Observations of the response time of high-latitude ionospheric convection to variations in the interplanetary magnetic field using EISCAT and IMP-8 data

Abstract

Abstract. We have combined $300 h of tristatic measurements of the ®eld-perpendicular F region ionospheric¯ow measured overhead at Tromsù by the EISCAT UHF radar, with simultaneous IMP-8 measurements of the solar wind and interplanetary magnetic ®eld (IMF) upstream of the Earth's magnetosphere, in order to examine the response time of the ionospheric ow to changes in the north-south component of the IMF (B z ). In calculating the¯ow response delay, the time taken by ®eld changes observed by the spacecraft to ®rst eect the ionosphere has been carefully estimated and subtracted from the response time. Two analysis methods have been employed. In the ®rst, the¯ow data were divided into 2 h-intervals of magnetic local time (MLT) and cross-correlated with the``half-wave recti®er'' function V 2 B s , where V is the solar wind speed, and B s is equal to IMF B z if the latter is negative, and is zero otherwise. Response delays, determined from the time lag of the peak value of the cross-correlation coecient, were computed versus MLT for both the east-west and north-south components of¯ow. The combined data set suggests minimum delays at $1400 MLT, with increased response times on the nightside. For the 12-h sector centred on 1400 MLT, the weighted average response delay was found to be 1.3 0.8 min, while for the 12-h sector centred on 0200 MLT the weighted average delay was found to increase to 8.8 1.7 min. In the second method we ®rst inspected the IMF data for sharp and enduring (at least $5 min) changes in polarity of the north-south component, and then examined concurrent EISCAT¯ow data to determine the onset time of the corresponding enhancement or decay of the¯ow. For the case in which the¯ow response was timed from whichever of the¯ow components responded ®rst, minimum response delays were again found at $1400 MLT, with average delays of 4.8 0.5 min for the 12-h sector centred on 1400 MLT, increasing to 9.2 0.8 min on the nightside. The response delay is thus found to be reasonably small at all local times, but typically $6 min longer on the nightside compared with the dayside. In order to make an estimate of the ionospheric information propagation speed implied by these results, we have ®tted a simple theoretical curve to the delay data which assumes that information concerning the excitation and decay of¯ow propagates with constant speed away from some point on the equatorward edge of the dayside open-closed ®eld line boundary, taken to lie at 77°magnetic latitude. For the combined cross-correlation results the best-®t epicentre of information propagation was found to be at 1400 MLT, with an information propagation phase speed of 9.0 km s A1. For the combined event analysis, the best-®t epicentre was also found to be located at 1400 MLT, with a phase speed of 6.8 km s A1.

Excitation of twin-vortex flow in the nightside high-latitude ionosphere during an isolated substorm

“…Solar wind and IMF data were obtained by the SWEPAM and MAG instruments, respectively (McComas et al, 1998;Smith et al, 1999). The propagation delay of field changes from ACE to the dayside ionosphere has been estimated using the method of Khan and Cowley (1999). The estimate includes the propagation time in the solar wind upstream of the bow shock, the frozen-in transit time across the subsolar magnetosheath, and the Alfvénic propagation time along open field lines from the subsolar magnetopause to the ionosphere.…”

“…Etemadi et al, 1988;Todd et al, 1988;Ruohoniemi et al, 1993;Ridley et al, 1998;Khan and Cowley, 1999). Even the substructure of characteristically pulsed dayside reconnection on 5-10 min time scales (i.e.…”

Two-dimensional electric field measurements in the ionospheric footprint of a flux transfer event

“…2). The time delay of the southward turning from the spacecraft to the magnetopause was determined using the method of Khan and Cowley (1999). The plane of the discontinuity in the IMF at 10:45 UT was projected back to the GSE x axis, and from there the time delay to the subsolar point on the magnetopause was calculated, using the solar wind plasma parameters.…”

“…The plane of the discontinuity in the IMF at 10:45 UT was projected back to the GSE x axis, and from there the time delay to the subsolar point on the magnetopause was calculated, using the solar wind plasma parameters. The IMF conditions measured by WIND resulted in a calculated delay of 12 min to the subsolar magnetopause and 2 min were added to account for propagation to the ionosphere (Khan and Cowley, 1999). Therefore the IMF delay to the ionosphere is estimated to be 14 AE 1 min.…”

A joint Cluster and ground-based instruments study of two magnetospheric substorm events on 1 September 2002

“…These data have been lagged in order to take into account the time taken by the solar wind to reach the Earth's magnetopause from where it is measured at the ACE spacecraft. The lag time for this interval was calculated as 75 min, using both magnetic field data from the Magnetic Fields Measurement (MGF) instrument on the Geotail spacecraft (data not shown) and the technique of Khan and Cowley (1999), from solar wind velocity and density data from the Solar Wind Electron, Proton and Alpha Monitor (SWEPAM) instrument (McComas et al, 1998) on ACE.…”