INTERPLANETARY MAGNETIC-FIELD CONTROL OF MAGNETOTAIL FIELD - IMP-8 DATA AND MHD MODEL COMPARED

Magnetic field patterns in the magnetotail's cross-sectional plane at similar to 30 R(E) derived from IMP 8 data are compared with correspon ding patterns derived from the Fedder-Lyon MHD model of magnetosphere - solar wind interaction. The comparisons emphasize features attributa ble to the influence of the interplanetary magnetic field (IMF). They reveal considerable correspondences in field asymmetries, nonuniform f ield perturbations, and current sheet twisting. Both data-based and MH D-based patterns are qualitatively similar to patterns obtained by sup erposing a uniform field on a dipole field, but they show that the fie ld perturbations are stronger at the equatorial flanks of the tail tha n in the high-latitude lobes. These quantitative details go beyond the superposition model. From the point of view of interpretation, the fa ct that the MHD-based patterns reproduce the distinctive nonsuperposit ion features seen in the data-based patterns indicates that these feat ures result from MHD motions within the magnetosphere distributing the field that is merged at the magnetopause (i.e., the ''penetrating'' f ield), From the point of view of modeling, the agreement indicates tha t MHD modeling captures essential aspects of the solar wind - magnetos phere coupling.