Qiugang Zong

Interactions between magnetosonic waves and radiation belt electrons: Comparisons of quasi‐linear calculations with test particle simulations

Quasi-linear theory (QLT) has been commonly used to study the Landau resonant interaction between radiation belt electrons and magnetosonic (MS) waves. However, the long-parallel wavelengths of MS waves can exceed their narrow spatial confinement and cause a transit-time effect during interactions with electrons. We perform a careful investigation to validate the applicability of QLT to interactions between MS waves, which have a distribution in frequency and wave normal angle, and radiation belt electrons using test particle simulations. We show agreement between these two methods for scattering rate of intense MS waves at L = 4.5 inside the plasmapause, but find a significant inconsistency for MS waves outside the plasmapause, due to the broad transit-time region in (Ek,α) space. Consequently, we introduce a particle-independent criterion to justify the validity of QLT for MS waves: the wave spatial confinement should be longer than two parallel wavelengths.

Pitch-angle distribution evolution of energetic electrons in the inner radiation belt and slot region during the 2003 Halloween storm

This injection of energetic electrons into the slot region may be associated with the plasmapause movement and Hiss/Chorus enhancement. This flux enhancement is possibly associated with convective transport from the plasma sheet, enhanced radial diffusion and local wave-particle interaction acceleration. By adopting a fitting parameter of loss time tL we solved the bounce-averaged pitch angle diffusion equation driven by field-aligned whistler-mode waves (including chorus and hiss). We show that pitch-angle scattering can account for the pitch-angle distribution evolution in 30–500 keVelectrons in the innermost radiation belt near L = 1.7 (as observed by Polar satellite) and the slot region 2 < L <3 . Inparticular,simulatedresultsindicatethattheloss-coneregionisalmostempty,andoutside the loss-cone region both flux and anisotropy of energetic electrons are reduced with the gyroresonant time. The obtained time scale for the pitch-angle distribution evolution is found to be approximately tens of hours, consistent with observation.

The transition to overshielding after sharp and gradual interplanetary magnetic field northward turning

Overshielding is referred to a shielding status, during which the dawnward shielding electric field dominates over the duskward penetration electric field in the inner magnetosphere, typically appearing when the interplanetary magnetic field (IMF) suddenly turns northward after a prolonged southward orientation. It is expected that the transition to overshielding after IMF northward turning can be affected by the shape of northward turning (sharp or gradual). Moreover, the initial shielding status (undershielding or goodshielding) prior to the transition may also have influence on the transition. Here we analyze two groups of cases, in which the transitions appear after sharp (duration less than 5 min) and gradual (duration more than 30 min) northward turning. Each group includes two cases, in which the transition initiated from undershielding and goodshielding. These cases show that (1) the beginning of the transition to overshielding coincides with sharp IMF northward turning but appears in the midst of gradual IMF northward turning; (2) the transition from goodshielding to overshielding is always associated with convection electric field drop and/or polar cap shrinkage, regardless of the shape of IMF northward turning; and (3) the typical solar wind condition in which the IMF suddenly turns northward after a prolonged southward orientation is neither a necessary condition nor a sufficient condition for overshielding. Furthermore, we will discuss the effect of substorm processes on overshielding.

Electric fields associated with dipolarization fronts

Electric fields associated with dipolarization fronts (DFs) have been investigated in the magnetotail plasma sheet using Cluster observations. We have studied each term in the generalized Ohms law using data obtained from the multispacecraft Cluster. Our results show that in the plasma flow frame, electric fields are directed normal to the DF in the magnetic dip region ahead of the DF as well as in the DF layer but in opposite directions. Case and statistical studies show that the Hall electric field is important while the electron pressure gradient term is much smaller. The ions decouple from the magnetic field in the DF layer and dip region (E + Vi×B ≠ 0), whereas electrons remain frozen-in (E + Ve×B=∇pe/nee).

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.

