Foreshock dynamics controlled by Hot Flow Anomaly: relation between dawn and dusk sector events

On 21 January 2005, the passage in the magnetospheric dawn sector of a tilted solar wind current sheet at the periphery of an Interplanetary Coronal Mass Ejection is not observed by Cluster in the dusk sector, where the Interplanetary Magnetic Field points in such a way to form a foreshock. It coincides however with the momentary transition of Cluster from a field-aligned beam region into a Ultra-Low-Frequency (ULF) wave field, with first ever reported 6s-periods in the spacecraft frame. The 4-spacecraft Cluster analysis shows that the ULF waves are intrinsically left-handed and propagating downstream in the proton plasma frame, and thus cannot be produced by the local upstream-directed field-aligned proton beam. Moreover, on each side of the magne- tosphere, Double Star TC-1 and Geotail experience large variations in magnetosheath plasma parameters consistent with the tailward effects of a Hot Flow Anomaly (HFA) expected as a result of the interaction with the current sheet. Modelling of the bowshock and fore- shock dynamics shows that the entry into the foreshock ULF wave field requires an expansion of the bow shock shape, consistent with the expected formation of a HFA bulge. We propose that the generation of the identified ULF waves on the dusk side is controlled by the production of beams of energetic particles, sufficiently hot for the ion/ion left-hand resonant instability to emerge, consistent with those particles backstreaming downstream towards Cluster from a presumed HFA expansion in the solar wind.