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[PEM17-P08] Role of magnetic field component of kinetic Alfvén waves in Landau resonant interaction with electrons in the magnetosphere
Keywords:dispersive Alfvén waves, kinetic Alfvén waves, electron acceleration process, terrestrial magnetosphere, Landau resonance, test particle simulation
Kinetic Alfvén wave (KAW) is a type of dispersive Alfvén wave with a long wavelength parallel to the magnetic field line and a perpendicular wavelength comparable to the ion Larmor radius. KAWs have an electric field parallel to the magnetic field line δE|| and accelerate electrons and ions along the magnetic field line through Landau resonance [e.g., Hasegawa and Chen, 1975; Kletzing, 1994; Artemyev et al., 2015]. KAWs are often observed in the terrestrial magnetosphere during substorms [e.g., Stasiewicz et al., 2000]. In particular, it has been pointed out that a few keV electrons produced by KAWs cause the auroral brightening during the substorm expansion phase [e.g., Hasegawa, 1976; Chaston et al., 2003; Duan et al., 2016]. The electron acceleration by KAWs has also attracted attention as an electron acceleration process in the Jovian magnetosphere [e.g., Mauk et al., 2017; Saur et al., 2018]. While the importance of the electron acceleration process by KAWs in magnetized planets is increasing, there are still unresolved questions regarding the details of the electron acceleration process by KAWs, such as the conditions determining the efficiency of the electron acceleration.
Although δE|| is known to play an important role in the electron acceleration by KAWs, the contribution of δB||, the magnetic field component parallel to the magnetic field, is not fully investigated. At the equatorial region in the L-shell value equal to 9, where the electron acceleration by KAWs occurs, the plasma β, on which δB|| depends, is mi/me < β < 1, δB|| becomes significant and is estimated to be approximately 8% of the background magnetic field. In this study, we perform test particle simulations for the electron acceleration by KAWs in the L-shell of 9 for the case where only δE|| is considered and the case where both δE|| and δB|| are considered, focusing on the contribution of each wave component to the electron acceleration. Here we focus on the change in the energy of an electron. The calculations were performed for electrons with an initial position at the magnetic equator, an initial energy of 600 eV, and an initial pitch angle of 85 degrees. Depending on the initial profile of the wave phase, the energy increase through the Landau resonance is different when only δE|| is considered and when δE|| and δB|| are considered. For example, while the heating is up to 4 keV when only δE|| is considered, 4.6 keV when δE|| and δB|| are considered. It indicates that the heating with δB|| is about 600 eV greater than without δB||. On the other hand, under the different initial wave phase settings, there is a case with only δE|| where the electron energy reaches 4.6 keV. In contrast, in the case with δE|| and δB||, the electron energy increased by only 2.7 keV. This difference is explained by the effect of the mirror force due to δB||, suggesting that the effect of δB|| should be considered in the electron acceleration process of KAWs in the large L-shell where mi/me ≪ β. In addition to the above results, we report analysis results on the time variation of forces, the wave phase as seen by the electron, and the electron distribution function, and discuss the contribution of δB|| to the Landau resonance between KAW and electron.
Although δE|| is known to play an important role in the electron acceleration by KAWs, the contribution of δB||, the magnetic field component parallel to the magnetic field, is not fully investigated. At the equatorial region in the L-shell value equal to 9, where the electron acceleration by KAWs occurs, the plasma β, on which δB|| depends, is mi/me < β < 1, δB|| becomes significant and is estimated to be approximately 8% of the background magnetic field. In this study, we perform test particle simulations for the electron acceleration by KAWs in the L-shell of 9 for the case where only δE|| is considered and the case where both δE|| and δB|| are considered, focusing on the contribution of each wave component to the electron acceleration. Here we focus on the change in the energy of an electron. The calculations were performed for electrons with an initial position at the magnetic equator, an initial energy of 600 eV, and an initial pitch angle of 85 degrees. Depending on the initial profile of the wave phase, the energy increase through the Landau resonance is different when only δE|| is considered and when δE|| and δB|| are considered. For example, while the heating is up to 4 keV when only δE|| is considered, 4.6 keV when δE|| and δB|| are considered. It indicates that the heating with δB|| is about 600 eV greater than without δB||. On the other hand, under the different initial wave phase settings, there is a case with only δE|| where the electron energy reaches 4.6 keV. In contrast, in the case with δE|| and δB||, the electron energy increased by only 2.7 keV. This difference is explained by the effect of the mirror force due to δB||, suggesting that the effect of δB|| should be considered in the electron acceleration process of KAWs in the large L-shell where mi/me ≪ β. In addition to the above results, we report analysis results on the time variation of forces, the wave phase as seen by the electron, and the electron distribution function, and discuss the contribution of δB|| to the Landau resonance between KAW and electron.