Modulation of Very Low Frequency Whistler Waves by Ultra Low Frequency Waves
This dissertation focuses on the study of the modulation of very low frequency (VLF) whistler mode waves in the Earth’s magnetosphere by ultra low frequency (ULF) waves. First, I provide an in-situ observation of chorus wave modulated by ULF waves deep in the inner magnetosphere. The observed ULF wave can modulate the distribution of both protons and electrons and amplify the intensity of chorus waves. Then I build a two-dimensional selfconsistent magnetic field (SCB) model to analyze the eigenmode of ULF field line resonance (FLR) with the effect of the anisotropic ring current pressure included. The results show that the eigenfrequency is reduced at the negative radial pressure gradient while increases at the positive pressure gradient. The compressional component of FLR magnetic field perturbation can be found in both the positive and negative gradient regions of the pressure and enhanced by larger plasma β and smaller anisotropy. Using about 2 years’ observations of three THEMIS satellites and over 5.5 years of observations of two Van Allen Probes satellites, I perform a statistical study of the chorus wave modulation events. The results indicate that in most of the modulation events, the intensity of chorus wave correlates to the background magnetic field negatively and plasma density positively. The intensity of chorus wave strongly depends on the amplitude of the background magnetic field perturbation but weakly depends on the amplitude of plasma density perturbation. Besides the work on VLF whistler mode waves modulated by ULF waves, I also perform two other relevant studies. The first one is using the two-dimensional self-consistent magnetic field (SCB) model to study the effects of localized thermal pressure on the magnetic field configuration and the formation of magnetic dip structure. The modeling results demonstrate that the magnetic perturbation increases with increasing plasma β and decreasing width of pressure distribution. The formation of magnetic dip requires a critical β value that increases with increasing width of pressure distribution and decreasing L shell. The other study is using the observations of DEMETER satellite to investigate propagation characteristics of low altitude ionospheric hiss. The ionospheric hiss can propagate from the high latitude regions to the equator within a waveguide near the region of cutoff frequency and plasma density peak, which results in the narrow frequency banded spectrum of ionospheric hiss waves with the central frequency around the local proton cyclotron frequency. The power of ionospheric hiss is stronger on the dayside than the nightside, under higher geomagnetic activity, in local summer and confined near the region where the local proton cyclotron frequency is equal to the wave frequency.