G. Q. Wang

Nonlinear Landau resonant scattering of near equatorially mirroring radiation belt electrons by oblique EMIC waves

In response to solar wind disturbances, radiation belt (a few hundreds of keV to several MeV) electron fluxes can be depleted significantly over the entire equatorial pitch angle range. The frequently mentioned cyclotron resonant scattering is applicable only for electrons mirroring off the equator. Here we propose a new physical mechanism, nonlinear Landau resonance with oblique electromagnetic ion cyclotron (EMIC) waves, to effectively scatter the near equatorially mirroring electrons. Our test particle simulations show that the nonlinear Landau trapping can occur over a wide energy range and yield the net decrease in equatorial pitch angle Δαeq≈10° within several seconds. Our parametric studies further reveal that this nonlinear Landau-trapping process is favored by a low plasma density, an intense wave field, a high wave frequency close to ion gyrofrequencies, and a large wave normal angle.

A statistical analysis of Pi2‐band waves in the plasma sheet and their relation to magnetospheric drivers

We use the Cluster data from 2001 to 2009 to investigate the occurrence of Pi2-band waves in the plasma sheet. To study the generation mechanisms of these waves, we examine the association between Pi2-band waves and dynamic processes in the plasma sheet (fast flows and substorm activity) and the direction of the solar wind velocity. For a total of 80 large-amplitude Pi2-band waves in the plasma sheet, we find that Cluster records fast flows during 62 events, 11 waves without fast flows occur during substorm time, 3 events occur when the solar wind velocity significantly changes its direction, and 4 events are not associated with any of the above activities. Most of the observed Pi2-band waves are predominantly compressional, while 2 events are transverse. Based on this statistical study, we suggest that fast flows maybe the main driver of Pi2-band waves/oscillations in the plasma sheet, especially considering that most of these waves are compressional. The relatively small number of other events indicates that other mechanisms also play a role in creating Pi2-band waves/oscillations in the plasma sheet but are relatively rare. In all wave events of this study, the plasma pressure and magnetic pressure vary in antiphase, suggesting that these waves have the slow-mode feature.

Flapping current sheet with superposed waves seen in space and on the ground

A wavy current sheet event observed on 15 October 2004 between 1235 and 1300 UT has been studied by using Cluster and ground-based magnetometer data. Waves propagating from the tail center to the duskside flank with a period ~30 s and wavelength ~1 RE are superimposed on a flapping current sheet, accompanied with a bursty bulk flow. Three Pi2 pulsations, with onset at ~1236, ~1251, and ~1255 UT, respectively, are observed at the Tixie station located near the foot points of Cluster. The mechanism creating the Pi2 (period ~40 s) onset at ~1236 UT is unclear. The second Pi2 (period ~90 s, onset at ~1251 UT) is associated with a strong field-aligned current, which has a strong transverse component of the magnetic field, observed by Cluster with a time delay ~60 s. We suggest that it is caused by bouncing Alfven waves between the northern and southern ionosphere which transport the field-aligned current. For the third Pi2 (period ~60 s) there is almost no damping at the first three periods. They occur in conjunction with periodic field-aligned currents one-on-one with 72 s delay. We suggest that it is generated by these periodic field-aligned currents. We conclude that the strong field-aligned currents generated in the plasma sheet during flapping with superimposed higher-frequency waves can drive Pi2 pulsations on the ground, and periodic field-aligned currents can even control the period of the Pi2s.

Spatial distribution of magnetic fluctuation power with period 40 to 600 s in the magnetosphere observed by THEMIS

Ultralow frequency (ULF) fluctuations are ubiquitous in the magnetosphere and have significant influence on the energetic particle transport. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) data to give the spatial distribution of the Pi2/Pc4 and Pc5 band magnetic fluctuation amplitude near the magnetic equator in the magnetosphere. Statistical results can be summarized as follows: (1) strong ULF fluctuations are common in the magnetotail plasma sheet; the amplitude of all three components of magnetic fluctuations decreases with decreasing radial distance; (2) during periods of high AE index, fluctuations can propagate toward the Earth as far as the data cutoff in the nightside of the magnetosphere, and the amplitude of magnetic fluctuations is clearly stronger near the dusk sector of the synchronous orbit than that near the dawn sector, suggesting that the substorm particle injection has significant contribution to these fluctuations; (3) intense compressional Pc5 band magnetic fluctuations are a persistent feature near two flanks of the magnetosphere. Clear peaks of the compressional Pi2/Pc4 band magnetic fluctuation power near two flanks can be found during periods of fast solar wind, while the power of compressional Pi2/Pc4 band fluctuations is weak when the solar wind is slow. (4) Solar wind dynamic pressure and its variations can globally affect the ULF fluctuation power in the magnetosphere. Magnetic fluctuations near the noonside can penetrate from the magnetopause to the synchronous orbit or inner when solar wind pressure variations are large.

Nonlinear fundamental and harmonic cyclotron resonant scattering of radiation belt ultra-relativistic electrons by oblique monochromatic EMIC waves†

Cyclotron resonant scattering by electromagnetic ion cyclotron (EMIC) waves has been considered to be responsible for the rapid loss of radiation belt high-energy electrons. For parallel-propagating EMIC waves, the nonlinear character of cyclotron resonance has been revealed in recent studies. Here we present the first study on the nonlinear fundamental and harmonic cyclotron resonant scattering of radiation belt ultra-relativistic electrons by oblique EMIC waves on the basis of test-particle simulations. Higher wave obliquity produces stronger nonlinearity of harmonic resonances but weaker nonlinearity of fundamental resonance. Compared to the quasi-linear prediction, these nonlinear resonances yield a more rapid loss of electrons over a wider pitch-angle range. In the quasi-linear regime, the ultra-relativistic electrons are lost in the equatorial pitch-angle range αeq 87.5∘ at ψ = 20∘ and 40∘. At the resonant pitch-angles αeq<75∘, the difference between quasi-linear and nonlinear loss timescales tends to decrease with the wave normal angle increasing. At ψ = 0∘ and 20∘, the nonlinear electron loss timescale is 10% shorter than the quasi-linear prediction; at ψ = 40∘, the difference in loss timescales is reduced to <5%.

Statistical study on ultralow‐frequency waves in the magnetotail lobe observed by Cluster

Based on Cluster data, we investigate 263 waves with periods between 40 and 150 s (Pi2 band events) and 161 waves with periods between 150 and 600 s (Pc5 band events) in the magnetotail lobe. Our findings are as follows: (1) 90% of the mean wave amplitudes within 40–150 s (150–600 s) are below ~0.25 (0.36) nT for the transverse components and ~0.16 (0.39) nT for the compressional components; (2) 69.6% (35.4%) of the compressional ratios of the waves with periods 40–150 s (150–600 s) are less than 0.5 with the maximum occurrence at ~0.3 (0.8); (3) waves within 40–150 s are more likely to occur in the lobe region close to the plasma sheet; (4) the wave amplitudes and the AE index are weakly correlated; however, the amplitudes tend to be larger when the AE index is larger; and (5) the amplitudes also tend to be larger when the solar wind velocity and the solar wind dynamic pressure or its variations (∆PSW) are larger; the correlation coefficient between the wave amplitudes with periods between 150 and 600 s and ∆PSW is up to ~0.58. We suggest that both dynamic processes in the plasma sheet boundary layer or plasma sheet (inner sources) and solar wind conditions (outer sources) can contribute to the generation of the lobe ULF waves; waves within 40–150 s are affected more by inner sources, while strong ∆PSW can drive compressional waves within 150–600 s in the magnetotail lobe.