Ionospheric response to changes in the interplanetary magnetic field observed by EISCAT and AMPTE-UKS

A major question in solar–terrestrial physics concerns the nature of the interaction between the solar wind and the Earth's magnetosphere and ionosphere, and how this depends on the direction of the interplanetary magnetic field (IMF). We report here simultaneous observations of the IMF by the AMPTE–UKS (Active Magnetospheric Particle Tracer Explorers–UK Satellite), and of plasma flow in the high-latitude ionosphere by the EISCAT radar with better time resolution than has been achieved hitherto. We discuss one event in which a sharp southwards turning of the IMF is followed by the onset of rapid ionospheric plasma flow near local noon, demonstrating the close control of ionospheric flows by the IMF. We discuss the causes of the observed 13-min delay, and study the ionospheric heating that accompanies the acceleration.