Geomagnetic activity effects on semidiurnal winds in the lower thermosphere

Abstract

Over the period from July 1976 to November 1977 regular incoherent scatter observations of E and F region ion‐drifts were conducted at Millstone Hill (42.6°N) by using the steerable (L band) and fixed (UHF) radars. Data were collected on a total of 46 days and analyzed to give the ionospheric electric field and daytime neutral winds in the lower thermosphere at 105, 115, 125, and 135 km. The wind measurements characteristically exhibited strong 12‐hour oscillations with the eastward wind component reaching a maximum 3 hours ahead of the southward wind component. These results imply that the neutral winds are being dominated by semidiurnal tides. The geomagnetic activity dependence of the semidiurnal winds was determined by dividing the observations into quiet and disturbed periods on the basis of the Kp index. During disturbed conditions the winds were found to be significantly altered from the quiet time behavior, with the semidiurnal amplitudes reduced by 20 to 50% at the lowest altitudes and the altitude of maximum wind increased to be above 125 km. The vertical wavelength also appeared to decrease under disturbed conditions. Some of these effects might be explained by geomagnetic‐activity induced changes in lower thermospheric temperature and density, which alter the dissipation of the upward propagating tidal component. Changes in the prevailing winds also appear to be associated with Kp increases, implying a modification of the general thermospheric circulation by high‐latitude heating. The incremental winds are small (∼5m/s) and eastward/equatorward at 105 km, becoming stronger ( ∼25m/s) and westward/equatorward at higher altitudes.