Sawtooth and dune auroras simultaneously driven by waves around the plasmapause

The dune aurora,at a scale of~30 kilometers,was reported recently using ground camera.The small-scale dune aurora occurs on the duskside and exhibits a monochromatic oscillation in the auroral emission,implying fundamental energy conversions.However,whether the dune auroras correspond to atmospheric waves or are associated with magnetospheric dynamics should be determined.This paper reports a dune aurora that occurred during a storm;further,we demonstrate that it was the substructure of the sawtooth aurora that was generated by plasmapause surface waves.Conjugate observations in the magnetospheric source region suggest that the exohiss waves,which are periodically modulated by the plasmapause surface wave-excited ultralow frequency wave,might be responsible for the generation of the dune aurora.Most reported dune aurora events have occurred simultaneously with sawtooth auroras,suggesting that both are plasmapause-driven cross-scale auroral structures.

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.

Recent advances in the magnetic reconnection,dipolarization,and auroral processes at giant planets from the perspective of comparative planetology

Magnetic reconnection and dipolarization are crucial processes driving magnetospheric dynamics,including particle energization,mass circulation,and auroral processes,among others.Recent studies have revealed that these processes at Saturn and Jupiter are fundamentally different from the ones at Earth.The reconnection and dipolarization processes are far more important than previously expected in the dayside magnetodisc of Saturn and potentially Jupiter.Dayside magnetodisc reconnection was directly identified by using Cassini measurements(Guo RL et al.,2018b)and was found to be drizzle-like and rotating in the magnetosphere of Saturn(Delamere et al.,2015b;Yao ZH et al.,2017a;Guo RL et al.,2019).Moreover,magnetic dipolarization could also exist at Saturn’s dayside(Yao ZH et al.,2018),which is fundamentally different from the terrestrial situation.These new results significantly improve our understanding of giant planetary magnetospheric dynamics and provide key insights revealing the physics of planetary aurorae.Here,we briefly review these recent advances and their potential implications for future investigations.

Evolution of clustered magnetic nulls in a turbulent-like reconnection region in the magnetotail

t Magnetic null points and flux ropes play important roles in the three-dimensional process of magnetic reconnection. In this study, a cluster of null points are reconstructed in the reconnection region in the magnetotail by applying a fitting-reconstruction method to measurements from the Cluster mission. The number of recon- structed null points varies rapidly, presenting a turbulentlike evolution of the magnetic structure. The electron density and the flux of the accelerated electrons were enhanced in this turbulent-like region. During this unstable reconnection process, a B-As-B null structure was formed, showing flux rope features and resembling a secondary island in the observation.