A New Perspective to the Daytime h’F variations and its Role in Modulating the Mesopause Energetics over Equatorial latitudes Sumod.S.G 1 , T. K. Pant 1 , C.Vineeth 1 and M.M.Hossain 1 1 Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India sgsumodh@gmail.com,tarun_kumar@vssc.gov.in,cnvins@gmail.com,mosarraf_hossain@vssc.gov.in Abstract This study reports unique observations, illustrating the vertical coupling between the daytime mesopause and F region of the ionosphere over a magnetic dip equatorial station Trivandrum in India. For the ‘first time’, it has been shown that the temporal variations in the mean daytime mesopause temperatures (MPT), during geomagnetically quiet days corroborate well with that of the base height changes (h’F) of the ionosphere. However, there exist some characteristic time delays between these two, which vary from 0 to 90 minutes. The observed time delays are attributed to the role of chemistry/dynamics in modulating atomic oxygen at these altitudes. 1. Introduction In recent years, the atmosphere – ionosphere coupling has emerged as an important area of research globally. Comprehension of the nature of this coupling requires an understanding of various neutral and plasma processes. As is known, the mesopause is a crucial transition region that facilitates the coupling between the neutral dynamics dominated atmosphere and the electro-dynamically controlled ionosphere above. In general, the day-to- day variability in this atmosphere-ionosphere system during geomagnetically quite times are believed to be governed primarily by the dynamical forcings from below like the tides, gravity and planetary waves, and their interactions. It has been shown that the aforesaid forcings induce significant changes in the mesopause energetics and dynamics [14]. At the same time, the mesopause energetics itself was found to exhibit substantial changes in conjunction with the ionospheric process like equatorial Counter Electrojet [16]. By now it is well understood that the investigation of the variability in the mesopause region vis-à-vis changes in the ionosphere can lead to a better understanding of the prevailing atmosphere-ionosphere coupling at any given time. However, it had been difficult to measure the neutral parameters like the temperature in the mesopause, more so during daytime, due to the relative inaccessibility of this region to the existing ground-based techniques. In this context, the unique dayglow photometer developed in India had been successfully used for making daytime mesopause temperature measurements over the Indian longitudes in recent years [10, 16]. Presently, this dayglow photometer is operated from Trivandrum (8.5 o N; 77 o E; dip lat. 0.5 o N), India to get the estimate and, also the temporal variability of the mesopause temperature over the dip equator. Simultaneous, co-located ionospheric measurements are also made using a ground-based digital ionosonde. These simultaneous measurements indicate that during geomagnetically quiet days the variability in the mean trends of the daytime mesopause temperature (MPT) significantly corroborate with those of the simultaneously measured bottomside ionospheric height (h’F), but after a time delay.. The present paper aims at reporting these first observations and exploring plausible physical mechanisms for explaining the same. 2. Dataset The daytime mesopause temperatures were estimated from the OH Meinel (8-3) band dayglow intensities of rotational lines at 731.6 and 740.2 nm following the method of Meriwether [7]. The characteristics of the photometer and data analysis procedure had been published elsewhere [15]. The measurements at every ~7 minute intervals typically span ~7-8 hours during daytime between 0900 to 1700 hrs for the zenith sky. For the present study, the MWDPM observations of magnetically quiet days (Ap<7) for the period 2005-2007 are considered, Ap being the index representing planetary level of geomagnetic disturbance. Simultaneous measurements on the F region height variations (h’F) as scaled from the quarter hourly ionograms from the collocated digital ionosonde have been used for representing the ionospheric variability. 978-1-4244-6051-9/11/$26.00 ©2011 IEEE