How can the Solar Wind Turn Sunward?

Abstract:

Hot flow anomalies (HFA’s) have been an unresolved mystery of solar wind - magnetosphere interaction for several decades. HFA’s are observed in the solar wind upstream of the Earth’s bow shock, and they are characterized by a hot plasma and strong flow deflections that result in extreme cases to sunward flow in the event. It had been suggested that these events are caused by the interaction of tangential discontinuities (TD’s) with the bow shock where the motional electric field points into the TD. However, observations of many HFA’s without apparent TD’s and of many events with TD’s but inconsistent electric field configurations shed doubt on this mechanism. This presentation demonstrates that transient depletions of the solar wind dynamic pressure can cause HFA-like signatures without the requirement of kinetic physics. Any significant transient solar wind dynamic pressure change requires the reformation of the bow shock. It is suggested that during this interaction a new shock is formed, which travels rapidly sunward, and the newly shocked plasma has many of the properties consistent with HFA observations. The presentation examines these properties, such as density changes and plasma heating, and it determines the dependence of these changes as a function of pre-existing solar wind properties and of solar wind dynamic pressure changes. Results from this analysis are compared with HFA observations and statistics.