Subir Mandal

and 5 more

We investigated the variability of short-period (<1 hour) atmospheric gravity waves (AGWs) in the high-latitude mesosphere-lower-thermosphere/ionosphere (MLTI) region using OH (3,1) band emission data from the Advanced Mesospheric Temperature Mapper (AMTM) at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) in Norway. These OH intensity maps from January 2014, 2015, and 2016 were analysed to characterise AGW activity at these altitudes. We derived phase velocity spectra of AGWs by applying the Matsuda-transform to the OH intensity maps and calculated spectral power across different phase speed ranges and propagation directions to study the day-to-day and intraday variability of AGWs. Our results reveal significant differences in AGW activity between these years, with 2015 exhibiting lower spectral power and less variability compared to 2014 and 2016. The vertical propagation efficiency of AGWs was estimated using principles of critical-level filtering, incorporating winds from the ERA5 dataset for altitudes between 0–50 km. The relatively lower AGW activity and spectral power observed in the MLTI region in 2015 were associated with higher Arctic Oscillation (AO) index values, suggesting gravity wave filtering by eastward winds in the upper stratosphere. In contrast, lower AO index values in 2014 and 2016 indicated minimal filtering, leading to more diverse spectra of AGWs observed in the OH images. These findings highlight the strong influence of stratospheric wind structures on AGW variability in the high-latitude MLTI region during winter.

Masaru Kogure

and 9 more

Fabio Vargas

and 10 more

This paper presents the results of a campaign covering a week of observations around the July 2, 2019, total Chilean eclipse. The eclipse occurred between 1922–2146 UTC, with complete sun disc obscuration happening at 2038–2040 UTC (1638–1640 LT) over the Andes Lidar Observatory (ALO) at (30.3$^\circ$S,70.7$^\circ$W). Observations were carried out using ALO instrumentation to observe eclipse–induced effects on the mesosphere and lower thermosphere region (MLT) (75–105 km altitude). Several mesosphere-sounding sensors were utilized to collect data before, during, and after the eclipse, including a narrow‐band resonance‐fluorescence 3D winds/temperature Na lidar with daytime observing capability, a meteor radar observing horizontal winds continuously, a multi-color nightglow all-sky camera monitoring the OH(6,2), O$_2$(0,1), O($^1S$), and O($^1D$) emissions, and a mesosphere temperature mapper (MTM) observing the OH(6–2) brightness and rotational temperature. We have also utilized TIMED/SABER temperatures and ionosonde measurements taken at the University of La Serena’s Juan Soldado Observatory. We discuss the effects of the eclipse in the MLT, which can shed light on a sparse set of measurements during this type of event. Our results point out several effects of eclipse–induced changes in the atmosphere below and above but not directly within the MLT. These effects include an unusual fast, bow–shaped gravity wave structure in airglow images, MTM brightness as well as in lidar temperature, strong zonal wind shears above 100 km, the occurrence of a sporadic E layer around 100 km, and finally variations in lidar temperature and density and the presence of a descending sporadic sodium layer near 98 km.
Gravity waves (GWs) generated by orographic forcing, also known as mountain waves (MWs) have been studied for decades. First measured in the troposphere, then in the stratosphere, they were only imaged at mesospheric altitude in 2008. Their characteristics have been investigated during several recent observation campaigns, but many questions remain concerning their impacts on the upper atmosphere, and the effects of the background environment on their deep propagation. An Advanced Mesospheric Temperature Mapper (AMTM) and the Southern Argentina Agile MEteor Radar (SAAMER) have been operated simultaneously during the Austral winter 2018 from Rio Grande, Argentina (53.8°S). This site is located near the tip of South America, in the lee of the Andes Mountains, a region considered the largest MW hotspot on Earth. New AMTM image data obtained during a 6-month period show almost 100 occurrences of MW signatures penetrating into the upper mesosphere. They are visible ~30% of time at the height of the winter season (mid-May to mid-July). Their intermittency is highly correlated with the zonal wind controlled by the semi-diurnal tide, revealing the direct effect of the atmospheric background on MW penetration into the Mesosphere Lower Thermosphere (MLT, altitude 80-100 km). Measurements of their momentum fluxes (MF) were determined to reach very large values (average ~250 m/s), providing strong evidence of the importance and impacts of small-scale gravity waves at mesospheric altitudes.