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Inertial Alfven Waves as a Possible Driver for Auroral Kilometric Radiation

Jose Mauricio Blanco Benavides

2019

The inner magnetosphere hosts a variety of different plasma environments. The transition from one region to the next extends over considerable lengths where adjacent plasmas merge gradually. Plasma waves play an important role in coupling these regions by facilitating particle and energy flows necessary to maintain or restore a state of dynamic equilibrium. Most types of plasma waves undergo dispersion as they move through regions with changing properties, affecting energetic particle
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... particle populations in the process. In this thesis, the effects of Dispersive Alfvén Waves (DAW) on electron plasmas are investigated for different scenarios using numerical simulations. To this end, the Drift Kinetic (DK1D) Vlasov solver [Watt et al., 2004] has been extended to include inhomogeneous background plasma conditions while preserving self-consistency between field and particles. A density model has been added which consists of a mixture of two plasmas: an ionospheric contribution composed of singly ionized oxygen which decays quickly with altitude, and a magnetospheric hydrogen plasma that is assumed spatially uniform. The resulting density variation gives rise to a realistic temperature profile along geomagnetic field lines. The occurrence of regular and inverse suprathermal electron energy dispersion reported by Cameron [2015] is addressed using a simpler version of the code valid for uniform plasmas. Regular energy dispersion is divided between cases with a single suprathermal component and those accompanied by a locally enhanced thermal population. Simulations reveal that the first kind of signatures form primarily under conditions of low wave phase speed and strong wave dispersion, ultimately producing electron acceleration to energies significantly higher than that predicted by Fermi-like interactions. Regular energy dispersion, on the other hand, shows evidence of an enhanced thermal population at larger wave phase speeds. The occurrence of high energy electron dispersion over Fermi-like electron energy dispersion is favored by a decrease of perpendicular iii wavelength and an increase of the plasma temperature and wave amplitude. Recent observations of inverse electron energy dispersion by the Canadian ePOP micro-satellite are explained as being due to the relative motion of the satellite and the source of wave emission. It is demonstrated that, for a source moving in the cross-plane of the background magnetic field and emitting Alfvén waves parallel to the field, inverse electron energy dispersion will be observed by a satellite whose trajectory is also in the cross-plane of the background field. The DK1D code is also used to determine the efficiency of electron trapping by Shear Alfvén Waves (SAW) in the magnetosphere. This process is shown to be limited by Landau damping at short perpendicular wavelengths. For the range of parameters considered, simulations reveal that waves do not survive to reach the inertial region. This strong influence of particle trapping and self-consistent Landau damping is an indication of possible over-or under-estimates of the energy gain of accelerated electrons in studies that disregard self-consistent wave-particle interactions. Lastly, the efficiency of the Electron Cyclotron Instability (ECI) resulting from fieldaligned electron acceleration by inertial Alfvén waves within and above the Ionospheric Alfvén Resonator (IAR) is investigated. Since the motion of these accelerated electrons preserves their magnetic moment the mirror force induces the formation of unstable horseshoe distributions. Electron distributions from simulation data are fitted to an analytical representation that enables the convective length associated with wave amplification of Auroral Kilometric Radiation (AKR) to be calculated. Simulation results show that AKR generation is most efficient where the ratio ω pe /ω ce is a minimum, and exclusively for electron number densities ≤ 10 5 cm −3 , in accordance with observations. Enhanced efficiency of AKR generation can be obtained by increasing both the background plasma temperature and the perpendicular wavelength. At altitudes above the IAR, the interference of reflected and incident waves coincides with a sudden termination of the conditions for AKR amplification. iv Preface Appendix C of this thesis has been published as A. V. Artemyev, R. Rankin, and M. Blanco, "Electron trapping and acceleration by kinetic Alfvén waves in the inner magnetosphere. I was responsible for deriving the two-fluid equations in the appendix of the paper and the expression for the effective potential in terms of the scalar potential through substitution of the vector potential from the wave equation. These equations indicate that electrons escape the wave at an altitude which depends on wave parameters, which motivated the analysis presented in the publication. v To Segundo y María Ester.

doi:10.7939/r3-mf59-pv59
fatcat:w2hs5t5qd5hmthcasexdyupvpy