Hot flow anomaly formation and evolution: Cluster observations

Abstract

In this study, we have examined the formation and evolution of 513 hot flow anomalies (HFAs) from 2003 to 2009 observed by the Cluster spacecraft. Our results show that an original upstream discontinuity in the vicinity of an HFAs and/or at least one side of the HFA with the convective electric field pointing toward the discontinuity may help an HFA growing, but it is not a necessary condition to generate an HFA. It is shown that a significant part of the thermal energy inside HFAs is converted from the kinetic energy of the solar wind, although additional heating process(es) is required to heat the plasma inside an HFA. In order to learn the evolution of an HFA, we have examined the electron spectrum and ion velocity distribution function (VDF) inside young and mature HFAs. It is found that the particle spectra are good indicators of a young or mature HFA. Inside young HFAs, electron spectra can be fitted by a single drift‐κ distribution, while inside mature HFAs it can be fitted by the combination of a drift‐Maxwellian distribution with the peak energy below ∼10 eV and a heated electron distribution. On the other hand, ion VDF inside mature HFAs shows a single distribution, whereas the VDF inside young HFAs shows two clear ion populations—one original solar wind and a reflected ion population. It is found that the reflected ion population inside young HFAs can be scattered to more than 180° in the Vpara‐Vperp1 plane, where Vperp1 is in the V‐B plane but perpendicular to B, which is similar to the foreshock distribution. This indicates that the reflected ion population could be diffusive from all directions rather than the unidirectional beam when an HFA is forming.