IMP 8 magnetosheath ®eld comparisons with models

This study presents cross-sectional vector maps of the magnetic ®eld derived from IMP 8 magnetometer in the magnetosheath at 30 Re behind the Earth. In addition the vector patterns of the magnetosheath ®eld for northward, southward, and east-west interplanetary magnetic ®eld (IMF) directions are qualitatively compared with those obtained from the Spreiter-Stahara gas dynamic (GD) and Fedder-Lyon magnetohydrodynamic models (MHD). The main purpose is to display the cross-sectional di€erences in relation to the dayside merging with di€erent IMF directions, allowing the reader to make direct visual comparisons of the vector patterns. It is seen that for east-west IMF directions, the data-based andMHD-based patterns di€er noticeably in a similar way from the GD model, presumably re ̄ecting the in ̄uence of dayside magnetic merging of the Earth's magnetic ®eld with the y-component of the interplanetary magnetic ®eld. All three northward IMF cross sections show comparable ®eld draping patterns as expected for a closed magnetosphere. For southward IMF case, on the other hand, di€erences between the three cross-sectional patterns are greater as seen in the ®eld vector sizes and directions, especially closer to the magnetopause where more disturbed magnetospheric conditions are known to be exist. The data comparisons with the MHD and GD models show that the di€erences result from the magnetic ®eld̄ow coupling and that the e€ects of dayside reconnection are present in IMP 8 magnetic ®eld measurements.