E-region temperature measurements at Millstone Hill

Abstract

An incoherent scatter radar experiment involving the transmission of pairs of short pulses at various spacings was employed at Millstone Hill to study the daytime thermal structure of the lower thermosphere. An altitude range from 110 to 130 km was covered with a height resolution of 6 km. The daily temperature variation exhibited oscillations having dominant periods in the range 6–14 hr and amplitudes as large as 28 per cent of the mean temperature. These waves possessed a downward phase velocity indicating an energy source at lower heights. A spectral analysis carried out on measurements gathered on successive days provided sufficient frequency resolution to resolve the different tidal frequencies and indicated the 12-hr period as the main component. Interpreting the measured oscillations as being primarily a manifestation of the solar semidiurnal tide, the observed phase and vertical wavelength are in agreement with theoretical predictions for the thermal (2, 4) tidal mode propagating from the lower atmosphere. On average, the semidiurnal wave amplitude is about 40 K with a time of maximum near 0700 hr LT at 115 km and is damped above that height. Large variability in amplitude and phase are found from one observing period to another. The height gradient of the average daytime temperature near 120 km is about 15 K/km. This value is larger than given by some theoretical treatments but is consistent when the energy carried into the region by the semidiurnal tide is allowed for.