Magnetosphere Seminars

Apr 15, 2014 – “The Role of Magnetosheath Plasma Beta in the Formation of Alfven Wings and the Saturation of the Region 1 Field-Aligned Current System”

Speaker:Frederick Wilder (LASP)
Date:Tuesday, Apr 15, 2014
Time:4:00 PM
Location:LASP Space Science Building, room N248 upstairs. Map

Seminar Abstract:

The phenomena of cross-polar cap potential (CPCP) and ionospheric field-aligned current (FAC) saturation remains largely unexplained. In recent years, empirical observations and statistical studies have been consistent with the Alfven Wing model. In this hypothesis the electric field of the solar wind MHD generator is partially reflected from the ionosphere due to a mismatch in conductance. There are several unanswered questions regarding the Alfven Wing model, especially regarding how a simple wave reflection mechanism can be explained using the velocity-magnetic field (VB) paradigm. Additionally, while the Model does a good job of predicting the saturation of the CPCP, the Alfvenic Mach number in the solar wind rarely drops below 1, which is considered a requirement for the formation of Alfven Wings. To address these issues, we propose a mechanism for CPCP and FAC saturation whereby open flux tubes are less able to deform under low magnetosheath plasma beta conditions. This, in turn, makes them less able to exert Maxwell stress on the polar cap ionosphere in order to accelerate ionospheric plasma against ion-neutral collisional drag. We show how this is captured in the Alfven Wing model, and use the LFM MHD model to demonstrate that this bending of the field can lead to the non-linear response of the Region 1 FAC system to increasingly southward IMF.