Open Geospace General Circulation Model simulation of a substorm: Axial tail instability and ballooning mode preceding substorm onset

摘要

Recent global simulations of substorms show that before the onset of near-Earth reconnection the pressure equilibrium in the tail breaks down. This instability has no cross-tail variation and is thus not a ballooning mode, and it is also distinct from the tearing mode. Here, we analyze an Open Geospace General Circulation Model simulation run of the 23 March 2007 substorm and find the same instability. Because this mode has no significant cross-tail variation associated with it we call it the KYO mode. Besides the KYO mode we also find the classical ballooning mode in the simulation. It has a wavelength of –0.5 RE and is marginally, but sufficiently, resolved as shown by a higher-resolution control run. These results suggest a new scenario for the substorm expansion phase onset. During the growth phase magnetic flux is added to the lobes and the plasma sheet thins but remains in equilibrium. When force balance is no longer possible the KYO instability grows and accelerates plasma tailward. The divergence of the resulting tailward flow reduces the normal magnetic field and thereby makes the current sheet tearing unstable. The tearing mode grows right out of the KYO mode. The classical ballooning mode grows at the same time and is superimposed on the KYO mode, but its role in initiating reconnection is still unclear. The growth time of the KYO mode, ～2min, is both consistent with the notion of an explosive growth phase and with recent ground-based observation of the initial growth of auroral arcs before auroral breakup.