Three‐dimensional plasma structures with anomalous flow directions near the Earth's bow shock

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We have examined AMPTE IRM data obtained in the solar wind near the Earth's bow shock and found 16 well‐defined cases where a region of hot subsonic plasma is embedded in the solar wind. Such structures had been observed first with instruments on ISEE 1 and 2 and later on AMPTE UKS and distinguished from bow shock crossings. Our observations confirm some of the earlier findings, notably the event profile, showing a hot, low‐density core flanked by narrow regions of high density and strong magnetic field. We also find the low (∼200 km/s) flow velocities, strongly deflected from the solar wind, and we substantially strengthen the local time dependence of the flow which invariably is directed dawnward for prenoon events and duskward for postnoon events. Our results differ from the reported ISEE results in two respects. First, the flows we observe tend to have larger angles relative to the solar wind, and they often even have a sunward component. Second, the events we have selected cannot be described as diamagnetic cavities. On the contrary, the magnetic fields are usually significantly enhanced. This apparent discrepancy may simply result from different event selection criteria. A quantitative analysis of the regions flanking the hot core shows they consist of fast, nearly perpendicular, supercritical shocks on the outside and tangential discontinuities on the inside. We find a systematic difference between the orientations of the leading and trailing edge boundary normals. While the former are directed largely transverse to the solar wind flow, the latter are more nearly aligned with the solar wind. Another new finding concerns the presence of enhanced fluxes of >70‐keV electrons which appear to be of magnetospheric origin. The majority of events are associated with directional discontinuities in the interplanetary magnetic field. We have also found events which are not embedded in the solar wind but occur between the solar wind and the magnetosheath, at times replacing the regular bow shock. Among the mechanisms discussed as causes for these events are the formation of sunward directed plasma jets from magnetopause reconnection, or from amplification of magnetic stresses associated with rotational discontinuities in the interplanetary medium; sudden and localized enhancements of bow shock reflection; and the interaction of the bow shock with tangential discontinuities having a specific internal structure.