MEASUREMENT OF THE MAGNETOTAIL RECONNECTION RATE

A technique to measure the magnetotail reconnection rate from the grou nd is described and applied to 71 hours of measurements from 20 nights . The reconnection rate is obtained from the ionospheric flow across t he polar cap boundary in the frame of reference of the boundary, measu red by the Sondrestrom incoherent scatter radar. For our measurements, the polar cap boundary is located using 6300 Angstrom auroral emissio ns and E region electron density. The average experimental uncertainty of the reconnection rate measurement is 11.6 mV m(-1) in the ionosphe ric electric field. By using a large data set, we obtain the dependenc e of the reconnection rate on magnetic local time, the interplanetary magnetic field, and substorm activity, with much higher accuracy. We f ind that two thirds of the average polar cap potential drop occurs ove r the 4-hour segment of the separatrix centered on 2330 MLT, that the linear correlation between the reconnection electric field and the hal f-wave rectified dawn-dusk solar wind electric field VB, peaks between 1.0 and 1.5 hours, with a maximum linear correlation coefficient of 0 .46 at 70 min; and that following substorm expansion phase onset, the reconnection electric field becomes larger than the experimental uncer tainty, with an average delay of 23 min. The 70-min delay of the recon nection rate with respect to VB, is a typical convection time for a fl ux tube across the polar cap. This result indicates that reconnection in the magnetotail is influenced by the solar wind electric field VB, on the field line being reconnected.