Dr. Ge Wang, IFS, UT Austin

"Onset of Whistler Chorus in the Magnetosphere"

Abstract: Whistler chorus waves are discrete very low frequency (VLF) waves that propagate in the Earth’s magnetosphere, which are usually excited during magnetic substorm periods by plasma-sheet energetic electrons injected to the inner magnetosphere near the magnetic equator. Fine coherent structures are observed as the chorus frequency chirps as both a rising or falling tone during the injection of the anisotropic plasma sheet electrons. We developed a model to study dispersive whistler waves in the magnetosphere, propagating in multiple modes with a narrow spread in spectrum bandwidth. Without any perturbation, three adiabatic invariants in dipole magnetic field exist as constants of motion. With a perturbation, these adiabatic invariants will evolve in response to the chorus. However, a single particle Hamiltonian in our approximation uncovers a slow and fast scale separation along the dipole magnetic field, where only one canonical action depends on the fast scale and needs to be updated during the nonlinear evolution. The scale separation enables a self-consistent full Earth-scale simulation of chorus waves in the magnetosphere. In our simulation, a hole in phase space arises because the magnetic mirror force acts as a drag on the cyclotron-resonant electrons, and forces phase space motion on the phase space structures that have developed. A phase space hole propagates along the magnetic field. Once it approaches the magnetic equator, a strong rising tone is triggered with an amplifying amplitude. The excited rising tone chorus propagates away from the equator. Eventually the hole is found to fade away in phase space when it crosses the equator. The strong correlation between the rising tone and phase space hole during the onset of the chorus can be further investigated using the satellite data.