5:15 PM - 6:30 PM
[PEM09-P12] Properties of dispersive Alfven waves and their roles in electron acceleration process in the terrestrial magnetosphere based on Plasma Distribution Solver
Keywords:Terrestrial magnetosphere, Alfven waves, Alfvenic acceleration, dispersive Alfven waves, electron acceleration mechanism, numerical experiment
Observations from the FAST satellite revealed the downward electrons in the energy range from tens of eV to a few keV in the terrestrial high latitude region [Chaston et al., 2002]. It is suggested that “Alfvenic acceleration” process by dispersive Alfven waves is responsible for the auroral electron acceleration. Alfvenic acceleration has also attracted attention as the electron acceleration mechanism in Jupiter [e.g., Mauk et al., 2017; Saur et al., 2018], and the importance of Alfvenic acceleration in the auroral acceleration process in magnetized planets is increasing. While Alfvenic acceleration occurring in the terrestrial environment has been studied for decades [e.g., Chaston et al., 2002; Watt et al., 2006], physical processes controlling the characteristic energy and pitch angle distributions have been still unclear.
For the study of the efficiency and energy dependence of Alfvenic acceleration, understanding of the characteristics of dispersive Alfven waves is essential. In the present study, we quantitatively examine the characteristics of dispersive Alfven waves and their spatial variation in the magnetosphere by using originally developed “Plasma Distribution Solver”, which calculates the plasma and pressure distributions along a magnetic field line. We use results from three different conditions; Case 1 (L=4, Ne_eq=240 cm-3), Case 2 (L=4, Ne_eq=24 cm-3), and Case 3 (L=15, Ne_eq=12 cm-3), where Ne_eq is the electron number density at the magnetic equator, and the background magnetic field is assumed to be a dipole field. First, the ratio of the plasma pressure to the magnetic pressure (plasma β) is calculated to determine whether kinetic or inertial Alfven wave dispersion relation should be used at each position along a magnetic field line. As a result, the characteristics of kinetic Alfven waves appears in the Case 1 in the region below 20 degrees of the magnetic latitude, and the kinetic Alfven wave region extends up to about 30 and 50 degrees of magnetic latitude in Cases 2 and 3, respectively.
Next, with the aim of considering the effect of Alfvenic acceleration on high-energy electrons, we investigate the relationship between the Larmor radius of energetic electrons and the perpendicular wavelength of dispersive Alfven waves, referring to the discussion in Chaston et al. (2004). Chaston et al. studied the relationship between the wavelength of the dispersive Alfven wave and the spatial scale of the cyclotron motion of ions, and showed that stochastic acceleration can occur when the wavelength perpendicular to the background magnetic field is smaller than the ion Larmor radius. In Case 1, the Larmor radius of electrons at 100 keV is larger than that thermal ions at the magnetic equator, and the effect of kinetic Alfven waves is expected to be significant. On the other hand, the Larmor radius of 100 keV electrons is smaller than that of thermal ions, suggests limited effect of kinetic Alfven waves in Case 2. In Case 3, the effect of kinetic/inertial Alfven waves is expected for 100 keV electrons in the region of 20-60 degrees magnetic latitude. In this presentation, we also discuss the energy dependence of the electron acceleration process with the results obtained by the Plasma Distribution Solver.
For the study of the efficiency and energy dependence of Alfvenic acceleration, understanding of the characteristics of dispersive Alfven waves is essential. In the present study, we quantitatively examine the characteristics of dispersive Alfven waves and their spatial variation in the magnetosphere by using originally developed “Plasma Distribution Solver”, which calculates the plasma and pressure distributions along a magnetic field line. We use results from three different conditions; Case 1 (L=4, Ne_eq=240 cm-3), Case 2 (L=4, Ne_eq=24 cm-3), and Case 3 (L=15, Ne_eq=12 cm-3), where Ne_eq is the electron number density at the magnetic equator, and the background magnetic field is assumed to be a dipole field. First, the ratio of the plasma pressure to the magnetic pressure (plasma β) is calculated to determine whether kinetic or inertial Alfven wave dispersion relation should be used at each position along a magnetic field line. As a result, the characteristics of kinetic Alfven waves appears in the Case 1 in the region below 20 degrees of the magnetic latitude, and the kinetic Alfven wave region extends up to about 30 and 50 degrees of magnetic latitude in Cases 2 and 3, respectively.
Next, with the aim of considering the effect of Alfvenic acceleration on high-energy electrons, we investigate the relationship between the Larmor radius of energetic electrons and the perpendicular wavelength of dispersive Alfven waves, referring to the discussion in Chaston et al. (2004). Chaston et al. studied the relationship between the wavelength of the dispersive Alfven wave and the spatial scale of the cyclotron motion of ions, and showed that stochastic acceleration can occur when the wavelength perpendicular to the background magnetic field is smaller than the ion Larmor radius. In Case 1, the Larmor radius of electrons at 100 keV is larger than that thermal ions at the magnetic equator, and the effect of kinetic Alfven waves is expected to be significant. On the other hand, the Larmor radius of 100 keV electrons is smaller than that of thermal ions, suggests limited effect of kinetic Alfven waves in Case 2. In Case 3, the effect of kinetic/inertial Alfven waves is expected for 100 keV electrons in the region of 20-60 degrees magnetic latitude. In this presentation, we also discuss the energy dependence of the electron acceleration process with the results obtained by the Plasma Distribution Solver.