Longitude variations of the solar semidiurnal tides in the mesosphere and lower thermosphere at low latitudes observed from ground and space

We present an analysis of longitudinal variation in the solar semidiurnal tide observed in the nocturnal thermal structure of the low-latitude mesopause region (83-103 km), with a focus on two sites: Arecibo, Puerto Rico (18.3 degrees N, 66.8 degrees W) and Maui, Hawaii (20.7 degrees N, 156.3 degrees W). Localized observations made by lidars are combined with longitudinal measurements by Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and compared with the Global-Scale Wave Model-2002 (GSWM-02) for 6 months: January, April, May, July, August, and October. In winter-spring, lidar-measured amplitudes are larger than those recorded by SABER, whereas in summer the amplitudes are similar and in autumn the lidars observe smaller amplitudes than SABER. GSWM amplitudes are between the lidar and SABER measurements for January, are comparable to the measurements in spring, underpredict for summer, and agree with the lidars in autumn. The phase structure is consistent among lidars and SABER for all of the months except January, when there is a large phase shift, and the local time of the temperature maximum is different between Arecibo and Maui. GSWM shows better agreement with phase measurements in winter-spring than in summer-autumn. Modal analysis of the SABER data indicates that the (2, 2) Hough mode is present throughout the year. In winter, it is joined by the (2, 4) mode, while in summer it is accompanied by the (2, 3) mode. This change in the predominant Hough modes may be related to the winter-summer phase difference.