Simultaneous Development of Multiple Auroral Substorms: Double Auroral Bulge Formation

The expansion phase of auroral substorms is characterized by the formation of an auroral bulge, and it is generally considered that a single bulge forms following each substorm onset. However, we find that occasionally two auroral intensifications takes place close in time but apart in space leading to the formation of double auroral bulges, which later merge into one large bulge. We report three such events. In those events the westward auroral electrojet intensified in each auroral bulge, and geosynchronous magnetic field dipolarized in the same sector. It appears that two substorms took place simultaneously, and each substorm was accompanied by the formation of its own substorm current wedge system. This finding strongly suggests that the initiation of auroral substorms is a local process, and there is no global reference frame for their development. For example, ideas such as (i) the auroralbreakup takes place in the vicinity of the Harang reversal and (ii) the westward traveling surge maps to the interface between the plasma sheet and low-latitude boundary layer, do not necessarily hold for every substorm. Even if those ideas may be suggestive of causal magnetospheric processes, the reference structures themselves are probably not essential. It is also found that despite the formation of two distinct auroral bulges, the overall magnetosphere-ionosphere current system is represented by one globally coherent system, and we suggest that its structure is determined by the relative intensities and locations of the two substorm current wedges that correspond to the individual auroral bulges.