Statistical study of favorable foreshock ion properties for the
formation of hot flow anomalies and foreshock bubbles
Abstract
Hot flow anomalies (HFAs) and foreshock bubbles (FBs) are frequently
observed in Earth’s foreshock, which can significantly disturb the bow
shock and therefore the magnetosphere-ionosphere system and can
accelerate particles. Previous statistical studies have identified the
solar wind conditions (high solar wind speed and high Mach number, etc.)
that favor their generation. However, backstreaming foreshock ions are
expected to most directly control how HFAs and FBs form, whereas the
solar wind may partake in the formation process indirectly by
determining foreshock ion properties. Using Magnetospheric Multiscale
mission and Time History of Events and Macroscale Interactions during
Substorms mission, we perform a statistical study of foreshock ion
properties around 275 HFAs and FBs. We show that foreshock ions with a
high foreshock-to-solar wind density ratio
(>~3%), high kinetic energy
(>~600eV), large ratio of kinetic energy to
thermal energy (>~0.1), and large
perpendicular temperature anisotropy
(>~1.4) favor HFA and FB formation. We also
examine how these properties are related to solar wind conditions:
higher solar wind speed and larger (angle between the interplanetary
magnetic field and the bow shock normal) favor higher kinetic energy of
foreshock ions; foreshock ions are less diffuse at larger ; small , high
Mach number, and closeness to the bow shock favor a high
foreshock-to-solar wind density ratio. Our results provide further
understanding of HFA and FB formation.