Magnetic field patterns in the magnetotail's cross-sectional plane at ~30 RE derived from IMP 8 data are compared with corresponding patterns derived from the Fedder-Lyon MHD model of magnetosphere-solar wind interaction. The comparisons emphasize features attributable to the influence of the interplanetary magnetic field (IMF). They reveal considerable correspondences in field asymmetries, nonuniform field perturbations, and current sheet twisting. Both data-based and MHD-based patterns are qualitatively similar to patterns obtained by superposing a uniform field on a dipole field, but they show that the field perturbations are stronger at the equatorial flanks of the tail than in the high-latitude lobes. These quantitative details go beyond the superposition model. From the point of view of interpretation, the fact that the MHD-based patterns reproduce the distinctive nonsuperposition features seen in the data-based patterns indicates that these features result from MHD motions within the magnetosphere distributing the field that is merged at the magnetopause (i.e., the ''penetrating'' field). From the point of view of modeling, the agreement indicates that MHD modeling captures essential aspects of the solar wind--magnetosphere coupling. ¿ American Geophysical Union 1995