Magnetic loop behind an interplanetary shock: Voyager, Helios, and IMP 8 observations

Abstract

Magnetic field and plasma data from five spacecraft (Voyager 1 and 2, Helios 1 and 2, and IMP 8) were used to analyze the flow behind an interplanetary shock. The shock was followed by a turbulent sheath in which there were large fluctuations in both the strength and the direction of the magnetic field. This in turn was followed by a region (magnetic cloud) in which the magnetic field vectors were observed to change by rotating nearly parallel to a plane, consistent with the passage of a magnetic loop. This loop extended at least 30° in longitude between 1 and 2 AU, and its radial dimension was approximately 0.5 AU. In the cloud the field strength was high, and the density and temperature were relatively low. Thus the dominant pressure in the cloud was that of the magnetic field. The total pressure inside the cloud was higher than outside, implying that the cloud was expanding as it moved outward, even at the distance of 2 AU. The momentum flux of the cloud at 2 AU was not higher than that of the preshock plasma, indicating that the cloud was not driving the shock at this distance. It is possible, however, that the shock was driven by the cloud closer to the sun where the cloud may have moved faster. An extraordinary filament was observed at the rear of the cloud. It was bounded by current sheets whose orientations were preserved over at least 0.12 AU and which were related to the plane of maximum variance of the magnetic field in the cloud.