Fazlul I. Laskar

Vertical coupling of atmospheres: dependence on strength of sudden stratospheric warming and solar activity

Comprehensive behavior of the low-latitude upper atmosphere during sudden stratospheric warming (SSW) events at varying levels of solar activity has been investigated. The equatorial electrojet (EEJ) strength and the total electron content (TEC) data from low latitudes over Indian longitudes during the mid-winter season in the years 2005 to 2013 are used in this study. Five major and three minor SSW events occurred in the observation duration, wherein the solar activity had varied from minimum (almost no sunspots) to mini-maximum (approximately 50 sunspots of the solar cycle 24). Spectral powers of the large-scale planetary wave (PW) features in the EEJ and the TEC have been found to be varying with solar activity and SSW strengths. Specially, the spectral powers of quasi-16-day wave variations during the three very strong SSW events in the years 2006, 2009, and 2013 were found to be very high in comparison with those of other years. For these major events, the amplitudes of the semi-diurnal tides and quasi-16-day waves were found to be highly correlated and were maximum around the peak of SSW, suggesting a strong interaction between the two waves. However, this correlation was poor and the quasi-16-day spectral power was low for the minor events. A strong coupling of atmospheres was noted during a relatively high solar activity epoch of 2013 SSW, which was, however, explained to be due to the occurrence of a strong SSW event. These results suggest that the vertical coupling of atmospheres is stronger during strong major SSW events and these events play an important role in enabling the coupling even during high solar activity.

Does sudden stratospheric warming induce meridional circulation in the mesosphere thermosphere system

Oxygen dayglow emissions at multiple wavelengths that emanate from different heights (from around 130 km to peak altitude of the ionospheric F region) over a low-latitude location showed systematic enhancements in intensities throughout the daytime hours during four sudden stratospheric warming (SSW) events that occurred in the years 2010–2013. The lower thermospheric temperatures at 120 km obtained from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument are found to be enhanced during SSW events at arctic latitudes and show a gradient with a decrease toward low-latitudes. During these events, the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics Doppler Interferometer measurements showed equatorward winds in the mesosphere lower thermosphere (MLT) altitudes over high-latitudes. Both the high-latitude lower thermospheric temperature enhancements and the MLT region equatorward winds occur nearly simultaneously with the observed enhancements in the atomic oxygen dayglow emission intensities at all the wavelengths over low-latitudes. Based on these measurements and other supporting information, it is proposed that a new cell of meridional circulation in the MLT winds is set up during SSW events, which enables transport of atomic oxygen from high-to-low latitudes. Such an additional contribution of oxygen density over low-latitudes interacts with lower thermospheric daytime dynamics in that region and is attributed to be the cause for the observed enhancement in the oxygen daytime optical emission intensities over low-latitudes. The results presented here provide experimental evidence to such circulation alluded to by earlier simulation studies.

Daytime wave characteristics in the mesosphere lower thermosphere region: Results from the Balloon‐borne Investigations of Regional‐atmospheric Dynamics experiment

Results obtained from a joint INDO-US experiment on the investigations of mesosphere/lower thermosphere wave dynamics using balloon-borne optical dayglow measurements in combination with ground-based optical, radio, and magnetometer data are presented. Ultraviolet OI 297.2 nm dayglow emissions that originate at ~ 120 km were measured from low-magnetic latitudes from onboard a balloon on 8 March 2010. This paper describes the details of a new spectrograph that is capable of making high spectral resolution (0.2 nm at 297.2 nm) and large (80°) field of view ultraviolet dayglow emission measurements and presents the first results obtained from its operation onboard a high-altitude balloon. Waves of scale sizes ranging from 40 to 80 km in the zonal direction were observed in OI 297.2 nm emissions. Meridional scale sizes of similar waves were found to be 200 km as observed in the OI 557.7 nm emissions that originate from ~ 100 km. Periodicities were also derived from the variations of equatorial electrojet strength and ionospheric height on that day. Common periodicities of waves (in optical, magnetic, and radio measurements) were in the range of 16 to 30 min, which result in intrinsic horizontal wave speeds in the range of 21 to 77 m s−1. It is argued that gravity waves of such scale sizes and speeds at these heights are capable of propagating well into the thermosphere because the background wind directions were favorable. These waves were potentially capable of forming the seeds for the generation of equatorial plasma irregularities which did occur on that night.