The Relative Importance of Alfven Wings and Geo-effective Length in Governing the Saturation of the Reverse Convection Potential Under Northward Interplanetary Magnetic Field (IMF)

Understanding the saturation of the ionospheric cross-polar cap potential (CPCP) is a crucial topic in understanding system-level solar wind-magnetosphere-ionosphere coupling. Recently there have been two competing hypotheses attempting to explain the saturation phenomenon. The Alfvén wing model predicts that saturation occurs under low solar wind Alfvénic Mach number, where the Earth's lobes form a wing-like structure with an Alfvénic conductance mismatched from the ionospheric conductance, leading to a reflection of the incident electric field from the polar cap [Kivelson and Ridley, 2008]. The sheath force balance model suggests that under strong driving, the magnetosheath plasma beta is reduced, changing the structure of the magnetosheath flow and leading to a diversion of magnetic flux around the flanks of the magnetosphere. This, in turn, reduces the geo-effective length in the solar wind as less magnetic flux reaches the dayside reconnection line [Lopez et al., 2010]. Recent MHD simulations have suggested that both mechanisms can occur simultaneously [Wilder et al., 2015]. To investigate the relative importance of each mechanism, we run the Lyon-Fedder-Mobarry (LFM) under strong northward IMF (20 nT) twice: once with a solar wind number density of 20 /cc and once with a density of 2 /cc. We find that the reverse convection potential is reduced for the low-density run, as predicted by both models. We find that for the low-density run, Alfvén wing-like structures form in the solar wind flow, but the embedded magnetic field does not appear to be connected to the polar ionosphere. The sunward reconnection exhausts that connect to the reverse convection in the polar ionosphere flow are near the local Alfvén speed for both runs, suggesting that they can be predicted by the local reconnection physics, and the length of the x-line pole-ward of the cusp is reduced by a factor of two for the low density run. This suggests that the reverse convection potential is governed by the local reconnection physics at the x-line and the geo-effective length, and that wave reflection phenomena in the Alfvén wings are not necessary to explain the saturation of the reverse convection potential.