Hot flow anomalies at Saturn's bow shock

摘要

We present evidence for the occurrence of hot flow anomalies (HFAs) at Saturn's bowshock. A survey of Cassini magnetic field and electron data taken upstream of the dawnflank bow shock is carried out in order to identify Kronian HFAs. Seventeen events areidentified that were all associated with energization of the solar wind electrons and satisfied the majority of the conditions for HFA formation. The majority of the eventspossessed a central cavity of rarefied plasma; however, for two of the events the centralcavity was over dense, possibly indicating that these examples were at an early stage offormation. For the event that occurred on 8 November 2004 the calculation of ionmoments is possible, revealing an ion temperature increase by a factor of —800 in thecentral region of the event that was associated with a significant deflection of the solarwind bulk flow. The spatial extent of the event was —4.6 Saturn radii in the directionnormal to the current sheet underlying the event. Pressure calculations imply that theheated central region was expanding at the time of the encounter. We conclude that the8 November 2004 event was a spacecraft encounter with an HFA at Saturn's bow shockand we propose that the other 16 events identified by the survey were also HFAencounters. These observations suggest that HFAs are a solar-system-wide phenomenon.