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

• Published in: Annales Geophysicae
• Main Authors: H. Khan, S. W. H. Cowley
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 1999
• Subjects: Science; Q; Physics; QC1-999; Geophysics. Cosmic physics; QC801-809; Tromsø; EISCAT
• Online Access: https://doi.org/10.1007/s00585-999-1306-8; https://doaj.org/article/420f257184e54d1284639dd6f8d2bd06

We have combined ~300 h of tristatic measurements of the field-perpendicular F region ionospheric flow measured overhead at Tromsø by the EISCAT UHF radar, with simultaneous IMP-8 measurements of the solar wind and interplanetary magnetic field (IMF) upstream of the Earth's magnetosphere, in order to examine the response time of the ionospheric flow to changes in the north-south component of the IMF ( B z ). In calculating the flow response delay, the time taken by field changes observed by the spacecraft to first effect the ionosphere has been carefully estimated and subtracted from the response time. Two analysis methods have been employed. In the first, the flow data were divided into 2 h-intervals of magnetic local time (MLT) and cross-correlated with the "half-wave rectifier" 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 coefficient, were computed versus MLT for both the east-west and north-south components of flow. 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 first 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 flow data to determine the onset time of the corresponding enhancement or decay of the flow. For the case in which the flow response was timed from whichever of the flow components responded first, 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 ...