Xu-Zhi Zhou

Tail Reconnection Triggering Substorm Onset

Magnetospheric substorms explosively release solar wind energy previously stored in Earths magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at ∼10 RE (RE: Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at ∼20 to 30 RE. We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 RE, at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

Magnetic flux transport by dipolarizing flux bundles

A dipolarizing flux bundle (DFB) is a small magnetotail flux tube (typically  65% of BBF flux transport, even though they last only ~30% as long as BBFs. The rate of DFB flux transport increases with proximity to Earth and to the premidnight sector, as well as with geomagnetic activity and distance from the neutral sheet. Under the latter two conditions, the total flux transport by a typical DFB also increases. Dipolarizing flux bundles appear more often during increased geomagnetic activity. Since BBFs have been previously shown to be the major flux transporters in the tail, we conclude that DFBs are the dominant drivers of this transport. The occurrence rate of DFBs as a function of location and geomagnetic activity informs us about processes that shape global convection and energy conversion.

On the origin of pressure and magnetic perturbations ahead of dipolarization fronts

Dipolarization fronts (DFs), earthward-propagating structures in the Earths magnetotail current sheet with sharp enhancements of the northward magnetic field Bz, are typically preceded by minor decreases in Bz. Other characteristic DF precursor signatures, including earthward flows and plasma density/pressure enhancements, have been explained in the context of ion acceleration and reflection at dipolarization fronts. In the same context here we simulate the spatial distribution of plasma pressure earthward of a convex DF. The resultant pressure distribution, which shows clear dawn-dusk asymmetries with greater enhancements at the DF duskside, agrees with statistical observations. The simulation further reveals that the reflected ions can carry a secondary current earthward of the advancing DF, which explains the characteristic signature of the Bz dip immediately ahead of the DF.

Substorm current wedge composition by wedgelets

Understanding how a substorm current wedge (SCW) is formed is crucial to comprehending the substorm phenomenon. One SCW formation scenario suggests that the substorm time magnetosphere is coupled to the ionosphere via “wedgelets,” small building blocks of an SCW. Wedgelets are field-aligned currents (FACs) carried by elemental flux transport units known as dipolarizing flux bundles (DFBs). A DFB is a magnetotail flux tube with magnetic field stronger than that of the ambient plasma. Its leading edge, known as a “dipolarization front” or “reconnection front,” is a product of near-Earth reconnection. Dipolarizing flux bundles, and thus wedgelets, are localized—each is only <3 RE wide. How these localized wedgelets combine to become large-scale (several hours of magnetic local time) region-1-sense SCW FACs is unclear. To determine how this occurs, we investigated wedgelets statistically using Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. The results show wedgelet asymmetries: in the dawn (dusk) sector of the magnetotail, a wedgelet has more FAC toward (away from) the Earth than away from (toward) the Earth, so the net FAC is toward (away from) the Earth. The combined effect of many wedgelets is therefore the same as that of large-scale region-1-sense SCW, supporting the idea that they comprise the SCW.

Asymmetric braking and dawnward deflection of dipolarization fronts: Effects of ion reflection

Dipolarization fronts (DFs), earthward propagating structures in the Earths magnetotail with sharp enhancements of the northward magnetic field, can reflect and accelerate ions in the ambient plasma sheet. The ion reflection and acceleration process, which generates earthward flows ahead of the DF, also imposes a dynamic pressure on the DF to decelerate its earthward motion. It has been shown that the ion reflection process is not symmetric, with stronger ion accelerations at the evening side of the DF than at its morning side, which implies dawn-dusk asymmetric reaction of the ambient plasma and consequently dawnward deflection of DFs. In this paper, we examine this scenario in detail, by carrying out statistical studies based on Time History of Events and Macroscale Interactions during Substorms observations from 2008 to 2011. We demonstrate the important role of the ion reflection process in the longstanding problems regarding DF evolution and bursty flow braking in the near-Earth plasma sheet.

On the generation of magnetic dips ahead of advancing dipolarization fronts

Dipolarizing flux bundles transport magnetic flux to the inner and dayside magnetosphere, heat the plasma sheet, and provide a seed population to the radiation belt. The magnetic perturbation ahead of them, often referred to as a dipolarization front (DF), is asymmetric with a small Bz dip followed by a sharp Bz enhancement. The Bz dip is thought to be generated from dawnward currents carried by DF-reflected ions; after reflection, these earthward moving ions gyrate clockwise and contribute to dawnward diamagnetic currents ahead of the front. Using observations of hundreds of DFs, we investigate this hypothesis. We find that the depth of the Bz dip as a function of the front azimuth depends on DF propagation speed and ambient plasma density. These statistical signatures support the hypothesis that the Bz dip is caused by ion reflection and suggest that secondary currents carried by these reflected ions can reshape the front significantly.

