3:30 PM - 3:45 PM
[PEM17-07] Theory and simulation of the auroral electron acceleration processes by kinetic Alfvén waves
Keywords:dispersive Alfvén waves, kinetic Alfvén waves, electron acceleration process, wave-particle interaction, Landau resonance
Kinetic Alfvén waves (KAW) are electromagnetic waves characterized by long wavelengths parallel to magnetic field lines and perpendicular wavelengths comparable to the ion Larmor radius. KAWs have a parallel electric field component (δE||) which accelerates electrons and ions along magnetic field lines [e.g., Hasegawa, 1976]. KAWs are often observed in the terrestrial magnetosphere during substorms [e.g., Stasiewicz et al., 2000], and accelerate electrons parallel to magnetic field lines to energies of several keV, thereby enhancing auroral brightness [e.g., Duan et al., 2016; Keiling et al., 2002]. In the process of electron acceleration by KAW, electrons satisfying the Landau resonance condition between the parallel velocity of electrons (v||) and the wave phase speed (Vph||) can be trapped by the wave and transported to higher latitudes while being accelerated [e.g., Artemyev et al., 2015; Damiano et al., 2016]. Additionally, it has been pointed out that trapped electrons with a small first adiabatic invariant (μ) can reach the ionosphere without much influence from the mirror force [Watt and Rankin, 2009]. However, there are still unresolved questions regarding the details of the process, such as the conditions for electron trapping and the criteria for electrons to reach the ionosphere.
In this study, we apply the theory of the second-order resonance of charged particles trapped by coherent electromagnetic waves [e.g., Omura et al., 2008] to the electron acceleration process by KAWs. We can describe the motion of electrons trapped by KAWs in the velocity phase space by considering a simple harmonic motion of the wave phase as viewed from the electron (ψ) and the inhomogeneity factor (S) due to the background magnetic field gradients. As trapped electrons move parallel to magnetic field lines, the background magnetic field gradient and S as viewed from the electron change accordingly. The range of energies at which electrons can be trapped narrows with increasing S so that it becomes easier for electrons to escape from the trapping region as they move to higher latitudes with larger magnetic field gradients. From theoretical calculations we find that electrons can be trapped by KAWs up to approximately 36° of magnetic latitude on the L=9 magnetic field line in the terrestrial magnetosphere, assuming δE|| is approximately 1 mV/m at the magnetic equator. We perform test particle simulations and confirm that while v|| of trapped electrons remain at around Vph||, the energy of electrons significantly changes after they escape from the trapping region. We focus our analysis on the motion of electrons that can reach the ionosphere after being detrapped from the wave. In the results, we find that electrons, which are detrapped between 10° and 20° of magnetic latitude with energies ranging from a few hundred eV to 2 keV, are accelerated to up to approximately 10.5 keV at the ionosphere while located at ψ ∈ (-π,0) where the wave accelerates the electron. Considering the maximum kinetic energy at the ionosphere and μ conservation through the electron acceleration, it is necessary for the electron to reach the ionosphere that μ is less than approximately 0.23 eV/nT. In addition to the above theoretical considerations and results, we discuss the region where electrons are efficiently accelerated and the amount of energy gained through the electron acceleration process.
In this study, we apply the theory of the second-order resonance of charged particles trapped by coherent electromagnetic waves [e.g., Omura et al., 2008] to the electron acceleration process by KAWs. We can describe the motion of electrons trapped by KAWs in the velocity phase space by considering a simple harmonic motion of the wave phase as viewed from the electron (ψ) and the inhomogeneity factor (S) due to the background magnetic field gradients. As trapped electrons move parallel to magnetic field lines, the background magnetic field gradient and S as viewed from the electron change accordingly. The range of energies at which electrons can be trapped narrows with increasing S so that it becomes easier for electrons to escape from the trapping region as they move to higher latitudes with larger magnetic field gradients. From theoretical calculations we find that electrons can be trapped by KAWs up to approximately 36° of magnetic latitude on the L=9 magnetic field line in the terrestrial magnetosphere, assuming δE|| is approximately 1 mV/m at the magnetic equator. We perform test particle simulations and confirm that while v|| of trapped electrons remain at around Vph||, the energy of electrons significantly changes after they escape from the trapping region. We focus our analysis on the motion of electrons that can reach the ionosphere after being detrapped from the wave. In the results, we find that electrons, which are detrapped between 10° and 20° of magnetic latitude with energies ranging from a few hundred eV to 2 keV, are accelerated to up to approximately 10.5 keV at the ionosphere while located at ψ ∈ (-π,0) where the wave accelerates the electron. Considering the maximum kinetic energy at the ionosphere and μ conservation through the electron acceleration, it is necessary for the electron to reach the ionosphere that μ is less than approximately 0.23 eV/nT. In addition to the above theoretical considerations and results, we discuss the region where electrons are efficiently accelerated and the amount of energy gained through the electron acceleration process.