11:30 AM - 11:45 AM
[PEM17-10] Implementation of Kappa Distribution Functions in the δf Gyrokinetic Code GKV
Keywords:aurora, gyrokinetic theory, plasma physics
We have developed a gyrokinetic model of the magnetospheric-ionosphere (M-I) coupling system by extending a gyrokinetic code GKV [1,2]. GKV is a δf code, which solves a set of gyrokinetic equations for the fluctuating part of the distribution function while keeping the background distribution fixed [3]. The simulation model is based on the assumption that the background distribution is a Maxwellian. In this study, we further extend the code by implementing the option to select a kappa distribution function as the background, allowing simulations of superthermal space plasmas.
We first examine the effects of the presence of superthermal particles on the dispersion relations of dispersive Alfvén waves obtained from the gyrokinetic set of equations. It turns out that the effects are negligible for small wavenumber modes, while the damping rates are modified at kinetic scales from Maxwellian cases. This suggests that the linear instabilities with small characteristic wavenumbers, such as the feedback instability [4,5] remain practically unaffected, whereas energy transfer at kinetic scales can be appreciably altered.
As an application of the kappa distribution model, we then simulate the nonlinear acceleration of auroral electrons by dispersive Alfvén waves driven by the feedback instability in the M-I coupling system. The resulting differential energy fluxes are compared with Maxwellian counterpart cases and observation data. Finally, based on the results, we discuss limitations of the current model and perspective on reproducing characteristic velocity distribution structures of auroral electrons associated with the Alfvénic aurora.
[1] T.-H. Watanabe, “A unified model of auroral arc growth and electron acceleration in the magnetosphere-ionosphere coupling”. Geophys. Res. Lett. 41, 6071 (2014).
[2] K. Fujita and T.-H. Watanabe, “Spontaneous generation of parallel electric field and auroral growth in a nonlinear gyrokinetic M-I coupling model”. 45th COSPAR Scientific Assembly. (2024).
[3] T.-H. Watanabe and H. Sugama, “Velocity–space structures of distribution function in toroidal ion temperature gradient turbulence”. Nucl. Fusion 46(1), 24 (2005).
[4] T. Sato, “A theory of quiet auroral arcs”. J. Geophys. Res. 83, 1042 (1978).
[5] T.-H. Watanabe, “Feedback instability in the magnetosphere-ionosphere coupling system: Revisited”. Phys. Plasmas 17, 022904 (2010).
We first examine the effects of the presence of superthermal particles on the dispersion relations of dispersive Alfvén waves obtained from the gyrokinetic set of equations. It turns out that the effects are negligible for small wavenumber modes, while the damping rates are modified at kinetic scales from Maxwellian cases. This suggests that the linear instabilities with small characteristic wavenumbers, such as the feedback instability [4,5] remain practically unaffected, whereas energy transfer at kinetic scales can be appreciably altered.
As an application of the kappa distribution model, we then simulate the nonlinear acceleration of auroral electrons by dispersive Alfvén waves driven by the feedback instability in the M-I coupling system. The resulting differential energy fluxes are compared with Maxwellian counterpart cases and observation data. Finally, based on the results, we discuss limitations of the current model and perspective on reproducing characteristic velocity distribution structures of auroral electrons associated with the Alfvénic aurora.
[1] T.-H. Watanabe, “A unified model of auroral arc growth and electron acceleration in the magnetosphere-ionosphere coupling”. Geophys. Res. Lett. 41, 6071 (2014).
[2] K. Fujita and T.-H. Watanabe, “Spontaneous generation of parallel electric field and auroral growth in a nonlinear gyrokinetic M-I coupling model”. 45th COSPAR Scientific Assembly. (2024).
[3] T.-H. Watanabe and H. Sugama, “Velocity–space structures of distribution function in toroidal ion temperature gradient turbulence”. Nucl. Fusion 46(1), 24 (2005).
[4] T. Sato, “A theory of quiet auroral arcs”. J. Geophys. Res. 83, 1042 (1978).
[5] T.-H. Watanabe, “Feedback instability in the magnetosphere-ionosphere coupling system: Revisited”. Phys. Plasmas 17, 022904 (2010).