Explorer 12 magnetopause observations: Large-scale nonuniform motion

A number of events have been studied that provide evidence that relatively small portions of the magnetopause occasionally become severely distorted. The largest distortions are generally elongated in the north-south direction, and the magnetosheath magnetic field is usually either nearly parallel or antiparallel to the magnetospheric field. As a result, the distortions do not involve severe bending of any magnetic field lines. The distortion is frequently triggered by a change in the properties of the solar wind. A model involving propagation of a thin discontinuity past the earth is consistent with the observations. Observationally, the severe distortions are distinct from the small-scale more or less uniform radial excursion of the magnetopause that are seen on a large fraction of all magnetopause crossings. They are also distinct from the expansions and contractions of the entire magnetosphere in response to changing solar wind pressure.