Ballooning instability of azimuthally small scale coupled Alfvén and slow magnetoacoustic modes in two-dimensionally inhomogeneous magnetospheric plasma

Abstract

This paper investigates the conditions of the ballooning instability of the coupled Alfvén and slow magnetoacoustic modes in the dipole model of Earth's magnetosphere taking into account plasma and magnetic field inhomogeneity in the direction along the magnetic field lines. The diamagnetic condition (meaning vanishing perturbation of the total pressure) is satisfied. It was shown that the instability develops on the slow magnetoacoustic oscillation branch, but the instability threshold is determined by the coupling with the Alfvén mode. The symmetric (with respect to the magnetic equator) modes were found to be more unstable than antisymmetric ones. In this case, the instability threshold depends on plasma compressibility: the finite sound velocity raises the instability threshold. For all other equal conditions, the instability threshold decreases with the decrease in the field line curvature radius on the equator.

The increase in the curvature radius of geomagnetic field lines preceding a classical dipolarization

“…The westward electric fields produce earthward flow bursts referred to as convection surge. The inductive electric fields produced by the dipolarization are of the same order of magnitude observed in DF (Runov et al, 2011). Figure 5.…”

“…The westward electric fields in the dipolarization front (DF) (Runov et al, 2011) embedded in the leading edge of bursty bulk flow (BBF) can be assumed as an external stimulus for triggering ballooning instability. In this case westward electric fields in the DF temporarily amplified the parallel flux flowing towards the end point of the flux tube in the equatorial plane and further steepened the earthward pressure gradient.…”

Ballooning Instability in the Magnetospheric Plasma: Two‐Dimensional Eigenmode Analysis

“…In this case, the frequency is purely imaginary: , Re(Ω − )=0. The instability develops when the pressure gradient is negative and lower than some threshold, which was approximately determined in Rubtsov, Mager, et al () for different particular cases. The longitudinal structure of the coupled Alfvén and slow mode disturbances was considered also by Mazur et al ().…”

“…Earlier, eigenmode analysis of ballooning perturbations in the Earth's magnetosphere was performed in Chan et al (), Cheremnykh and Parnowski (), Parnowski (), Agapitov et al (), Xia et al (), and Rubtsov, Mager, et al (). These works allowed to determine the threshold of the ballooning instability under different assumptions on the magnetospheric plasma and boundary conditions on the ionosphere.…”

“…These works allowed to determine the threshold of the ballooning instability under different assumptions on the magnetospheric plasma and boundary conditions on the ionosphere. In particular, Rubtsov, Mager, et al () considered the parallel structure of the unstable ballooning oscillations (coupled Alfvén and slow modes) at the realistic boundary conditions, when the electric field and plasma parallel displacement vanish on the ionosphere. However, those studies considered only one‐dimensional (field‐aligned) structure of the perturbation, without taking into account the transverse structure.…”