Physics and Astronomy PhD Thesis Defense - Shuo Wu, Dartmouth College

Title: "Whistler Chorus Waves: Linear Theory and Nonlinear Simulations in Dipole Geometry"

July 31, 2015
10 am - 12 pm
Location
Wilder 202
Sponsored by
Physics & Astronomy Department
Audience
Public
More information
Tressena Manning
603-646-2854

Abstract: Whistler chorus waves have recently drawn tremendous attention as an important mechanism in controlling the energetic electron flux in Earth's radiation belt. This dissertation aims to answer questions on whistler chorus waves, such as "What is the effect of cold plasma density on linear whistler instability? How do whistler mode chorus waves evolve in a meridional plane? How would chorus waves occur if the magnetosphere is compressed?"

First, we derive the real dispersion relation and linear growth rate of whistler mode in mixed hot and cold plasma. We find that there is a peak in the temporal and convective growth rates varying cold plasma density. We model the relation between the linear growth rate and various plasma parameters and use this model to explain the observed modulation of chorus intensity by cold plasma density.

Second, we simulate the nonlinear growth of whistler chorus waves in a dipole field using a hybrid code. The hybrid code uses the particle-in-cell technique in generalized orthogonal coordinates. A small fraction of electrons are treated as particles with anisotropic temperature that leads to the whistler instability. Other electrons are treated as a cold fluid without mass. The validity of our model is confirmed by using both our hybrid code and a full dynamics particle in cell code. Our 1-D simulations along the dipole field line reproduce chorus generation in agreement with observations and past studies. We find that it is easier to simulate temporal frequency variation in a scaled down system with greater magnetic field inhomogeneity. Our 2-D simulations reveal features of chorus propagation in a meridional plane and the effects of background plasma density on that propagation. These are the first first-principles 2-D self-consistent nonlinear simulations of whistler waves in Earth's dipole field. Our preliminary simulation in a 1-D compressed dipole field reproduces the generation of off-equator chorus that have been observed on the dayside. It is the first attempt to self-consistently simulate the nonlinear growth of chorus when the magnetosphere is compressed by the solar wind.

 
Location
Wilder 202
Sponsored by
Physics & Astronomy Department
Audience
Public
More information
Tressena Manning
603-646-2854