The semiannual variation of transpolar arc incidence and its relationship to the Russell–McPherron effect

Earth’s aurora is a luminescent phenomenon generated by the interaction between magnetospheric precipitating particles and the upper atmosphere;it plays an important role in magnetosphere–ionosphere(M-I)coupling.The transpolar arc(TPA)is a discrete auroral arc distributed in the noon-midnight direction poleward of the auroral oval and connects the dayside to the nightside sectors of the auroral oval.Studying the seasonal variation of TPA events can help us better understand the long-term variation of the interaction between the solar wind,the magnetosphere,and M-I coupling.However,a statistical study of the seasonal variation of TPA incidence has not previously been carried out.In this paper,we have identified 532 TPA events from the IMAGE database(2000–2005)and the Polar database(1996–2002),and calculated the incidence of TPA events for different months.We find a semiannual variation in TPA incidence.Clear peaks in the incidence of TPAs occur in March and September;a less pronounced peak appears in November.We also examine seasonal variation in the northward interplanetary magnetic field(IMF)over the same time period.The intensity and occurrence rate of the northward IMF exhibit patterns similar to that of the TPA incidence.Having studied IMF Bz before TPA onset,we find that strong and steady northward IMF conditions are favorable for TPA formation.We suggest that the semiannual variation observed in TPA incidence may be related to the Russell–McPherron(R-M)effect due to the projection effect of the IMF By under northward IMF conditions.

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.

Poleward-moving recurrent auroral arcs associated with impulse-excited standing hydromagnetic waves

In Earth's high-latitude ionosphere, the poleward motion of east–west elongated auroral arcs has been attributed to standing hydromagnetic waves, especially when the auroral arcs appear quasi-periodically with a recurrence time of a few minutes. The validation of this scenario requires spacecraft observations of ultra-low-frequency hydromagnetic waves in the magnetosphere and simultaneous observations of poleward-moving auroral arcs near the spacecraft footprints. Here we present the first observational evidence from the multi-spacecraft THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission and the conjugated all-sky imager to support the scenario that standing hydromagnetic waves can generate the quasi-periodic appearance of poleward-moving auroral arcs. In this specific event, the observed waves were toroidal branches of the standing hydromagnetic waves, which were excited by a pulse in the solar wind dynamic pressure. Multi-spacecraft measurements from THEMIS also suggest higher wave frequencies at lower L shells (consistent with the distribution of magnetic field line eigenfrequencies), which indicates that the phase difference across latitudes would increase with time. As time proceeds, the enlarged phase difference corresponds to a lower propagation speed of the auroral arcs, which agrees very well with the ground-based optical data.

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.

月球表面及空间环境对太阳风与地球风的响应

月球空间作为日-地-月空间的重要组成部分,是研究基础空间物理以及等离子体与类月天体(无大气且无全球磁场)相互作用的天然实验室,也是影响月球探测任务实施的重要环境因素。面向月球科研站等月球探测任务的国家重大战略需求,亟需聚焦月球空间环境对太阳风与地球风的响应,其中不仅包括常规监测月球空间天气(包括监测太阳风、地球风、能量带电粒子和中子辐射及波动环境、月尘和外逸层等)以保障人类探月登月的空间环境安全,也包括研究各种空间粒子与月表物质和电磁场的相互作用。可利用多卫星观测、遥感监测、月表就地探测、数据分析、数值模拟等手段发现新的物理现象或揭示新的物理机制,探寻太阳系中不同天体空间物质循环过程与辐射环境的差异及其产生原因,从而拓展我们对地球和行星宜居性的认识,并为月球科研站等月球后续任务的实施提供参考。

THEMIS observation of a magnetotail current sheet flapping wave

A flapping wave was observed by THEMIS-B(P1)and THEMIS-C(P2)probes on the dawn side of the magnetotail,while the solar wind was generally stable.The magnetic activity was quite weak,suggesting that this flapping wave was generated by an internal instability,which normally occurs during magnetic quiet times.Our analysis shows that the flapping wave was propagating downward with a tail-aligned scale of at least 3.7 R E and did not show much change in shape during its propagation from P1 to P2.Correlation analysis employed to estimate the time lag between the corresponding half waveforms of P1 and P2 shows that the propagating velocities along the current sheet normal directions were close to each other in the beginning,but increased linearly later on.The average wavelength of the flapping wave is approximately 4 R E.Theoretical analysis suggests that the ballooning type wave model may not be the mechanism for the observed flapping wave,but that the magnetic double-gradient instability model is a more plausible candidate.

Braking of high-speed flows in the magnetotail:THEMIS joint observations

The motion and deceleration processes of plasma sheet high-speed flows have great significance to magnetospheric particle acceleration,magnetic field perturbation,magnetic flux transport,triggering of substorm,and the current system formation in the magnetotail.From February to April 2009,two satellites of the Time History of Events and Macroscale Interactions during Substorms mission,THA and THE,were often separated largely in Z direction,but had small X and Y separations.Such special configuration allows simultaneous observations of highspeed flows at the center and boundary of the plasma sheet.Based on selected case study and statistical analysis,it is found that for about 89%of the events we selected,the probe further away from the neutral sheet observed the high-speed flow earlier than the one close to the center,and the flow is mainly field aligned.And for about 95%events the probe further away from the neutral sheet observed higher X component of the plasma flow.With the hypothesis that parallel flow keeps the same speed during its earthward propagation while central plasma sheet stream uniformly or suddenly brakes on its way to the earth,we deduced the position where the deceleration begins to be between 13 Re and 17 Re downtail,where thenear-earth reconnection is supposed to occur.In addition,our statistical results show that dipolarization fronts observed in the central plasma sheet are more prominent than those observed in the plasma sheet boundary layer ahead of the high-speed flow.