Propagation of Pi2 Pulsations through the Braking Region: Results from UCLA and Lyon-Fedder-Mobarry Global MHD Simulations

Abstract:

The relationship between the onset of Pi2 pulsations in ground magnetometers and substorm onset has been used extensively over the years to determine when and where substorm onset takes place. However, there are still many questions concerning how and where the Pi2 pulsations themselves are generated, how they propagate through the system, and their relationship to flow bursts in the plasma sheet.

In order to investigate Pi2 generation and propagation through the magnetosphere, and their relationship to bursty flows, we have modeled a substorm on 14-Sep-2004 using both the Lyon-Fedder-Mobarry (LFM) and the UCLA 3D global magnetohydrodynamic (MHD) models. By using two different models we show that the perturbations develop as a result of the equations, and do not depend on the set-up or numerical methods used in a particular model. The results from both models show that, in the braking region, the region where flows are slowed as they approach the inner edge of the plasma sheet, Pi2 period pulsations develop in the magnetic field, velocity, and pressure, concurrently with the arrival of fast earthward flows and their associated dipolarization fronts. The disturbance generated by the fast flows at the inner edge of the plasma sheet creates Alfvén waves that map to auroral regions and compressional waves that propagate deeper into the inner magnetosphere. The fast flows also generate Pi2 period fluctuations in the field aligned currents at the inner edge of the plasma sheet and in the inner magnetosphere.