Data obtained from the WATS (Wind and Temperature Spectrometer) and LP (Langmuir Probe) experiments on board DE‐2 (Dynamic Explorer) during high solar activity show evidence of anomalous latitudinal variations in the zonal winds and temperature at low latitudes. The zonal winds exhibit a broad maximum centered around the dip equator, flanked by minima on either side around 25 degrees; while the temperature exhibits a pronounced bowl‐shaped minimum at the dip equator which is flanked by maxima. The two minima in the zonal winds and the corresponding maxima in the temperature are nearly collocated with the crests of the well known Equatorial Ionization Anomaly (EIA). The maximum in the zonal winds and the minimum in the gas temperature are collocated with the trough of the EIA. The differences between the maxima and minima in temperature and zonal winds, on many occasions, are observed to exceed 100 K and 100 m/s, respectively. The characteristics of this new phenomenon have eluded present day empirical models of thermospheric temperature and winds. The connection among these variables can be understood from the ion‐neutral drag effect on the motions of the neutrals that in turn affect their energy balance.

A comparative study is made of the changes in the latitudinal structure of the F region electron density at fixed altitudes in the Indian equatorial region on days with and without postsunset onset of equatorial spread F, using (N‐h) profile data of Ahmedabad (dip latitude 18.6°N), Waltair (dip latitude 10.6°N), and Kodaikanal (dip latitude 1.5°N). It is found that on spread F days the ratio of the electron density in the altitude region 270 ‐ 300 km between Ahmedabad and Waltair showed a sudden enhancement starting at 1700 LT by a factor of 8 to 30 (at 1900 LT) from a near‐constant value of about 2 during the daytime. No such enhancement of the density ratio was evidenced on days without spread F. The enhancement of the electron density ratio prior to the onset of spread F is interpreted as an intensification of the northern crest of the equatorial anomaly, with the ionization in the bottomside F region as far north as 9° from the dip equator participating in the crest intensification process. The rapid intensification of the ionization anomaly engenders a similar augmentation of the neutral anomaly around sunset hours. This in turn creates a localized cell of altitude dependent equatorward neutral wind that aids further intensification of the crests of both the anomalies and augmentation of ionization in the magnetic field line tube passing through the height of maximum plasma density of the F2 layer over the equator. The net result of these coupling processes is a weakening of the ambient transequatorial wind (particularly during northern winter months) and reduction of the north‐south asymmetry of the ionization anomaly crests, a condition favorable for the onset of spread F (Maruyama and Matuura, 1984). The present study thus indicates a significant role of the anomaly intensification at altitudes much below the F region peak and associated neutral dynamics in the initiation of spread F.

Data obtained from the WATS (Wind and Temperature Spectrometer) instrument on DE‐2 (Dynamics Explorer) during high solar activity, show new evidence for the presence of vertical winds of a significant magnitude in the equatorial thermosphere. They reveal a latitudinal structure that can be related to the recently discovered phenomena of the Equatorial Temperature and Wind Anomaly (ETWA). In the local evening hours, the vertical winds usually are downward around the dip equator and collocated with the temperature minimum of ETWA. In general, they are upward at about 24° dip latitude away from the dip equator and are collocated with the ETWA temperature crests. The magnitude of the vertical winds is in the 10–40 m/s range. It is proposed that the temperature and pressure ridges, formed by the excess ion drag on the zonal winds around the two crests and ordered by the relatively lower ion drag at the trough of the well known Equatorial Ionization Anomaly (EIA), drive a new wind system in the meridional plane and that the measured vertical winds form part of this wind system.

A unique OI 630.0 nm dayglow photometer operated from Waltair (10.0 ø N clip lat.), a-low-latitude station in India, in a bidirectional mode, i.e., over zenith and at 20 ø elevation pointing north, has revealed features associated with the evolution of the EquatoriM Ionization Anomaly (EIA). The estimated strength of the: EIA on a particular day based on these features reveal that significant differences exist in the EIA contribution to OI 630.0 nm on equatorial spread-F (ESF) and non-ESF days. There exists a precursor in the OI 630.0 nm clayglow which en•[bles the prediction of ESF at least 3 hours prior to its actual occurrence and hence points to the significant control of daytime EIA-related processes in the triggering of the post-sunset ESF.

In view of the present understanding of Equatorial Spread F (ESF) through the generalized Rayleigh‐Taylor instability mechanism, the importance of dynamical parameters is examined by means of a nonlinear numerical simulation technique. The investigation reveals that agencies like downward wind and eastward electric field of measured magnitudes accelerate the evolutionary process of ESF even beyond 350 km height, where according to the linear theories they are considered to be less significant. It is also shown that the acceleration must be due to the presence of in‐situ winds or electric fields. The variabilities in the occurrence characterstic of ESF are discussed in connection with the possible day‐to‐day variabilities of vertical winds.