Determining Substorm Onset Location Using 3D Empirical Force-Balanced Pressure and Magnetic Field Models for Substorm Growth Phase
Tuesday, 16 December 2014
Where substorm onset occurs in the plasma sheet is one of the central questions to substorm research but remains undetermined. Current magnetic field models are not developed specifically for the substorm growth phase, and thus are not reliable for determining the equatorial mapping location of the auroral onset. Recently, we have established empirical modeling of 3D force-balanced pressure and magnetic field configurations for the substorm growth phase. Our model first predicts pressure corresponding to time integrated solar wind loading and AE, solar wind dynamic pressure, and sunspot number. It then computes 3D magnetic field in force balance with the modeled pressure. In this study, we empirically modeled the growth phase of one substorm event on March 5, 2008 at 06:04 UT, during which three THEMIS probes were at the duskside plasma sheet with large radial separations and two GOES satellites were near midnight. There was also good aurora coverage showing onset occurring at ~67° near midnight. As the substorm growth phase develops, the overall pressure increases in response mainly to the increasing solar wind energy loading, while field lines become more stretched with Bz decreasing in the near Earth region but increasing in the tail. The modeled pressure and magnetic field and their evolutions match reasonably well with the observations at all THEMIS and GOES locations. In addition, the latitude of computed proton isotropic boundary, an indicator of the equatorial boundary of proton precipitation due to current sheet scattering, also matches well with the equatorward edge of observed proton aurora and its equatorward motion. These comparisons clearly show that our model realistically represents the overall magnetic field configuration during this event. The equatorial mapping of the auroral onset thus reliably indicates that the onset of this substorm should have occurred at r = ~13 RE in the tail plasma sheet. More events will be modeled to statistically determine the equatorial onset location.