Comparison of observed and model magnetic fields at high altitudes above the polar cap: POLAR initial results

Data obtained from the high altitude, polar orbiting spacecraft, POLAR, are compared with the latest version of the data-based magnetospheric magnetic field model. The data generally agree well with the model. The major directional discrepancies at high altitudes are near the dayside cusp and on the “open” field lines over the polar cap, especially close to the boundary of the polar cap. Near the cusp, the agreement is improved if a stronger solar wind dynamic pressure and more negative IMF By and Bz are used as the model input parameters, than was actually observed. The field measured in the vicinity of the polar cusps is generally weaker than predicted by the model. Close to noon the spacecraft enters a region of additional structured field depression that appears to be the polar cusp proper. Within the limited statistics presented here, the invariant latitude of the cusp appears to be controlled by the north-south component of the IMF and the broad depression appears to be controlled by the tilt of the dipole.

Cross-tail expansion of dipolarizing flux bundles

Dipolarizing flux bundles (DFBs), magnetotail flux tubes with stronger, more dipolar magnetic field than the background magnetotail plasma, which are led by dipolarization fronts, typically propagate earthward within bursty bulk flows. Although DFBs are localized (typically 1–3 RE) in the cross-tail direction, many of them may combine to cause global effects, such as the substorm current wedge. Knowledge of how this local-to-global coupling happens is crucial for understanding magnetotail dynamics. We investigate how the coupling happens using multipoint observations from the Time History of Events and Macroscale Interactions during Substorms mission. Our results show that most DFBs expand in the cross-tail direction as they propagate earthward. Through such expansion, azimuthally localized DFBs can cover a wide region to cause global effects when they arrive at the inner edge of the plasma sheet.

Charged Particle Behavior in Localized Ultralow Frequency Waves: Theory and Observations

The formation and variability of the Van Allen radiation belts are highly influenced by charged particles accelerated via drift-resonant interactions with ultralow frequency (ULF) waves. In the prevailing theory of drift resonance, the ULF wave amplitude is assumed independent of magnetic longitude. This assumption is not generally valid in Earths magnetosphere, as supported by numerous observations that point to the localized nature of ULF waves. Here, we introduce a longitude dependence of the ULF wave amplitude, achieved via a von Mises function, into the theoretical framework of ULF wave-particle drift resonance. To validate the revised theory, the predicted particle signatures are compared with observational data through a best-fit procedure. It is demonstrated that incorporation of non-local effects in drift-resonance theory provides an improved understanding of charged particle behavior in the inner magnetosphere through the intermediary of ULF waves.

Radial propagation of magnetospheric substorm-injected energetic electrons observed using a BD-IES instrument and Van Allen Probes

In cases where substorm injections can be observed simultaneously by multiple spacecraft, they can help elucidate the potential mechanisms of particle transport and energization, of great importance to understanding and modeling the magnetosphere. In this paper, using data returned from the BeiDa-IES (BD-IES) instrument onboard a satellite in an inclined (55°) geosynchronous orbit (IGSO), in combination with two geo-transfer orbiting (GTO) satellite Van Allen Probes (A and B), we analyze a substorm injection event that occurred on the 16th of October 2015. During this substorm injection, the IGSO onboard BD-IES was outbound, while both Van Allen Probe satellites (A and B) were inbound, a configuration of multiple trajectories that provides a unique opportunity to simultaneously investigate both the inward and outward radial propagation of substorm injection. Indicated by AE/AL indices, this substorm was closely related to an IMF/solar wind discontinuity that showed a sharp change in IMF Bz direction to the north. The innermost signature of this substorm injection was detected by Van Allen Probes A and B at L-3.7, while the outermost signature was observed by the onboard BD-IES instrument at L~10. These data indicate that the substorm had a global, rather than just local, effect. Finally, we suggest that electric fields carried by fast-mode compressional waves around the substorm injection are the most likely candidate mechanism for the electron injection signatures observed in the inner- and outermost inner magnetosphere.