Dynamic response of the cusp morphology to the solar wind: A case study during passage of the solar wind plasma cloud on February 21, 1994

Abstract

On February 21, 1994, both Geotail and IMP 8 satellites detected an interplanetary plasma cloud with intense interplanetary magnetic field (IMF > 50 nT) and high dynamic pressure (> 50 nPa). During this interval the Freja satellite detected intense cusp‐like plasma injections in four out of six dayside traversais. The first two traversais are carefully studied. During the first traversal the overall morphology of the ion injection is characterized by a “multiple‐injection” signature over a wide magnetic local time (MLT) range, whereas it is characterized by a “single‐injection” signature with narrow injection region at 8 MLT in the second traversal. The solar wind conditions were also quite different between these two periods: while both dynamic and magnetic pressures stayed high during entire period, the dynamic beta was much higher during the first Freja traversal than during the second traversal. Between these two traversais, the cusp plasma injection is detected by the Søndre Strømfjord radar. The radar signature of the plasma injection is identified using the satellite particle data when the satellite and the radar were conjugate (the satellite's footprint was in the radar's field of view.) The cusp position and dynamics observed by the Søndre Strømfjord radar again show a very good correlation to the solar wind condition, especially to the dynamic pressure. The result indicates the following. (1) During southward IMF the cusp morphology differs for conditions of high or low solar wind dynamic pressure. High dynamic pressure widens the cusp (with multiple injections), whereas high magnetic pressure narrows it (with single injection). The effect of the IMF on the cusp locations and morphology becomes dominant only when the dynamic pressure is not very high. (2) Such a morphological difference reflects dynamic pressure more than dynamic beta during southward IMF at least during times of high solar wind dynamic pressure. (3) The cusp morphology responds very quickly to the changes in the solar wind conditions.