Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data

Abstract

The daily variations of the meridional wind at ±18° latitude have been obtained for summer and winter between 1977 and 1979 using the in situ measurements from the Atmosphere Explorer-E (AE-E) satellite. The AE-E altitude increased from about 250 to about 450 km during this period, with solar activity increasing simultaneously. Data are presented at three altitudes, around 270, 350 and 440 km. It was possible to average the data to obtain the 24 h variations of the meridional wind simultaneously at northern and southern latitudes and thereby study the seasonal variation of the meridional wind in the altitude range covered. Two features are found showing significant seasonal variation: (a) a late afternoon maximum of the poleward wind occurring only in winter at 1800 LT at all three altitudes; (b) a night-time maximum in the equatorward wind—the summer equatorward wind abating earlier (near 2130 LT) and more rapidly than the winter wind (after 2300 LT). Furthermore, in summer the night-time wind reaches higher amplitudes than in winter. The night-time feature is consistent with the observed seasonal variation of the equatorial midnight temperature maximum, which occurs at or before midnight in summer and after midnight in winter, showing a stronger maximum in summer. The observed night-time abatement and seasonal variations in the night-time winds are in harmony with ground based observations at 18° latitude (Arecibo). The time difference found between summer and winter abatements of the night-time equatorward wind are in large part due to a difference between the phases of the summer and winter diurnal (fundamental) components, and diurnal amplitudes are larger in summer than in winter at all threee altitudes. However, the higher harmonics play an important role, their amplitudes being roughly 50% of the diurnal and in some instances larger. The 24 h variation is mainly diurnal at all altitudes in both summer and winter, except in winter around 2700 km altitude where the semi- and ter-diurnal components are approximately equal to or larger than the diurnal.