Multisatellite observations of large magnetic depressions in the solar wind

Two large depressions in the magnitude of the interplanetary magnetic field, lasting similar to 10-20 min, have been observed in the solar wind just upstream of the Earth's bow shock by three spacecraft (Active Magnetospheric Particle Tracer Explorer UK Subsatellite (AMPTE UKS), AMPTE Ion Release Module (IRM), and ISEE 1). The multiple satellite observations show that the depressions are convecting with the ambient solar wind. Analysis of the depression boundaries shows that they are tangential discontinuities with normals aligned approximately parallel to the GSE x direction. The electron distributions measured within the structures are remarkably isotropic when compared to the more anisotropic distributions found in the ambient solar wind. There is also a reduction in plasma wave activity during the depressions. The depressions exhibit characteristics similar to those of solar wind magnetic holes but are much larger than has been typically observed. The depressions also display similarities with encounters of the heliospheric plasma sheet and heat flux dropouts, both of which are typically observed near sector boundaries, close to the heliospheric current sheet. The nature of these depressions is discussed in the context of magnetic hole and heliospheric plasma sheet observations. A large magnetic hole structure formed from a conglomeration of small holes appears the more likely scenario for the observed depressions.