ICS-13 Abstract View

 

Ballooning and feedback instabilities in the magnetosphere-ionosphere coupling
Watanabe, T.-H., watanabe.tomohiko@nagoya-u.jp (1)
In this study, we consider competition of two types of instabilities in the magnetosphere-ionosphere (M-I) coupling, that is, the ballooning and the feedback instabilities.

The ballooning instability has been investigated as a possible mechanism for triggering the substorm in the magnetosphere. It is known to be destabilized, when the pressure gradient coupled with the magnetic curvature overcomes the stabilization effect due to the field line bending. Simultaneously, in the M-I coupling system in polar regions, the shear Alfven waves (or the kinetic Alfven waves) can also be destabilized through the magnetospheric convection, if the ionospheric density change is taken into account with the feedback mechanism [1-3]. As the field line bending is related to the shear Alfven wave propagation, the above situation is regarded as a competition of the two types of instabilities with different energy sources.

We have investigated the ballooning and the feedback instabilities in the same theoretical model of the M-I coupling. Our linear analysis demonstrates that, as the interchange term enhances, the "unstable" shear Alfven waves with the opposite sign of the real eigenvalues in their lowest harmonic branch collide with each other and transit to the ballooning mode. When the magnetospheric convection is weak, however, the feedback instability can not grow while the ballooning mode remain to be excited by the pressure gradient.

The present theory implies that competition of the two instabilities may give a plausible explanation of auroral beading triggered through the M-I coupling.


[1] T. Sato, J. Geophys. Res., 83, doi:10.1029/JA083iA03p01042.
[2] T.-H. Watanabe, Phys. Plasmas, 17, 022904 (2010).
[3] T.-H. Watanabe, Geophys. Res. Lett., 41, doi:10.1002/2014GL061166 (2014).
(1) Department of Physics, Nagoya University, Nagoya 464-8602, Japan