Hot Flow Anomalies at Earth's Bow Shock

Hot flow anomalies (HFAs) are kinetic phenomena observed near the Earth's bow shock. They are characterized by a significant increase in particle temperature, substantial particle deflection from the solar wind flow direction, and a corresponding decrease in density.
As a spacecraft travels through a HFA, it will see heated plasma bounded by regions of enhanced magnetic field magnitude and density.

Two distinct types of HFAs have been identified, (Thomsen et al., 1986; Lucek et al., 2004). HFA observations with both a solar wind and reflected component have been interpreted as the signatures of an early stage of development. HFA observations with a single hot-ion population have been interpreted as the signature of the later stages evolution of a HFA. HFAs are important to study and understand because they may play an important role in solar windmagnetosphere coupling. They may also drive magnetopause motion, boundary waves, and flux transfer events (FTEs). They can excite ultra low frequency (ULF) waves in the magnetosphere, drive magnetic impulse events in the ionosphere, and trigger aurora brightening or dimming. Studying HFAs will aid in the understanding of fundamental processes that operate throughout the heliosphere such as particle energization and shocks. The flux rope signatures in some HFAs (Hasegawa et al., 2012) may also be linked to the understanding of magnetic reconnection physics.