UCLA

Pi2 pulsations are magnetic field fluctuations with periods between 40 an 150 seconds observed on the ground in conjunction with the onset of magnetospheric substorms. Pi2 period perturbations are also observed in magnetic field and plasma observations in space in conjunction with fast earthward flows leading to several theories concerning how and where the pulsations are generated. We investigate the source and propagation of Pi2 period pulsations through the magnetosphere, tracing the disturbances from their origin in the magnetotail through the inner edge of the plasma sheet and into the inner magnetosphere. Several models for the generation of Pi2 pulsations have been constructed by using satellite and ground-based observations. Our approach is to use global magnetohydrodynamic (MHD) computer codes to simulate the Earth's magnetosphere during substorms to determine where the Pi2 period perturbations are being generated. We use two different MHD models, the UCLA and Lyon-Fedder-Mobarry (LFM) models, in order to test the robustness of our conclusions about Pi2. The simulation results are compared with ground-based and satellite data for validation.

We find that Pi2 period perturbations are generated in both models by earthward propagating fast flows inside of x~-12 RE. As the flows propagate through the braking region, the region where flows slow down as they approach the inner edge of the plasma sheet, the pulsations begin to run ahead of the fast flow and its accompanying dipolarization front into the inner magnetosphere. This indicates that a compressional wave is being generated by the flow as it slows in the braking region. The speed of the flows, and the penetration of the flows and perturbations into the inner magnetosphere, depends strongly on the ionospheric models used at the inner boundary of the MHD models. When modeled Pedersen conductances are high with respect to typically observed values, strong line-tying slows the flows more quickly in the braking region. Therefore, perturbations are not able to propagate as freely into the inner magnetosphere.

When looking at the power spectral density (PSD) we find that the fluctuations in Bz propagate with the flow channel until they reach ~-8 RE, but as the disturbance approaches the inner edge of the plasma sheet the flow speeds decrease and the perturbations propagate earthward and spread azimuthally. As a result the perturbations generated by an azimuthally thin flow channel would be observed over a large area on the ground. Field line tracing shows that ionospheric perturbations do not always map to a flow channel, but they do map to a disturbance in the thermal pressure associated with the flow. Since the flows are responsible for the thermal pressure perturbations, they are also indirectly responsible for the perturbations observed in the simulated ionosphere.