The cross-polar potential drop and its correlation to the solar wind

The cross-polar potential drop Phi(pc) as derived from the FAST satellite i s used to study the average magnetospheric response to changes in the solar wind as monitored by the Wind spacecraft. The coupling of the solar wind w ith the magnetosphere is examined using the interplanetary magnetic field ( IMF) B-z, the model reconnection electric field vB(t)sin(k)(theta/2) for k= 3, 4, and the Akasofu-Perreault epsilon parameter. Initial results comprisi ng 37 cases of Phi(pc) show one major response of the magnetosphere to the solar wind forcing at 15 min time lag followed by two minor pulses at 55 mi n and 105 min, respectively, during times when the IMF polarity was mainly southward and the geomagnetic activity was moderate to low. Phi(pc) shows a very good correlation with typical models of the reconnection electric fie ld at 15 min time lag, reaching a maximum linear correlation coefficient of r=0.95 for vB(t)sin(3)(theta/2). In order to reach an understanding of the importance of individually calculated correlation coefficients, we introdu ce the statistical. bootstrap algorithm of Efron and Tibshirani [1993], whi ch allows us to estimate a correlation coefficient standard error. In defin ing a quality measure based on this method, the significance coefficient s, we are able to interpret a resulting correlation coefficient time lag seri es in terms of a linear prediction filter similar to earlier techniques. Th e results on the magnetospheric response for Phi(pc) are further compared w ith those obtained using the geomagnetic indices Dst, SYM-H, and ASY-H. The similar magnetospheric response to the solar wind electric field for Phi(p c) and ASY-H at time delays of more than 40 min together with a high correl ation coefficient between Phi(pc) and ASY-H suggest that these magnetospher ic parameters couple to one another. The average dynamic response of the io nospheric convection to the solar wind electric field is in essential agree ment with those reported by Klimas et al. [1994] on a normal magnetospheric mode with a recurrence frequency of 50 min.