EVIDENCE OF LANDAU AND CYCLOTRON RESONANCE BETWEEN PROTONS AND KINETIC WAVES IN SOLAR WIND TURBULENCE

The wave–particle interaction processes occurring in the solar wind provide crucial information to understand the wave dissipation and simultaneous particle heating in plasma turbulence. One requires observations of both wave fluctuations and particle kinetics near the dissipation range, which have, however, not yet been analyzed simultaneously. Here we show new evidence of wave–particle interactions by combining the diagnosis of wave modes with the analysis of particle kinetics on the basis of measurements from the WIND spacecraft with a high cadence of about 3 s. Solar wind protons appear to be highly dynamic in their velocity distribution consisting of varying anisotropic core and beam components. The basic scenario of solar wind proton heating through wave–particle interaction is suggested to be the following. Left-handed cyclotron resonance occurs continuously, and is evident from the observed proton core velocity distribution and the concurrent quasi-parallel left-handed Alfven cyclotron waves. Landau and right-handed cyclotron resonances are persistent and indicated by the observed drifting anisotropic beam and the simultaneous quasi-perpendicular right-handed kinetic Alfven waves in a general sense. The persistence of non-gyrotropic proton distributions may cast new light on the nature of the interaction between particles and waves near and beyond the proton gyro-frequency.

Current reduction in a pseudo‐breakup event: THEMIS observations

Pseudo-breakup events are thought to be generated by the same physical processes as substorms. This paper reports on the cross-tail current reduction in an isolated pseudo-breakup observed by three of the THEMIS probes (THEMIS A (THA), THEMIS D (THD), and THEMIS E (THE)) on 22 March 2010. During this pseudo-breakup, several localized auroral intensifications were seen by ground-based observatories. Using the unique spatial configuration of the three THEMIS probes, we have estimated the inertial and diamagnetic currents in the near-Earth plasma sheet associated with flow braking and diversion. We found the diamagnetic current to be the major contributor to the current reduction in this pseudo-breakup event. During flow braking, the plasma pressure was reinforced, and a weak electrojet and an auroral intensification appeared. After flow braking/diversion, the electrojet was enhanced, and a new auroral intensification was seen. The peak current intensity of the electrojet estimated from ground-based magnetometers, ~0.7 × 105 A, was about 1 order of magnitude lower than that in a typical substorm. We suggest that this pseudo-breakup event involved two dynamical processes: a current-reduction associated with plasma compression ahead of the earthward flow and a current-disruption related to the flow braking/diversion. Both processes are closely connected to the fundamental interaction between fast flows, the near-Earth ambient plasma, and the magnetic field.

Generation of proton aurora by magnetosonic waves.

Earths proton aurora occurs over a broad MLT region and is produced by the precipitation of low-energy (2–10 keV) plasmasheet protons. Proton precipitation can alter chemical compositions of the atmosphere, linking solar activity with global climate variability. Previous studies proposed that electromagnetic ion cyclotron waves can resonate with protons, producing proton scattering precipitation. A long-outstanding question still remains whether there is another mechanism responsible for the proton aurora. Here, by performing satellite data analysis and diffusion equation calculations, we show that fast magnetosonic waves can produce trapped proton scattering that yields proton aurora. This provides a new insight into the mechanism of proton aurora. Furthermore, a ray-tracing study demonstrates that magnetosonic wave propagates over a broad MLT region, consistent with the global distribution of proton aurora.

Determining the mechanism of cusp proton aurora

Earths cusp proton aurora occurs near the prenoon and is primarily produced by the precipitation of solar energetic (2–10 keV) protons. Cusp auroral precipitation provides a direct source of energy for the high-latitude dayside upper atmosphere, contributing to chemical composition change and global climate variability. Previous studies have indicated that magnetic reconnection allows solar energetic protons to cross the magnetopause and enter the cusp region, producing cusp auroral precipitation. However, energetic protons are easily trapped in the cusp region due to a minimum magnetic field existing there. Hence, the mechanism of cusp proton aurora has remained a significant challenge for tens of years. Based on the satellite data and calculations of diffusion equation, we demonstrate that EMIC waves can yield the trapped proton scattering that causes cusp proton aurora. This moves forward a step toward identifying the generation mechanism of cusp proton aurora.

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.