A Study of the Temporal and Spatial Development of Substorm Expansion Phase Onset Signatures

Determining a substorm expansion phase onset time is a problem not yet fully resolved. This study has correlated HYDRA electron flux and magnetic field data measured onboard the POLAR Satellite with FUV auroral image data observed on the IMAGE spacecraft, ground based magnetogram and Pi2 data from auroral zone latitudes, and solar wind data obtained on the ACE spacecraft. The location of POLAR expressed in GSM coordinates, in units of earth radii, was constrained as follows: -9 ≤ x ≤ -7, -2 ≤ y ≤ 2, and -1 ≤ z ≤ 1. This study suggests that a prominent signature of expansion phase onset is a well-defined rapid increase of the HYDRA electron flux in the energy range of 1 KeV to greater than 10 KeV. This energization or injection of electrons is usually, but not always, accompanied by a dipolarization of the magnetic field. Closely correlated in time with this onset signature is an auroral brightening, although not always prominent, and well-defined onsets of Pi2 activity and magnetic bays. Subsequently during the interval, typically within 30min - 1hr of the onset of increased particle flux at POLAR, additional more intense auroral brightenings occur with simultaneous increases in Pi2 amplitude and enhanced negative bays. The results of this study may help to further understand the concepts of multiple onset substorms, auroral brightenings and pseudo-breakups, and the timing of substorm expansion phase onsets.