Dong-Xiao Pan

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.

Understanding the ion distributions near the boundaries of reconnection outflow region

An intresting signature observed shortly after the onset of magnetotail reconnection is the gradual appearance of a local peak of ion phase space density (PSD) in the duskward and downstream direction separated from the colder, nearly-isotropic ion population. Such a characteristic ion distribution, served as a diagnostic signature of magnetotail reconnection and well reproduced by a particle-tracing Liouville simulation, are found to appear only near the off-equatorial boundaries of the reconnection outflow region. Further analysis on ion trajectories suggests that the ions within the local peak and within the neighboring PSD cleft both belong to the outflowing population; on top of their outflowing motion, they both meander across the neutral sheet to exhibit duskward velocities near the off-equatorial edges of their trajectories. The difference between them is that the local peak originates from ions previously constituting the pre-onset plasma sheet, whereas the cleft corresponds to the inflowing lobe ions before they are repelled in the downstream direction. As reconnection proceeds, the local PSD peak gradually attenuates and then disappears, which is a signature of reconnection flushing effect that depletes the ions in the pre-onset plasma sheet and eventually replaces them by lobe ions.

Electron flat-top distributions and cross-scale wave modulations observed in the current sheet of geomagnetic tail

We present new observations of electron distributions and the accompanying waves during the current sheet activities at ∼60 RE in the geomagnetic tail detected by the ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moons Interaction with the Sun) spacecraft. We find that electron flat-top distribution is a common feature near the neutral sheet of the tailward flowing plasmas, consistent with the electron distributions that are shaped in the reconnection region. Whistler mode waves are generated by the anisotropic electron temperature associated with the electron flat-top distributions. These whistler mode waves are modulated by low frequency ion scale waves that are possibly excited by the high-energy ions injected during the current sheet instability. The magnetic and electric fields of the ion scale waves are in phase with electron density variations, indicating that they are compressional ion cyclotron waves. Our observations present examples of the dynamical processes occu...

Characteristics of high latitude precursor flows ahead of dipolarization fronts

Dipolarization fronts (DFs), earthward-propagating structures in the magnetotail current sheet characterized by sharp enhancements of northward magnetic field, are capable of converting electromagnetic energy into particle kinetic energy. The ions previously accelerated and reflected at the DFs can contribute to plasma flows ahead of the fronts, which have been identified as DF precursor flows in both the near-equatorial plasma sheet and far from it, near the plasma sheet boundary. Using THEMIS (Time History of Events and Macroscale Interactions during Substorms) observations, we show that the earthward particle and energy flux enhancements ahead of DFs are statistically larger further away from the neutral sheet (at high latitudes) than in the near-equatorial region. High-latitude particle and energy fluxes on the DF dawnside are found to be significantly greater than those on the duskside, which is opposite to the dawn-dusk asymmetries previously found near the equatorial region. Using forward- and backward- tracing test-particle simulations, we then explain and reproduce the observed latitude-dependent characteristics of DF precursor flows, providing a better understanding of ion dynamics associated with dipolarization fronts.

THEMIS statistical study on the plasma properties of high-speed flows in Earth’s magnetotail

Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations from 2007 to 2011 tail seasons, we study the plasma properties of high speed flows (HSFs) and background plasma sheet events (BPSs) in Earth’s magnetotail (|YGSM|<13RE, |ZGSM|<5RE,–30RE<XGSM<–6RE), and their correlations with solar wind parameters. Statistical results show that the closer the HSFs and BPSs are to the Earth, the hotter they become, and the temperature increase of HSFs is larger than that of BPSs. The density and temperature ratios between HSFs and BPSs are also larger when events are closer to Earth. We also find that the best correlations between the HSFs (BPSs) density and the solar wind density occur when the solar wind density is averaged 2 (3.5) hours prior to the onset of HSFs (BPSs). The normalized densities of both HSFs and BPSs are correlated with the interplanetary magnetic field (IMF) θ angles

Ion acceleration and reflection on magnetotail antidipolarization fronts

\left( {\theta = \arctan \left( {{B_z}/\sqrt {B_x^2 + B_y^2} } \right)} \right.

On the generation of magnetic dips ahead of advancing dipolarization fronts: STATISTIC CHARACTERISTICS OF DIP REGION

which are averaged 3 hours before the observation time. Further analysis indicates that both HSFs and BPSs become denser during the northward IMF period.