The nonlinear response of the polar cap potential under southward IMF: A statistical view

We report the results of an investigation into the effect of solar wind properties on the saturation of the polar cap potential (CPCP) during periods of strongly southward IMF. We use propagated solar wind data to search for periods between 1998 and 2007 when the interplanetary electric field is stable for more than 50 min and placed further conditions on the availability of SuperDARN and DMSP velocity data. CPCP values are calculated from these data sets and various fits of the polar cap potential to the interplanetary electric field (IEF) are compared. It is found that the trend is nonlinear, with a square root function fitting better than a straight line, and that the CPCP does not appear to exhibit asymptotic behavior. The nonlinearity of the CPCP is then correlated with various interplanetary parameters to test the various models of polar cap potential saturation. It is also found that the deviation of the CPCP from a linear fit has statistically significant correlation with solar wind Alfvenic Mach number and no significant correlation with solar wind dynamic pressure.