OBSERVATIONAL TEST OF HOT FLOW ANOMALY FORMATION BY THE INTERACTION OF A MAGNETIC DISCONTINUITY WITH THE BOW SHOCK

The formation of a hot flow anomaly (HFA) observed near the Earth's bow shock appears to be due to the interaction between the bow shock and an impinging discontinuity in the upstream plasma. Recent single-particle and two-dimensional hybrid numerical studies have suggested that such an interaction will produce an HFA only if the motional electric field in the ambient plasma points toward the discontinuity, thereby focusing shock-reflected ions into it. We perform a test of this electric field orientation for a set of nine HFA events observed by the ISEE spacecraft and described previously in the literature. The principal difficulty with the test is the determination of the normals to the discontinuities. Application of a minimum variance analysis produces discontinuity normals which suggest that the majority of the discontinuities were probably tangential, even though it is doubtful that the conditions for validity are very well satisfied for these events. Under the assumption that the discontinuities were tangential, the predicted electric field orientation is found on at least one side of all nine observed HFAs (on the trailing edge of seven and the leading edge of five, and on both sides of three events). Further, there is evidence that asymmetries in the observed magnetic field signatures are related to the orientation of the motional electric field: The events in which the electric field points toward the discontinuity on both sides tend to be those with fairly symmetric flanking magnetic field enhancements. Two-dimensional hybrid simulations appear to confirm the association between the magnetic field signature and the electric field orientations, and they show further that rotational discontinuities appear to be less effective at producing distinct HFA-like signatures than tangential discontinuities.