Origin of the backstreaming ions in a young Hot Flow Anomaly

We analyze an event in front of the bow shock observed by Cluster spacecraft on 22.02.2006. This event has many attributes of Hot Flow Anomaly at early stage of development including strong upstream beam and disturbed magnetic profile with increased magnetic field at one or two sides as observed by 4 Cluster spacecraft. The angle between the magnetic field vectors at two sides of the current sheet was ~ 10{\deg}. The minimum magnetic field magnitude within HFA was ~ 1 nT. The shock at two sides of the HFA was quasi-perpendicular. Upstream beam was observed on the leading side of the HFA. Parameters and velocity distributions of solar wind ions and of upstream ions observed on C1 and C3 spacecraft are analyzed separately in order to trace their changes across the event. The goal of this analysis was to get more information about the source of upstream beam. The beam evolved from the start of its observation till the HFA encounter being initially energetic and nearly mono-energetic. Its mean energy continuously decreased and energy spectrum widened as HFA approached spacecraft. First observation of particular energy that diminished with approaching the HFA varied linearly with gyro-radius of ions. The energy spectra of integrated beam (for all observation) and the energy spectrum of the beam observed just in front of HFA are very similar to the magnetosheath ion energy spectrum observed after bow shock crossing at ~ 1 hour after observation of the HFA. Lowest energies in the beam were observed within HFA only. Highest density and pressure of upstream beam are found in the current sheet itself. We suggest that the upstream beam is the result of the magnetosheath ions leakage through the region of HFA crossing with the bow shock front.

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