Propagation and Excitation of Equatorial Electrostatic and Electromagnetic Emissions In the Magnetosphere

Abstract

The plasma density is an important parameter for affecting the distribution of magnetospheric waves. This dissertation addresses the effects of plasma density on the propagation of magnetosonic (MS) waves and generation and distribution of electron cyclotron harmonic (ECH) waves. These two waves are important for the dynamics of energetic electrons in the Earth’s magnetosphere. The plasma density effect on the radial propagation of MS waves is evaluated by the FiniteDifference Time-Domain (FDTD) method. We find that the radially propagating MS waves can propagate down to ionospheric altitude with a smoothly varying plasma density profile if no damping mechanism is considered. The fine-scale density structures near the outer edge of plasmapause can effectively block MS wave propagation, and therefore, such a structured plasmapause can serve as a terminating boundary of MS waves, which was often shown by Van Allen Probes observation. We extend the linear growth rate formula of the electromagnetic modes in previously assumed non-relativistic regimes to the relativistic regime. We also derive a general electrostatic wave linear growth rate solver for a realistic and arbitrary plasma distribution function and apply this to examine the instability of ECH waves. The ECH wave growth rate increases with loss-cone size, parallel temperature of hot electrons, cold electron temperature and electron density, and decreases with hot electron perpendicular temperature. Such a linear instability solver can be readily applied to simulation and observation. We, for the first time, simulate the global ECH wave evolution during a geomagnetic storm using a Ring current-Atmosphere interactions Model with Self-Consistent Magnetic field (RAM-SCB) combined with our linear growth rate solver. Our simulation shows that the ECH wave instability becomes more intensive with stronger geomagnetic activity or during the main phase of geomagnetic storms. ECH wave instability is much stronger at nightside and dawnside, compared to that at dayside and duskside. The unstable region of ECH waves extends to larger MLT and lower L shell regions as geomagnetic activity increase or geomagnetic storms evolve to the main phase, and the inner boundary of ECH wave instability is traced well by plasmapause location. Finally, we investigate the relation between ECH waves and the plasmapause using Van Allen Probes observation. Two categories of ECH waves are shown by their different behaviors near the plasmasphere boundary layer (PBL, i.e., plasmapause). Category I ECH waves are terminated at the outer boundary of PBL because of the rapid suppression of ECH wave instability due to a dramatic increase of cold plasma density and a dramatic decrease of hot electron flux across the PBL. Category II ECH waves, limited at nightside and dawnside, can be excited across the PBL. Lower harmonic bands can be excited further inside the plasmasphere. Such Category II ECH waves occur when the hot electron flux penetrates across the PBL/plasmasphere and cold plasma density increases gradually. Statistically, the wave power of Category II ECH waves is much more intense than that of Category I, and Category II ECH waves are accompanied by more injected energetic electrons overlapping with the cold dense electrons in the PBL.