2:00 PM - 2:15 PM
[PEM10-12] Nonlinear gyrokinetic simulation of auroral growth in the M-I coupling system
Keywords:aurora, gyrokinetic theory, plasma physics
Numerous theoretical models of the magnetosphere-ionosphere (M-I) coupling system have been developed to study auroral phenomena. The feedback instability in the M-I coupling system has been discussed as a plausible mechanism that can simultaneously explain the formations of local current systems and auroral structure growth.
Since the original works in the 1970s [1,2], the feedback instability has been investigated with various models. In Refs. [3,4] the secondary growth of the Kelvin-Helmholtz instability in the M-I coupling system is investigated by nonlinear simulations using reduced magnetohydrodynamic (MHD) models. However, the MHD model cannot account for the parallel electric field, which is crucial for auroral particle acceleration along magnetic field lines. To simulate the generation of the parallel electric field self-consistently, we need to extend the M-I coupling model to include kinetic effects so that Alfven waves coupling the magnetosphere and ionosphere induce the parallel electric field.
In the present study, we have developed a nonlinear gyrokinetic simulation model for the magnetosphere by extending a gyrokinetic simulation code GKV [5]. Nonlinear simulations using the newly developed model have reproduced similar evolution of the M-I coupling system to the previous results from the reduced MHD model, where on finds the primary growth of the feedback instability and the subsequent transition to turbulence due to the secondary growth of the Kelvin-Helmholtz instability. The left panel in Figure 1 below shows the emergence of Kelvin-Helmholtz vortices in the density profile at the top of the model magnetosphere (magnetic equator). While the vortices break into smaller structures, the Alfven waves with strong irregularities propagate down to the ionosphere and produce turbulent structures of auroral vortices.
In addition, we have found effective energy transfer from the electromagnetic fields to electrons through wave-particle interactions with the parallel electric field. The right panel in Figure 1 represents the time history of the entropy balance in the magnetosphere. The yellow curve shows the rate of energy exchange from the Alfven waves to electrons and ions integrated along the field line in the magnetosphere.
In the presentation, we will also discuss the spatio-tempotal structures of the parallel electric field and the velocity distribution function.
Since the original works in the 1970s [1,2], the feedback instability has been investigated with various models. In Refs. [3,4] the secondary growth of the Kelvin-Helmholtz instability in the M-I coupling system is investigated by nonlinear simulations using reduced magnetohydrodynamic (MHD) models. However, the MHD model cannot account for the parallel electric field, which is crucial for auroral particle acceleration along magnetic field lines. To simulate the generation of the parallel electric field self-consistently, we need to extend the M-I coupling model to include kinetic effects so that Alfven waves coupling the magnetosphere and ionosphere induce the parallel electric field.
In the present study, we have developed a nonlinear gyrokinetic simulation model for the magnetosphere by extending a gyrokinetic simulation code GKV [5]. Nonlinear simulations using the newly developed model have reproduced similar evolution of the M-I coupling system to the previous results from the reduced MHD model, where on finds the primary growth of the feedback instability and the subsequent transition to turbulence due to the secondary growth of the Kelvin-Helmholtz instability. The left panel in Figure 1 below shows the emergence of Kelvin-Helmholtz vortices in the density profile at the top of the model magnetosphere (magnetic equator). While the vortices break into smaller structures, the Alfven waves with strong irregularities propagate down to the ionosphere and produce turbulent structures of auroral vortices.
In addition, we have found effective energy transfer from the electromagnetic fields to electrons through wave-particle interactions with the parallel electric field. The right panel in Figure 1 represents the time history of the entropy balance in the magnetosphere. The yellow curve shows the rate of energy exchange from the Alfven waves to electrons and ions integrated along the field line in the magnetosphere.
In the presentation, we will also discuss the spatio-tempotal structures of the parallel electric field and the velocity distribution function.