Magnetospheric response to the solar wind as indicated by the cross-polar potential drop and the low-latitude asymmetric disturbance field

The cross-polar potential drop Phi (pc), and the low-latitude asymmetric ge omagnetic disturbance field, as indicated by the mid-latitude ASY-H magneti c index, are used to study the average magnetospheric response to the solar wind forcing for southward interplanetary magnetic field conditions. The s tate of the solar wind is monitored by the ACE spacecraft and the ionospher ic convection is measured by the double probe electric field instrument on the Astrid-2 satellite. The solar wind-magnetosphere coupling is examined f or 77 cases in February and from mid-May to mid-June 1999 by using the inte rplanetary magnetic field B-z component and the reconnection electric field . Our results show that the maximum correlation between Phi (pc) and the re connection electric field is obtained approximately 25 min after the solar wind has reached a distance of II R-E from the Earth, which is the assumed average position of the magnetopause. The corresponding correlation for ASY -H shows two separate responses to the reconnection electric field, delayed by about 35 and 65 min, respectively. We suggest that the combination of t he occurrence of a large magnetic storm on 18 February 1999 and the enhance d level of geomagnetic activity which peaks at Kp = 7(-) may explain the fa st direct response of ASY-H to the solar wind at 35 min, as well as the lac k of any clear secondary responses of Phi (pc) to the driving solar wind at time delays longer than 25 min.