Is the Field Line Dipolarization a Flux Tube Transition to a New Equilibrium of Larger Curvature Radius of Geomagnetic Field Lines?

Abstract:

In the midnight magnetosphere at geosynchronous altitudes, transitional intervals of substorms lasting about 10-min followed ground Pi2 onset ⁽¹⁾. During these intervals, the arrival of dipolarization front from the tail stimulated Ballooning instability which in turn excited a slow magnetoacoustic wave (SMA) ⁽²⁾. Perpendicular divergence of plasma flows in a dawn-dusk direction associated with SMA relaxed the radial inhomogeneity that developed in the growth phase ⁽³⁾. The relaxation of radial inhomogeneity reconfigured the flux tubes to a new equilibrium of larger curvature radius, i.e., field line dipolarization. Westward electric fields (~1mV/m) produced by the flux tube transition in the equatorial plane were transmitted earthward along the field lines and produced incomplete Cowling channel in the polar ionosphere through ionospheric dynamo processes driven by the transmitted electric fields ⁽⁴⁾. Regions of the field line dipolarization expanded in azimuth from 19 to 03LT during the transitional intervals. Thereafter, field magnitudes in the midnight magnetosphere increased by field line pileup indicating start of classical dipolarization ⁽¹⁾. Ionospheric current systems of substorms, such as westward electrojet, Harang discontinuity, and poleward expansion of aurora all occurred in the transitional intervals ⁽⁴⁾. The transitional intervals may be the most active periods in the substorm phase.

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