Effect of Field Line Stretching on its Resonance Frequency

A. T. Y. Lui and C. Z. Cheng
The Johns Hopkins University/Applied Physics Laboratory, USA.

Field line resonance (FLR) has the potential to account for magnetic and electric oscillations observed near the equatorial region of the magnetosphere and on the ground associated with auroral arcs. In particular, field line resonance has been proposed as a mechanism for the formation of auroral arcs. One main problem in associating FLR with the observed oscillations at the ground and in space is the disparity between the theoretical predicted frequencies for FLR and the observed ones. Previous theoretical works based on dipolar field lines give frequencies too high to match with the observed frequencies when typical mass density is used. In this paper, we present theoretical calculation on how the apparent discrepancy in frequency may be resolved by field line stretching and identification of the correct mode for the FLR. The calculation is performed with a magnetospheric magnetic field model based on a self-consistent equilibrium solution with provision for stretching of field lines by the development of a strong cross-tail current. We obtain quantitatively the resonance frequencies for the shear Alfvén and slow mode branches of FLR threading the auroral zone for field lines stretched to various distances in the nightside magnetosphere.

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