On the origin of hot diamagnetic cavities near the Earth's bow shock

Abstract

On occasion, hot diamagnetic cavities have been observed upstream from the Earth's bow shock. The origin of these cavities has been a mystery ever since their discovery several years ago. In this paper, we note that, although only a small number of such events have so far been identified (corresponding to an average of about one event per month of observing time), for of these previously identified events occurred on a single day (November 16, 1977). Additional plasma and field data from that day are examined in considerable detail for further clues to the possible source of these events. A remarkable characteristic of the interplanetary medium on November 16 was the presence throughout that day and the next of large‐amplitude, very low‐frequency, nonmonochromatic, and noncompressive magnetic field variations (i.e., rotations), which resulted in intermittent magnetic connections between the observing spacecraft and the bow shock. The previously identified hot diamagnetic cavity events (HDCs), as well as several other intervals of HDC‐like properties observed very near the shock itself, tend to occur near transitions between quasi‐perpendicular and quasi‐parallel shock geometries. Furthermore, several of the HDC‐like episodes seen near and within the shock are closely associated with intervals of dense, nearly specularly reflected ions. These observations lead us to suggest that HDCs may form from an unusually strong interaction between shock‐reflected ions and the incoming solar wind. It is proposed that this interaction stems from a temporary and fairly localized reflection of a larger‐than‐normal fraction of the incident ions, which is stimulated by sudden changes in the upstream field orientation. The consequences of such a temporary overreflection are examined and found to be consistent with many of the observed features of HDCs, including the strong slowing, deflection, and heating of the flow as well as the localization, internal recoveries, and occasional formation upstream from the shock itself. This hypothesis also appears capable of explaining the considerable diversity of signatures seen at the leading edge of the various observed HDCs, ranging from the presence of a fast forward shock to the absence of any leading magnetic compression at all.