Polar Observations Change Perception Regarding Location of Substorm Onset

1. Polar Observations Change Perception Regarding Location of Substorm Onset High resolution global imaging of the auroral oval by VIS has become a critical element in the search for substorm onset mechanisms. September 4, 1996 2129 UT Quiet oval 2135 UT Substorm Ignition 2140 UT Explosive Expansion L. A. Frank and J. B. Sigwarth, Findings concerning the positions of substorm onsets with auroral images from the Polar spacecraft, J. Geophys. Res., 105, 12,747-12,761, 2000.

2. Polar Observations Change Perception Regarding Location of Substorm Onset Measurements: High resolution auroral images from the VIS cameras. The images acquired with the VIS allow location of the auroral ignition or “onset” region to within 20 km in the ionosphere. Ring Current (3-8 Re) Observations: VIS images documenting the first glimmers of auroral brightenings define the footpoint of the local time and latitude associated with the substorm onset region. For the Sept. 4, 1996 event, tracing back along the magnetic field direction pinpoints the ignition or “onset” location in the ring current region just beyond geosynchronous altitudes and inside the near Earth plasma sheet. This, and other events analyzed by Frank et al. [2001] are consistent with a substorm onset location between 5 and 10 RE.

3. Polar Observations Change Perception Regarding Location of Substorm Onset Interpretation and Implications: • There is a long running debate in space physics as to the location and responsible mechanism for the auroral substorm ignition or onset. • One long-held idea is that the onset of magnetic reconnection in the magnetotail (Near Earth Neutral Line model) between 20 and 30 RE initiates auroral substorms. • The competing theory is that an instability occurs in the near Earth plasma sheet at distances of 6 -10 RE. Several current driven or ballooning mode instability mechanisms have been proposed and studied extensively. • The critical region for reconciling the two pictures is the transition at 6-15 RE where the field changes from dipolar to tail-like and plasma pressure gradients are largest. • The present observations support the new perception that the substorm trigger region is between 6 and 10 RE. • Future spacecraft conjunctions in the critical regions (Polar and Geotail in the transition region, Cluster farther down the tail)will provide the best opportunities for completing a full substorm theory.