A unified model for the feedback and ballooning instabilities in the magnetosphere‐ionosphere (M‐I) coupling is developed by means of the reduced magnetohydrodynamic and two‐fluid equations, ...involving the local current closure and the ionospheric conductivity change in a scale of auroral fine structures, self‐consistently. In a low pressure gradient case, the Alfvén harmonics are destabilized through the feedback mechanism, while the ballooning instability appears if the magnetospheric pressure gradient exceeds a critical value. Transition of the dominant instability between the feedback and ballooning modes is brought by change of the normalized pressure gradient or the convection electric field in the magnetosphere. The obtained results imply a variety of appearance of auroral arcs and beads in the M‐I coupling system.
Plain Language Summary
A novel explanation on generation of arc and beading structures of auroras is provided by means of theoretical analysis based on the first principle of plasma physics. The theory predicts competition of two different types of perturbation growth, leading to a variety of auroral appearance.
Key Points
A new magnetosphere‐ionosphere coupling model is derived, where the feedback and ballooning instabilities are simultaneously described
Transition of dominant instability mode is brought by change of pressure gradient or convection electric field in the magnetosphere
The magnetosphere‐ionosphere coupling theory predicts a novel scenario on the auroral beading and preexisting arcs
Gyrokinetic turbulence simulations are applied for the first time to the cross-scale interactions of microtearing modes (MTMs) and electron-temperature-gradient (ETG) modes. The investigation of the ...fluctuation response in a multiscale simulation including both types of instabilities indicates that MTMs are suppressed by ETG turbulence. A detailed analysis of nonlinear mode coupling reveals that radially localized current-sheet structures of MTMs are strongly distorted by fine-scale E×B flows of ETG turbulence. Consequently, electron heat transport caused by the magnetic flutter of MTMs is significantly reduced and ETG turbulence dominates electron heat transport.
Multiscale gyrokinetic turbulence simulations with the real ion-to-electron mass ratio and β value are realized for the first time, where the β value is given by the ratio of plasma pressure to ...magnetic pressure and characterizes electromagnetic effects on microinstabilities. Numerical analysis at both the electron scale and the ion scale is used to reveal the mechanism of their cross-scale interactions. Even with the real-mass scale separation, ion-scale turbulence eliminates electron-scale streamers and dominates heat transport, not only of ions but also of electrons. Suppression of electron-scale turbulence by ion-scale eddies, rather than by long-wavelength zonal flows, is also demonstrated by means of direct measurement of nonlinear mode-to-mode coupling. When the ion-scale modes are stabilized by finite-β effects, the contribution of the electron-scale dynamics to the turbulent transport becomes non-negligible and turns out to enhance ion-scale turbulent transport. Damping of the ion-scale zonal flows by electron-scale turbulence is responsible for the enhancement of ion-scale transport.
•A contour dynamics method is applied to the Vlasov-Poisson system.•An efficient implementation of the periodic boundary condition is proposed.•The new method is benchmarked for the linear and ...nonlinear Landau damping.•Energy and particle conservation is examined for the benchmark test.•Particle trapping process is well captured by the contour dynamics method.
We revisit the contour dynamics (CD) simulation method which is applicable to large deformation of distribution function in the Vlasov-Poisson plasma with the periodic boundary, where contours of distribution function are traced without using spatial grids. Novelty of this study lies in application of CD to the one-dimensional Vlasov-Poisson plasma with the periodic boundary condition. A major difficulty in application of the periodic boundary is how to deal with contours when they cross the boundaries. It has been overcome by virtue of a periodic Green's function, which effectively introduces the periodic boundary condition without cutting nor reallocating the contours. The simulation results are confirmed by comparing with an analytical solution for the piece-wise constant distribution function in the linear regime and a linear analysis of the Landau damping. Also, particle trapping by Langmuir wave is successfully reproduced in the nonlinear regime.
A linear eigenmode analysis of the magnetosphere‐ionosphere coupling shows that the feedback instability leading to spontaneous formation of auroral arc structures remains unstable even in a case ...with strong vertical shear of horizontal ion flows induced by ion‐neutral collisions in the E layer. For low‐order Alfvén harmonics, the linear frequency and growth rate obtained by means of a height‐resolved model of the ionosphere are comparable to those resulted from the height‐integrated one, where fine vertical structures of the ionospheric density and magnetic field perturbations are well resolved in the former. The present result confirms validity of the height‐integrated ionosphere model in the long parallel wavelength limit, as assumed in previous studies on the feedback instability.
Plain Language Summary
Spontaneous formation of auroral arcs can be explained in terms of a plasma instability in the magnetosphere‐ionosphere coupling. The present study confirms validity and robustness of the basic instability theory even in cases with inhomogeneous profiles of the ionospheric parameters such as the ion‐neutral collision.
Key Points
Feedback instability in the magnetosphere‐ionosphere coupling is unstable even in a case with strong vertical flow shear in the E layer
The linear eigenmode analysis captures entire profiles of the electromagnetic fields and the density perturbations along a field line
A conventional magnetosphere‐ionosphere coupling model with a height‐integrated ionosphere remains valid in a low‐frequency regime
Abstract
We investigate the effect of the electron temperature gradient (ETG) driven turbulence on the energy transport in JT-60U L-mode plasma by means of the multi-scale gyrokinetic simulation. In ...the core region at
r
/
a
=
0.5
, the instability in the ion scale is driven by the ion temperature gradient (ITG), meanwhile strong unstable ETG modes are found in the electron scale. The nonlinear multi-scale gyrokinetic simulation shows that ETG modes are stabilized in the nonlinear phase and the energy transport in the multi-scale simulation is similar to that obtained in the ion scale ITG simulation. In an outer region at
r
/
a
=
0.6
, the ion scale instability changes to be the trapped electron mode (TEM). The multi-scale simulation shows that both the ion and the electron energy flux are reduced by
∼
30
%
compared to those obtained in the single scale TEM simulation. Interestingly, the electron energy flux is close to the experimental value after this reduction. From the data analyses, we find that ETG turbulence damps the energy of TEM modes through the ion scale/electron scale coupling and the electron scale/electron scale coupling, and can be modeled as a turbulent diffusion of TEM modes. These results suggest that the single ion scale simulation seems to be still valid in the inner region with
r
/
a
<
0.5
. However, in the outer region it is necessary to include the ETG modes in the gyrokinetic simulations to explain the energy transport in this L-mode plasma. This is the first result showing that ETG turbulence can reduce the electron energy loss via the cross-scale interaction in a real tokamak equilibrium profile.
Abstract
Multi-scale gyrokinetic theory and simulations of a toroidal magnetized plasma have revealed the existence of cross-scale interactions of the trapped electron mode (TEM) and the electron ...temperature gradient (ETG) turbulence. Reduction of the TEM instability growth rate in the ETG turbulence is clearly identified, and is well represented in the form of effective diffusion. A theoretical model based on the stochastic forcing by the ETG turbulence well describes the turbulent diffusion coefficient observed in multi-scale turbulence simulations.
PDF
