Shan Wang

and 9 more

The 1-Hz whistler wave precursor attached to shock-like structures are often observed in foreshock. Using observations from the Magnetospheric Multiscale mission, we investigate the interactions between 1-Hz waves and ions. Incoming solar wind ions do not gyro-resonate with the wave, since typically the wave is right-handed in their frame. We demonstrate that solar wind ions commonly exhibit 180 gyro-phase bunching from the wave magnetic field, understanding it with a reconciled linear picture for non-resonant ions and non-linear trapping theory of anomalous resonance. Along the longitudinal direction, solar wind ions experience Landau resonance, exhibiting either modulations at small wave potentials or trapping in phase-space holes at large potentials. The results also improve our understanding of foreshock structure evolution and 1-Hz wave excitation. Shock-like structures start with having incoming solar wind and remotely-reflected ions from further downstream. The ion-scale 1-Hz waves can already appear during this stage. The excitation may be due to shock-like dispersive radiation or kinetic instabilities resonant with these remotely-reflected ions. Ions reflected by local shock-like structures occur later, so they are not always necessary for generating 1-Hz waves. The wave leads to ion reflection further upstream, which may cause reformation. In one event, locally-reflected ions exhibit anomalous resonance in the early stage, and later approach to the gyro-resonant condition with gyro-phases ~270 . The latter is possibly due to nonlinear trapping in regions with an upstream-pointing magnetic field gradient, linked to reformation. Some additional special features like frequency dispersions are observed, requiring better explanations in the future.

Terry Zixu Liu

and 4 more

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.