Wave-particle Interactions In Solar Wind Heating And Evolution Of Earth Radiation Belt

The wave-particle interactions have always been an important subject in the space plasma research. In this paper, we mainly focus on their roles in the solar wind heating and the evolution of the Van Allen radiation belt. Using one-and two-dimensional hybrid simulation models, we have studied the ion/ion instabilities and the parametric instabilities of Alfven waves in the fast solar wind. Moreover, large amounts of data, which are provided by the THEMIS, GOES, and POES satellites and OMNIWEB database, have been employed to analyze the bandwidths and coherence coefficients of chorus waves, investigate the generation mechanisms of chorus waves, and study the effect of solar winds upon relativistic electron flux dropouts. The principal results are summarized as follows.1. The ion/ion instabilities in the fast solar windTwo-dimensional hybrid simulations are performed to investigate the nonlinear evolution of the electromagnetic alpha/proton instability. We further find that the obliquely propagating Alfven waves excited by the instability have nearly linear polarization. The background proton component, as well as the alpha component, can be resonantly heated by the oblique Alfven waves.We also employ two-dimensional hybrid simulations to study the proton/proton instability. The obliquely propagating Alfven waves are found to be unstable to the instability. At first, the Alfven waves have a nearly linear polarization, and both the ambient protons and minor ions O6+can be resonantly heated. The heating is primarily in the direction perpendicular to the background magnetic field. With the evolution of the instability, the obliquely propagating Alfven waves gradually become left-hand polarized, and then cannot resonantly heat the ambient protons or minor ions. The effects of the plasma beta and temperature anisotropy of the ambient protons on the evolution of the instability are also considered.2. The parametric instabilities of Alfven waves in the fast solar windTwo-dimensional hybrid simulations are used to study the parametric decay of a monochromatic Alfven wave in low beta plasma. Both the linearly and left-hand polarized pump Alfven waves are considered. For the linearly polarized pump Alfven wave, either a parallel or oblique propagating wave can lead to the decay along the perpendicular direction. Initially, the parametric decay takes place along the propagating direction of the pump wave, and then the decay occurs in the perpendicular direction. With the increase of the amplitude and the propagating angle of the pump wave, the spectral range of the excited waves becomes broad in the perpendicular direction. But the effects of the plasma beta on the spectral range of the excited waves in perpendicular direction are negligible. However, for the left-hand polarized pump Alfven wave, when the pump wave propagates along the ambient magnetic field, the parametric decay occurs nearly along the ambient magnetic field, and there is no obvious decay in the perpendicular direction. Significant decay in the perpendicular direction can only be found when the pump wave propagates obliquely.The parametric decay of a circularly polarized Alfven wave in a proton-electron-alpha plasma system is investigated with one-dimensional hybrid simulations. In cases without alpha particles, with the increase of the wave number of the pump Alfven wave, the growth rate of the decay instability increases and the saturation amplitude of the density fluctuations slightly decreases. However, when alpha particles with a sufficiently large bulk velocity along the ambient magnetic field are included, at a definite range of the wave numbers of the pump wave, both the growth rate and the saturation amplitude of the parametric decay become much smaller and the parametric decay is heavily suppressed. At these wave numbers, the resonant condition between the alpha particles and the daughter Alfven waves is satisfied, therefore, their resonant interactions might play an important role in the suppression of the parametric decay instability.With a one-dimensional hybrid simulation model, we study the evolution of the parametric instabilities of a monochromatic left-hand polarized Alfven wave in a proton-electron-alpha plasma with a low beta. When the drift velocity between the protons and alpha particles is sufficiently large, the wave numbers of the backward daughter Alfven waves can be cascaded toward higher values due to the modulational instability during the nonlinear evolution of the parametric instabilities, and the alpha particles are resonantly heated in both the parallel and perpendicular direction by the backward waves. On the other hand, when the drift velocity of alpha particles is small, the alpha particles are heated in the linear growth stage of the parametric instabilities due to the Landau resonance with the excited in acoustic waves. Therefore, the heating occurs only in the parallel direction, and there is no obvious hearing in the perpendicular direction.3. Statistical results of whistler mode waves in the magnetosphereThe bandwidths and coherence coefficients of lower band whistler mode waves are analyzed using Time History of Events and Macroscale Interactions during Substorms (THEMIS) waveform data for rising tones, falling tones, and hiss-like emissions separately. We also evaluate their dependences on the spatial location, electron density, the ratio of plasma frequency to local electron gyrofrequency (fpe/fce), and the wave amplitude. Our results show that the bandwidth normalized by the local electron gyrofrequency of rising and falling tones is very narrow (～0.01fce), smaller than that of the hiss-like emissions (～0.025fce). Meanwhile, the normalized bandwidth of discrete emissions gradually decreases with increasing wave amplitude, whereas that of hiss-like emissions increases slowly. The coherence coefficient of rising and falling tones is extremely large (～1), while the coherence coefficient of hiss-like emissions is smaller but is still larger than0.5. For all categories of whistler mode waves, the normalized bandwidth increases at larger L shells. Furthermore, the normalized bandwidth is positively correlated with local fpe/fce but is inversely correlated with the electron density. Interactions between radiation belt electrons and whistler mode waves have been widely described by quasi-linear diffusion theory. Our results suggest that although quasi-linear theory is not entirely applicable for modeling electron interactions with rising and falling tones due to their narrow bandwidth and high coherence coefficient, it is suitable to treat wave-particle interactions between electrons and low-amplitude hiss-like emissions.Linear theory suggests that whistler mode wave growth rates are proportional to the ratio of hot electron (～1to30keV) density to total electron density (Nh/Nt), whereas nonlinear wave theory suggests that an optimum linear growth rate is required to generate rising tone chorus from hiss-like emissions. Using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) waveform data collected by three probes over the past～5years, we investigate the correlation between Nh/Nt and wave amplitude/wave occurrence rate for rising tone, falling tone, and hiss-like emissions separately. Statistical results show that the rising and falling tones preferentially occur in the region with a limited Nh/Nt range, whereas both the occurrence rate and wave amplitudes of hiss-like emissions become larger for higher values of Nh/Nt. Our statistical results not only provide an important clue on the generation mechanism of hiss-like emissions, but also provide supporting experimental evidence for the nonlinear theory of generating rising tone chorus.4. The effect of solar wind parameters upon the relativistic electron flux dropoutsSuperposed epoch analyses were performed on193significant relativistic electron flux dropout events to study the roles of different solar wind parameters in driving the depletion of relativistic electrons, using-16years of data from the POES and GOES missions, and the OMNIWEB solar wind database. We find that the solar wind dynamic pressure and IMF Bz play key roles in causing the relativistic electron flux dropouts, but either large solar wind dynamic pressure or strong southward IMF Bz alone is also found to be capable of resulting in the significant depletion of relativistic electrons. The relativistic electron flux dropouts occur not only when the magnetopause is compressed closer to the Earth, but also when the magnetopause located very (>～10Re). Importantly, our results show that in addition to the large solar wind dynamic pressure, which pushes the magnetopause inward strongly and causes the electrons to escape from the magnetopause, relativistic electrons can also be scattered into the loss cone and precipitate into the Earth’s atmosphere during periods of strong southward IMF Bz, which preferentially provides a source of free energy for electromagnetic ion cyclotron (EMIC) wave excitation. This is supported by the fact that the strongest electron precipitation into the atmosphere is found in the dusk sector, where EMIC waves are typically observed in the high-density plasmasphere or plume and cause efficient electron precipitation down to～1MeV.