Lower thermospheric structure from Millstone Hill Incoherent Scatter Radar Measurements: 2. Semidiurnal temperature component

Abstract

Daytime temperature determinations from 151 days of incoherent scatter radar measurements at Millstone Hill (42°N) from 1970 to 1975 were analyzed to characterize the semidiurnal temperature oscillation in the lower thermosphere (105–125 km). An analytical model fitted to the measured temperatures contained terms to specify the dependencies of the semidiurnal term on day of year (annual and semiannual terms), solar activity, and geomagnetic activity. The model representation, whose coefficients are tabulated, showed that seasonal effects were larger than effects associated with the other parameters. The annual mean semidiurnal oscillation had a maximum amplitude of 28 K at 115 km and a vertical wavelength of 43 km. Variations associated with season were large, for example, ±17 K in amplitude and ± 1.2 hours in phase at 115 km when referred to the annual mean semidiurnal vector. Generally, the altitude of maximum was lowest at the solstices, and the longest vertical wavelength occurred in winter. Semidiurnal temperature measurements from Saint Santin showed good agreement with the Millstone Hill model results in winter, but some significant amplitude and phase differences were apparent in other seasons. Theoretical predictions indicated that the observed semidiurnal oscillation at Millstone Hill is primarily the upward propagating tide rather than an in situ tide. Comparisons with thermospheric Hough mode extensions indicated a temperature structure best matched by the S2,4 mode, whereas previously reported wind measurements were found to be best matched by the S2,2 mode. Because of their different vertical structures it was postulated that it might be possible to reproduce both the observed temperature and wind measurements by a suitable synthesis of the S2,2 and S2,4 modes. The observed solar flux effects could be reasonably attributed to changes in tidal dissipation in the lower temperature produced by changes in mean density and temperature with solar cycle.