Multipoint observations of planar interplanetary magnetic field structures

Abstract

We present interplanetary magnetic field (IMF) data made on 1 November 1984 by three spatially well-separated spacecraft in the solar wind: the IMP 8, AMPTE-UKS and -CCE spacecraft. The IMF measured by each of the spacecraft is found to consist of a multiplicity of structures within which the magnetic field varies in parallel planes. The orientations of these planes at the three spacecraft locations are similar. The planes are inclined at a large angle to the ecliptic, and they lie almost perpendicular to the nominal Parker spiral direction in the ecliptic. Intercomparisons of the measurements at the various spacecraft show that the IMF features at one spacecraft are clearly reproduced at another with, however, time delays required for the propagation of signals. Using these time delays and the mutual separations of the spacecraft, we infer that the structures are convecting with the ambient flow. Simultaneous observations made downstream of the bow shock in the magnetosheath reveal that the magnetosheath magnetic field, too, is planar. However, the plane of maximum variation of the magnetosheath magnetic field is sheared with respect to the corresponding plane in the solar wind, and the field now lies approximately tangential to the magnetopause. Finally, our observations are contrasted to another IMF configuration showing gross departures from the nominal Parker spiral orientation: magnetic clouds.