Empirical Relationship between Nightside Reconnection Rate and Solar Wind / Geomagnetic Measurements

Abstract

According to the expanding-contracting polar cap paradigm (ECPC), dayside and nightside reconnection control magnetosphere-ionosphere dynamics at high latitudes by increasing and decreasing the open magnetic flux content, respectively. The dayside reconnection rate can be estimated using parameters measured in the solar wind, but there is no reliable and available proxy for the nightside reconnection rate. We aim to remedy this by using AMPERE to estimate a time series of open flux content. The AMPERE data set originates from the global Iridium satellite system, enabling continuous measurements of the field-aligned Birkeland currents, from which the open magnetic flux of the polar caps is derived. This method provides seven years of open flux change estimates. A series of nightside reconnection rates are then derived by directly relating the dayside reconnection rate to the estimated open flux change. Various proxies for the nightside reconnection rate are estimated by relating several geomagnetic indices with our estimated nightside reconnection rate through multivariate regression analysis. By comparing the estimated open flux change with solar wind conditions and geomagnetic indices, we find our open flux estimates are highly dependent on the magnitude of the Birkeland currents. During low activity periods, the estimated open flux proves to be highly inaccurate with improving accuracy for higher activity periods. We also find that the nightside reconnection rate proxies fail to explain the majority of the variation in our estimated nightside reconnection rate series and propose that this is mainly due to the inaccuracy of our open flux estimates.