Local time variation of equatorial temperature and zonal wind anomaly (ETWA)

Abstract

We describe the average and apparent local (solar) time (LST) variations in the zonal winds (Z) and temperatures (T) as measured with the Wind and Temperature Spectrometer (WATS) on-board the polar orbiting DE-2 satellite in the altitude range of 300–450 km during a near solar maximum period of 1981–1982. During this time period, the variations in the solar flux (F10.7) and magnetic activity (Ap) contribute significantly to the apparent LST variations in T; while those effects on the LST variations in Z are small. The observations are related to the equatorial ionization anomaly (EIA) as seen in the electron density data obtained from the same satellite with the LANG instrument. The latitudinal variations in Z always reveal a maximum at the dip equator where the EIA trough forms and minima (with velocities reduced by a factor of two) are seen on either side of the equator where the EIA crests form. At 20:00 LST, the largest wind velocities are observed, directed eastward, after changing direction at around 17:00 LST where the largest accelerations occur. We delineate the diurnal variations in the strength of the Equatorial Temperature and Wind Anomaly (ETWA) defined by the differences in the wind velocities (DZ) and temperatures (DT) at the crests and troughs of the ionization. The diurnal variations in DZ are similar to those in Z at the trough. The diurnal variations in DT differ from those apparent in T. Excess temperatures, DT, at the crests show up with the development of the EIA as early as 09:30 LST by which time the zonal wind has attained its westward maximum. But DT continues to increase with the EIA crest until 14:00 LST, followed by a dip at around 16:00 LST in phase with the zero crossing of Z. The highest value of DT is only reached at 20:00 LST when both Z and the crest intensification reach their respective maxima. This demonstrates that the development of the equatorial temperature anomaly critically depends both on the development of the EIA crests and the zonal winds, which clearly establishes the primary role of ion drag in generating the ETWA phenomenon.