-aligned flow conditions. On the basis it is suggested that magnetosheath thickness may decrease by ~10% as the IMF becomes increasingly flow aligned.

Geophys. Res. Lett., 23, No. 10, 1207-1210, May 1996