Hough Mode Decomposition of the SE2 Tide Extracted From TIMED Observations

Based on the daily SE2 temperature tide at MLT altitudes (70–108 km altitudes) from Li et al. (2017; https://doi.org/10.1002/2016JA023435), which is picked up from TIMED/SABER observations at 45°S–45°N latitudes during a solar cycle (2002–2012), we decompose the high resolution (1 day) SE2 tide into a set of orthogonal Hough functions, which are orthogonal, and analyze the characteristics (i.e., annual, interannual, and day-to-day variations) of their coefficients. Our results show that the first antisymmetric and second symmetric Hough functions ((2, 3) and (2, 4)) are stronger than other Hough functions. The properties of the climatological variation are that: the (2, 3) function manifests an obvious semiannual variation that the monthly mean values are larger during equinoxes than solstices; the (2, 4) function presents two peak values are, respectively, in June and November with June maxima. Actually, the higher resolution SE2 tide was utilized to reveal the meteorological (day-to-day variabilities) of the Hough functions. It is worth to point out that the major two ranks ((2, 3) and (2, 4)) both dominate in high altitudes and all manifest remarkable semiannual variations and weak annual variations that they are larger during solstices than equinoxes with June solstice maxima. The further discussion reveals the contributions from the dominant two ranks of Hough functions absolute amplitudes play more significant role in the day-to-day variability of their Hough coefficients, than their phases.