Feedback instability analysis for dipole configuration with ionospheric and magnetospheric cavities

Abstract

[1] Linear eigenmodes of ionospheric feedback instability in the dipole magnetic field geometry are investigated by considering the ionospheric and magnetospheric resonant cavities with inhomogeneous Alfven velocity profiles. The two-field reduced magnetohydrodynamic model is used to describe shear Alfven wave dynamics, associated with auroral arcs, in a strongly nonuniform magnetic flux tube. Linear properties of ionospheric cavity modes (eigenfrequency and eigenfunctions) are examined for a wider parameter range than the previous works. It is found that several harmonics, denoted by the mode numbers of n = 3, 6–7, 10–11, …, are trapped in the cavity region, and their magnetic fluctuations have a large amplitude at the ionosphere, leading to a high growth rate. By steepening a gradient of the Alfven velocity on the magnetospheric side, the properties of ionospheric cavity modes are fully dismissed from these low n harmonics, although their growth rates remain small positive values. On the other hand, the fundamental field line resonant mode (n = 0) has a high growth rate with a large amplitude of magnetic fluctuations at the ionosphere owing to the magnetospheric cavity. Possible spatial and temporal scales of auroral arcs and ULF wave resonances in a realistic situation are discussed.