7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

A nonlinear gyrokinetic model of themagnetosphere-ionosphere coupling system

Keiji Fujita1, Tomo-Hiko Watanabe1
1Department of physics, Nagoya university
e-mail (speaker): fujita.keiji.x6@f.mail.nagoya-u.ac.jp
The auroral emission is known to result from collisions
between neutral particles in the ionosphere andprecipitating electrons from the magnetosphere.
However, physical mechanisms behind a variety ofauroral phenomenon have not been fully understood.
For example, some fundamental questions, such as how
the auroral electrons are accelerated and how the auroral
structures develops, remain unresolved.
Since interactions between the magnetosphere and theionosphere play important roles, numerous theoreticalmodels of the magnetosphere-ionosphere (M-I) couplingsystem have been developed. Particularly, the feedback
instability in the M-I coupling system has been discussed
as a plausible model that can simultaneously address theissues mentioned above.
Since the original works in the 1970s [1,2], thefeedback instability has been investigated with variousmodels. In Refs. [3,4] the secondary growth of theKelvin-Helmholtz instability in the M-I coupling system
is investigated by nonlinear simulations using reduced
magnetohydrodynamic (MHD) models, which elucidate
spontaneous formation of auroral vortex structures. The
left figure in Figure 1 shows the time history ofelectromagnetic energy of the magnetosphere obtained inRef [4]. Around time=10, the nonlinear growth of thefeedback instability saturates, and transition toturbulence due to the secondary instability occurs.
Nevertheless, the parallel electric field, which canaccelerate the auroral electrons along field lines, is notincluded self-consistently in the MHD models. Thus, weneed to extend the M-I coupling model to include kineticeffects, which can produce and sustain the parallel
electric field.
The gyrokinetic theory is relevant to describe themagnetospheric dynamics self-consistently including theparallel electric field in the low-frequency regime. In Ref.
[5], the linear feedback instability is successfully

formulated by means of the gyrokinetic equations for themagnetospheric plasma, where the particle acceleration
by kinetic Alfvén wave is confirmed in terms of theenergy exchange rate through the wave-particleinteraction. The dynamical evolution of the feedback M-I
coupling is also simulated using a linearized gyrokinetic
model [6]. However, to investigate the auroral vortex
formation and the electron acceleration simultaneously,
one needs to explore nonlinear evolutions of thefeedback instability.
In the study, therefore, we develop the gyrokinetic
simulation model for the magnetosphere including
nonlinear effects of the 𝐸 × 𝐵 convection and theparallel advection terms. For this purpose, thegyrokinetic simulation code GKV has been extended
with a plug-in module for the M-I coupling. The rightfigure in Figure. 1 shows the history of electromagnetic
energy in the magnetosphere calculated with thegyrokinetic model. Although the initial conditions andbackground parameters are difference from those in thereduce MHD calculation, a qualitatively similar behavior
to the reduced MHD result is observed in the gyrokinetic
result. In the presentation, we will discuss the result in
more detail as well as the boundary condition for the M-I
coupling.
References[1]. G. Atkinson, J. Geophys. Res. 75, 4746 (1970).
[2]. T. Sato, J. Geophys. Res. 83, 1042 (1978).
[3]. T.-H. Watanabe, Phys. Plasmas 17, 022904 (2010).
[4]. T.-H. Watanabe, et al., New J. Phys. 18, 125010(2016).
[5]. T.-H. Watanabe, Geophys. Res. Lett. 41, 6071
(2014).
[6]. S. Nishimura and R. Numata, J. Phys. Soc. Japan 90,094901 (2021).

Figure 1: Time histories of electromagnetic field energy calculated with the reduce MHD model in Ref.[4](left) and with the gyrokinetic model in the present study (right).