Inertial Alfven Waves as a Possible Driver for Auroral Kilometric Radiation

• Author / Creator

  Blanco Benavides, Jose Mauricio

• 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 movethrough regions with changing properties, affecting energetic 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 includeinhomogeneous 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 singlesuprathermal component and those accompanied by a locally enhanced thermal population. Simulations reveal that the first kind of signatures form primarily under conditionsof 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 wavelength and an increase of the plasma temperature and wave amplitude. Recent observations of inverse electron energy dispersion by the Canadian ePOP microsatellite 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 limitedby 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 field-aligned 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.

• Subjects / Keywords

  • Inertial Alfven waves
  • Inner magnetosphere
  • Kinetic Alfven Waves
  • electron acceleration
  • Auroral Kilometric Radiation

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-mf59-pv59

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.