Pressure Variations and Particle Acceleration Associated with Foreshock Bubbles and Hot Flow Anomalies

Abstract:

The ion foreshock upstream of the quasi-parallel terrestrial bow shock is characterized by suprathermal plasma counter-streaming against the incident solar wind. Interactions between the foreshock and solar wind plasmas can enhance wave activity and produce large-scale transient phenomena, such as hot flow anomalies (HFAs) and foreshock bubbles (FBs). HFAs and FBs both result from discontinuities in the interplanetary magnetic field “sweeping up” and concentrating suprathermal foreshock ions via gyro-kinetic effects. These concentrations become the super-heated cores of HFAs and FBs and result in strong disruptions of the local solar wind plasma, including flow deflections and strong density and magnetic field variations. One of the consequences of these strong variations is a change in the total pressure incident on the bow shock. Here, we quantify these pressure variations from a series of cases of HFAs and FBs observed by NASA’s THEMIS spacecraft and discuss how these pressure variations can penetrate through the magnetosheath and impact the magnetosphere, resulting in significant magnetopause displacement. With evidence from multi-point observations, we also compare and contrast the nature of HFAs and FB impacts on the magnetopause, showing that HFAs tend to be more localized in nature while FBs impact the system more globally, similar to pressure pulses in the solar wind. Both phenomena also result in the formation of strong compressions and/or shocks, resulting in strong wave activity. We finish with a discussion of the potential for particle acceleration from HFAs and FBs, showing evidence that both are highly efficient at ion and electron acceleration.