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Abstract. This paper presents measurement of daytime Sodium (Na) airglow emissions at 589.6 nm wavelength using a ground based 1-m scanning monochromator from Trivandrum (8.5 • N, 77 • E, 0.5 • N dip latitude), a nearequatorial station in India. The Na emission intensity exhibits significant temporal variability within a day and from one day to another. The Na intensity has been compared with the daytime OH airglow intensity at 731.6 nm simultaneously measured using the unique Multiwavelength Dayglow Photometer (MWDPM). The temporal variabilities of the Na and OH intensities corroborate each other in most of the occasions without any appreciable time delay, although a small time delay up to ∼10 min has been observed at times. It appears that the variability of daytime Na emissions is influenced too by the photo-chemical excitations. The observed similarities in the Na and OH intensities are believed to be due to the similar roles played by [O] and O 3 (in their photochemical excitations) and the dynamics occurring at about the same altitude region (∼90 km). It has also been found that the gravity waves play significant role in producing the short-scale oscillations in these emissions.

Evidence for simultaneous changes in the thermosphere and mesopause associated with penetration electric fields using the OI 630.0 nm and OH (8, 3) dayglow measurements are presented. An unusual decrease in the thermospheric OI 630.0 nm dayglow and an intense heating (~30 K) at OH emission altitudes over a magnetic dip equatorial station, Trivandrum, India, were observed on 9 April 2006, coincident with the penetration of a noontime westward interplanetary electric field. The Sounding of the Atmosphere using Broadband Emission Radiometry observations on board TIMED satellite also revealed heating at the mesopause heights during this period. In addition, ionosonde and magnetometer observations indicated the presence of strong penetration electric fields. We propose that strong heating at the mesopause resulted in the intrusion of additional neutrals, like N 2 , into the emission altitudes and quenched the O( 1 D) atoms therein. The reduced Cowling conductivity at the E region inferred using magnetometer observations further corroborates this proposed mechanism.

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.

A case study is performed to investigate the probable reasons behind substantial daytime sodium (Na) D 1 airglow intensity (589.6 nm) variations measured using a ground-based monochromator during the three near consecutive days of February 2007 from Trivandrum (8.5°N, 77°E), India. The roles of both the resonance fluorescence and the chemistry have been considered in this study. It appears that fluorescence plays only a minor role towards the observed five to nine times of large intensity variations among these days. From investigations on the role of chemistry, it seems that through the Chapman chemical scheme, Na abundance contribute favorably, while the O 3 concentrations and the ambient temperature do not play any role as such for the observed intensity variations. From further investigations, it transpires that because of pressure differences (approximately 0.0002 to 0.0003 hPa/day) in the emitting altitude region among these days, the variations in collisional quenching of excited NaO * (A 2 Σ + ) (first excited electronic state of NaO that produces D line) contribute considerably towards the observed intensity variations. From consideration of all the possible factors, it appears that whereas resonance fluorescence plays only a minor role, chemistry has contributed to greater extent towards the observed significant intensity differences among these days.