Distributions of suprathermal ions near hot flow anomalies observed by RAPID aboard Cluster Original Research Article

A series of events having signatures of hot flow anomalies (HFA) observed by the magnetic field, plasma, and energetic particle instruments aboard the Cluster spacecraft upstream of the Earth’s bow shock in February–April 2003 are investigated. During this period, the separation of the four satellites (up to about 10,000 km) allowed determining the velocity of moving interplanetary plasma structures, which intersect the bow shock. About 50 candidates for HFA events were found and some statistical aspects are discussed. Two HFA events are analyzed in detail: one on 16 February 2003 at 10:48 UT with marked energetic ion signature and another on 7 March 2003 at 10:15 UT with a smaller increase. Both occurred in the vicinity of the bow shock, the plasma speed decreased, its flow deflected, its temperature increased, whereas a magnetic structure with rapidly changing field direction passed by and finally reached the bow shock. Both electrons and ions are accelerated; elevated fluxes of 28–68 keV ions appeared before and lasted longer than the HFA event as observed by all four RAPID instruments. The directional and pitch angle distributions of low-energy ions transformed into the plasma frame are discussed.