HR: 1340h
AN: SM43A-1074    [Abstracts]
TI: Magnetospheric Current Response to Solar Wind Dynamic Pressure Enhancements during Strong Magnetic Storms
AU: * Shi, Y
EM: yongshi@atmos.ucla.edu
AU: Zesta, E
EM: ezesta@atmos.ucla.edu
AU: Lyons, L R
EM: larry@atmos.ucla.edu
AB: Recent studies have found that solar wind dynamic pressure enhancements can cause clear dawn-dusk asymmetric H perturbations in low-latitude ground magnetometers, particularly when the IMF Bz has been southward for some time before the compression occurs. The asymmetry consists of negative H perturbations on the dusk side and positive H perturbations on the dawn side, a response that is uncharacteristic of what is expected from a typical magnetospheric compression. This response was qualitatively interpreted as the result of the intensification of the existing partial ring current by the pressure enhancement. The assumption is that the intensified partial ring current creates the negative perturbations on the dusk side, overwhelming the effect of all other magnetospheric (Chapman-Ferraro and R1 and R2) currents. The present study quantitatively investigates the response of the different magnetospheric current systems to the solar wind dynamic pressure enhancements and their contribution to the ground H perturbations by combining modeling and observational results during two pressure enhancement events that occurred during strong magnetic storms of similar strength. The magnitude of the pressure in the two events is the differing factor. We used the Tsyganenko storm-time magnetic field models (TS05), which includes separate modules for each magnetospheric current system, to fit and model the ground perturbations that result form the compressions. We first modified the TS05 by adding the present state of dynamic pressure to the parameterization scheme of the R1 and R2 field-aligned current modules and to that of the symmetric and asymmetric ring currents. We then fit the model to the low- and mid-latitude ground magnetometer observations for each of the two selected magnetic compressions. For the pressure enhancement occurring during the main phase of the September 25, 1998 storm, the modeling results show that the primary contributor to the ground asymmetric H perturbation is the intensified partial ring current and its closure field-aligned currents flowing downward in the morning sector and upward in the post-midnight sector. The other currents have much less contribution. The modeling results are consistent with the current pattern inferred from the global ground H and D magnetic perturbations. For the pressure enhancement occurring during the main phase of the May 29, 2003 storm, the modeling results are consistent with an intensified R1 current pattern twisting towards the dawn side, also inferred from the global ground H and D component perturbations. The difference between these two cases is that the pressure enhancement magnitude of the 2003 event was almost twice that of the 1998 event, which resulted in the magnetopause moving much closer to the Earth by comparison. We explore how this difference can explain the different current patterns.
DE: 2700 MAGNETOSPHERIC PHYSICS (6939)
DE: 2774 Radiation belts
DE: 2778 Ring current
DE: 2784 Solar wind/magnetosphere interactions
SC: SPA-Magnetospheric Physics [SM]
MN: 2007 Fall Meeting