Lower thermospheric structure from Millstone Hill Incoherent Scatter Radar Measurements: 1. Daily mean temperature

Abstract

Daytime temperature determinations from 151 days of incoherent scatter radar measurements at Millstone Hill (42.6°N) from 1970 to 1975 were analyzed to characterize the mean daily temperature in the lower thermosphere (105–125 km). An analytical model fitted to the measured temperatures contained terms to specify the dependencies of the mean daily temperature on day of year (annual and semiannual terms), solar cycle, solar rotation, and geomagnetic activity. The model representation, whose coefficients are tabulated, showed that seasonal, solar cycle, and geomagnetic effects were all of comparable magnitude and could each produce a variation of 5–10% in the mean temperature. The solar rotation effects were found to be small. The annual term and the smaller semiannual term combined to produce a summer or later summer temperature maximum. Geomagnetic activity effects were determined on the basis of a delayed Kp index, and a propagation delay of 3.7 hours was obtained from the model fit to the lower thermospheric temperature measurements. An increase in either mean solar flux or Kp produced an increase in the mean daily temperature, the magnitude of the increase generally being larger at the highest altitudes. Comparison of the model results with a model based on temperatures measured at Saint Santin (44°N) showed similar seasonal variations but a larger solar cycle dependence at Millstone Hill. The strong Kp dependence found at Millstone Hill is attributed to the relatively high geomagnetic latitude of this station.