Search for geomagnetic storm effects on lower thermospheric winds at midlatitudes

Neutral winds in the lower thermosphere during several geomagnetic storms of different intensity are derived from ion velocity measurements made by the Millstone Hill incoherent scatter radar (42.6°N, 71.5°W). The reliable detection of storm effects on midlatitude winds in the altitude region 100-130 km is difficult since the response at these altitudes is expected to be small. A search for such effects has been conducted using observations made during the past eight years during moderate and intense geomagnetic conditions. When geomagnetic activity is moderate, as that occurred in May 1995 and March 1999, there is no strong evidence for a response in the ion drifts or lower thermosphere winds. When the geomagnetic disturbances are intense, exceeding a geomagnetic index Kp of 6, such as that occurred in June 1991 and September 1998, there is clear evidence of a response. The primary finding is that the zonal wind component is enhanced, and the magnitude of the enhanced winds and their duration depends on the intensity and duration of the storm. In September 1998, both the ion and neutral velocities in the lower thermosphere are found to respond to the disturbance, and the largest enhancement of daytime zonal winds is observed roughly 13 h after the onset of the storm impulse.