The structure of hot flow anomalies in the magnetosheath

The interaction of the solar wind with the Earth's magnetosphere creates a population of backstreaming ions. When a tangential discontinuity contacts the bow shock, these ions can be focused to the discontinuity region and create so-called Hot Flow Anomalies (HFAs) - diamagnetic cavities filled with hot, tenuous, and deflected plasma population. These cavities are swept downstream and can be observed in the magnetosheath. We have analyzed INTERBALL-1 and MAGION-4 observations of magnetosheath HFAs with attention to their internal structure. We have found that HFAs often consist of two parts separated by an density enhancement. The particle behaviour can differ in these two parts. We demonstrate this peculiarity and discuss its possible origin.