Neutral winds in the lower thermosphere observed by WINDII during the April 4-5th, 1993 storm

Abstract

Data from a major geomagnetic storm in April 1993 were selected for the first WINDII study of storm-induced neutral winds in the lower thermosphere between 90 and 200 km. The storm commenced at ~1200 UT April 4th, when the Kp index increased from 1.3 to 7.7. The geomagnetic latitude range of WINDII measurements from the daytime O( 1 S) emission reaches the southern geomagnetic pole. The wind exhibits a dramatic change during the storm. In the polar region the maximum velocity of ~200 ms -1 at 140 km altitude on April 2nd increases to ~650 ms -1 at 200 km on April 5th. Winds plotted in geomagnetic coordinates show the familiar two-cell pattern in that winds are much stronger in the dusk sector than in the dawn sector. This pattern penetrates down to ~130 km. During the recovery period on April 8th, winds at higher altitudes return to normal while the two-cell pattern persists at lower altitudes. The thermospheric O( 1 S) emission rate is severely depleted by the storm to half of its normal value.