We present distributions of the zonal-mean temperature and static stability in the Venusian atmosphere obtained from Venus Express and Akatsuki radio occultation profiles penetrating down to an altitude of 40 km. At latitudes equatorward of 75°, static stability derived from the observed temperature profiles is consistent with previous in-situ measurements in that there is a low-stability layer at altitudes of 50-58 km and highly and moderately stratified layers above 58 km and below 50 km, respectively. Meanwhile, at latitudes poleward of 75°, a low-stability layer extends down to 42 km, which has been unreported in analyses of previous measurements. The deep low-stability layer in the polar region cannot be explained by vertical convection in the middle/lower cloud layer, and the present result thus introduces new constraints on the dynamics of the sub-cloud atmosphere. The Venusian atmosphere is in striking contrast to the earth's troposphere, which generally has a deeper low-stability layer at low latitudes than at mid-and high latitudes. The dynamics of the Venusian atmosphere remain unclear because Venus is completely globally covered by thick clouds at altitudes of 48-70 km. The thermal structure of the Venusian atmosphere across this cloud layer is important in terms of understanding aspects of the general circulation, such as the mean meridional circulation and baroclinic instability waves, which contribute to meridional heat transport and atmospheric super-rotation. In-situ measurements were made by entry probes around the equator and 60°N as part of the Venera and Pioneer Venus missions, revealing a low-stability (weakly or almost neutrally stratified) layer at an altitude of approximately 50-55 km, a highly stratified layer above 55 km and a moderately stratified layer below 50 km 1. Radio occultation measurements, one of the most useful methods of obtaining vertical temperature profiles, were made as part of the National Aeronautics and Space Administration's (NASA's) Pioneer Venus mission and the European Space Agency's (ESA's) Venus Express mission 2-4 to obtain latitude-height distributions of temperature. These observations revealed that a cold latitudinal band called a "cold collar", which is thought to be formed by dynamics 5 and/or the latitudinal cloud structure 6 , is located at roughly 65° latitude near the cloud top (at nearly 65 km altitude) and surrounds a warm polar region and that temperature increases with latitude above 65 km and decreases with latitude below 60 km. Furthermore, static stability profiles obtained from the Venus Express radio occultation measurements showed that the low-stability layer in the cloud layer is deeper at high latitudes than at low and mid-latitudes 4. However, no in-situ measurements have been made at latitudes poleward of 60°, and the thermal structure below the cloud layer at high latitudes thus remains unknown.

Temperature profiles of the Venus atmosphere obtained by the Akatsuki radio occultation measurements showed a prominent local time dependence above 65‐km altitude at low latitudes equatorward of 35°. A zonal wavenumber 2 component is predominant in the temperature field, and its phase (i.e., isothermal) surfaces descend with local time, suggesting its downward phase propagation. A general circulation model (GCM) for the Venus atmosphere, AFES‐Venus, reproduced the local time‐dependent thermal structure qualitatively consistent with the radio occultation measurements. Based on a comparison between the radio occultation measurements and the GCM results, the observed zonal wavenumber 2 structure is attributed to the semidiurnal tide. Applying the dispersion relationship for internal gravity waves to the observed wave structure, the zonally averaged zonal wind speed at 75‐ to 85‐km altitudes was found to be significantly smaller than that at the cloud top. The decrease of the zonal wind speed with altitude is attributed to the momentum deposition by the upwardly propagating semidiurnal tide excited in the cloud layer.

Abstract. Tidal activity in the Mesospheric Lower Thermosphere (MLT) region over Trivandrum (8.5 • N, 77 • E) is investigated using the observations from newly installed SKiYMET Meteor Radar. The seasonal variability and vertical propagation characteristics of atmospheric tides in the MLT region are addressed in the present communication.The observations revealed that the diurnal tide is more prominent than the semi/terdiurnal components over this latitude. It is also observed that the amplitudes of meridional components are stronger than that of zonal ones. The amplitude and phase structure shows the vertical propagation of diurnal tides with vertical wavelength of ∼25 km. However, the vertical wavelength of the semidiurnal tide showed considerable variations. The vertical propagation characteristics of the terdiurnal tide showed some indications of their generating mechanisms. The observed features of tidal components are compared with Global Scale Wave Model (GSWM02) values and they showed a similar amplitude and phase structure for diurnal tides. Month-to-month variations in the tidal amplitudes have shown significant seasonal variation. The observed seasonal variation is discussed in light of the variation in tidal forcing and dissipation.

This paper presents the first ever comparison of daytime Mesopause OH rotational temperatures as determined using Meinel (8‐3) dayglow emissions with those obtained using the recently established and collocated, meteor radar (SKiYMET) over the magnetic equator in India. The measurements were made over Thiruvananthapuram (8.5°N, 76.5°E, 0.5°N diplat.) for January–May 2005 period. Overall agreement had been good (±20 K) with all the gross features coming out fairly well. Further, the temperatures estimated using these two techniques, exhibit significant wavelike modulations with periods spanning ∼3–65 days with varying phase differences, presumably due to their interaction with the longer period waves like the semiannual oscillation (SAO). The consistency of the temperature variabilities at two different altitudes viz. 87 and 94 km enables investigation of the mesosphere lower thermosphere dynamic coupling.

The spatiotemporal evolution of the daytime mesopause temperature (MT) over a tropical station, Trivandrum (8.5°N, 77°E), has been investigated during the annular solar eclipse of 15 January 2010 using a meridional scanning Multi‐Wavelength Dayglow Photometer. This eclipse was a unique event by virtue of its exceptionally long duration and noontime occurrence over the present observational site. It has been observed that during the course of the eclipse, MT underwent significant enhancement (∼35 K). This increase in temperature was found to be present throughout the region of coverage of the photometer, i.e., ±200 km centered at Trivandrum in the meridional direction. It has also been found that the enhancement in MT revealed a preferential northward/southward movement before/after the main phase of the eclipse. In addition to this, a sudden launch of gravity waves with periodicity of ∼30 min was noticed during the first contact, which was amplified during the maximum phase of the eclipse. Another noteworthy observation was the amplification of a ∼2 hour wave during the course of the eclipse. These observations and their plausible mechanisms are discussed in detail in context of the daytime hydroxyl emissions and mesopause energetics and dynamics.