Statistical properties of kinetic-scale magnetic holes in terrestrial space

Kinetic-scale magnetic holes(KSMHs)are structures characterized by a significant magnetic depression with a length scale on the order of the proton gyroradius.These structures have been investigated in recent studies in near-Earth space,and found to be closely related to energy conversion and particle acceleration,wave-particle interactions,magnetic reconnection,and turbulence at the kineticscale.However,there are still several major issues of the KSMHs that need further study—including(a)the source of these structures(locally generated in near-Earth space,or carried by the solar wind),(b)the environmental conditions leading to their generation,and(c)their spatio-temporal characteristics.In this study,KSMHs in near-Earth space are investigated statistically using data from the Magnetospheric Multiscale mission.Approximately 200,000 events were observed from September 2015 to March 2020.Occurrence rates of such structures in the solar wind,magnetosheath,and magnetotail were obtained.We find that KSMHs occur in the magnetosheath at rates far above their occurrence in the solar wind.This indicates that most of the structures are generated locally in the magnetosheath,rather than advected with the solar wind.Moreover,KSMHs occur in the downstream region of the quasi-parallel shock at rates significantly higher than in the downstream region of the quasi-perpendicular shock,indicating a relationship with the turbulent plasma environment.Close to the magnetopause,we find that the depths of KSMHs decrease as their temporal-scale increases.We also find that the spatial-scales of the KSMHs near the subsolar magnetosheath are smaller than those in the flanks.Furthermore,their global distribution shows a significant dawn-dusk asymmetry(duskside dominating)in the magnetotail.

Kinetic scale magnetic holes in the terrestrial magnetosheath:A review

Magnetic holes at the ion-to-electron kinetic scale(KSMHs)are one of the extremely small intermittent structures generated in turbulent magnetized plasmas.In recent years,the explorations of KSMHs have made substantial strides,driven by the ultra-high-precision observational data gathered from the Magnetospheric Multiscale(MMS)mission.This review paper summarizes the up-to-date characteristics of the KSMHs observed in Earth’s turbulent magnetosheath,as well as their potential impacts on space plasma.This review starts by introducing the fundamental properties of the KSMHs,including observational features,particle behaviors,scales,geometries,and distributions in terrestrial space.Researchers have discovered that KSMHs display a quasi-circular electron vortex-like structure attributed to electron diamagnetic drift.These electrons exhibit noticeable non-gyrotropy and undergo acceleration.The occurrence rate of KSMH in the Earth’s magnetosheath is significantly greater than in the solar wind and magnetotail,suggesting the turbulent magnetosheath is a primary source region.Additionally,KSMHs have also been generated in turbulence simulations and successfully reproduced by the kinetic equilibrium models.Furthermore,KSMHs have demonstrated their ability to accelerate electrons by a novel non-adiabatic electron acceleration mechanism,serve as an additional avenue for energy dissipation during magnetic reconnection,and generate diverse wave phenomena,including whistler waves,electrostatic solitary waves,and electron cyclotron waves in space plasma.These results highlight the magnetic hole’s impact such as wave-particle interaction,energy cascade/dissipation,and particle acceleration/heating in space plasma.We end this paper by summarizing these discoveries,discussing the generation mechanism,similar structures,and observations in the Earth’s magnetotail and solar wind,and presenting a future extension perspective in this active field.