Fast shocks at the edges of hot diamagnetic cavities upstream from the Earth's bow shock

Abstract

Recently, several interesting events, described as hot expanding diamagnetic cavities, have been observed upstream from the earth's bow shock using the ISEE 1 and 2 spacecraft. It has been suggested that fast shocks may form at the edges of some of these events because of the rapid expansion of the cavities. We have examined plasma density, temperature, velocity, and total field changes across the edges of several events and find these changes to be qualitatively consistent with the presence of shocks there. The presence of flat‐topped electron distributions and occasional electron beams at and downstream from the edges provides additional evidence for shocks. Plasma wave observations also show shocklike electrostatic noise at the edges of several events. We conclude that the edges of diamagnetic cavity events are often shocks, with a range of shock strengths similar to that observed in the interplanetary medium. The range of shock strengths may be the result of different convection and/or expansion speeds of the cavities; these convection and/or expansion speeds in turn may ultimately be related to the age of the cavities.