The climatology of atomic oxygen (O) in the MLT (mesosphere and lower thermosphere) is balanced by O production via photodissociation in the lower thermosphere and recombination in the upper mesosphere. The motivation here is to establish the intra-annual variation in the eddy diffusion coefficients and eddy velocity in the MLT based on the constituent climatology of the region. The analysis method, originally developed in the 60’s (Colegrove et al. 1965), was refined for a study of MLT global inter-annual variations in global mean values (Swenson et al. 2018, 2019, respectively) ,(S19). In the this study, the intra-annual cycle was divided into twenty-six (two-week) periods for each of three zones, the northern hemisphere (NH, 15 to 55 degrees ), southern hemisphere (SH, -15 to -55 degrees ), and the equatorial region (EQ, 15 to -15 degrees ). Sixteen years of SABER O density measurements (2002-2018) and MSIS 2.0 model N , O and T profiles (80-96 km) were determined for each of the periods and zones for determination of O eddy diffusion velocities and fluxes. Atomic oxygen diffusive fluxes ([O]*v, 80-96 km) are balanced by the continuity of chemical loss, but the intra-annual variation of k$_{zz}$ (determined from v ) and [O] are determined separately. The major findings include: 1) A dominant AO below 87 km in the NH and SH zones, with the largest variation in amplitude between winter and summer at 83 km. 2) A dominant SAO at all altitudes (80-96 km) in the EQ zone. 3) Intra-annual variability in the global average [O] and k$_{zz}$ contribute to variability of O eddy transport in the MLT.

We employ in this work the first airglow dataset registered at the Remote Optical Facility (ROF) in Culebra, Puerto Rico, during the descending phase of the solar cycle #24. From November 4, 2015, to September 26, 2019, observations were carried out during 633 nights at ROF using a small all-sky imager, while events were identified in 225 of 499 nights classified as clear. A quantitative analysis of these and their dependency by geophysical parameters (solar and geomagnetic activities) are the main focus of this study. We introduce an original statistical methodology that examines the unique features of the dataset and minimizes the cross-contamination of individual modulators onto one another, avoiding bias in the results. Our findings include a primary peak of occurrence in the December solstice and a secondary peak in the June solstice. We observed a remarkable correlation in the occurrence rate of the with the geomagnetic activity. A notable modulation of the occurrence rate with the solar activity is also found, which includes periods of correlation and anti-correlation depending on the season. This modulation has an annual component that is ~33% and ~83% stronger than the semi-annual and terannual components, respectively. We discuss these findings based on the behavior of the thermospheric neutral winds derived from 30 years of Fabry-Perot interferometer observations. Our results, which are valid for low to moderate solar activity, point out circumstances that might explain differences in previous climatological studies of nighttime

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.

Atomic oxygen (O) in the MLT (mesosphere and lower thermosphere) results from a balance between production via photo-dissociation in the lower thermosphere and chemical loss by recombination in the upper mesosphere. The transport of O downward from the lower thermosphere into the mesosphere is preferentially driven by the eddy diffusion process that results from dissipating gravity waves and instabilities. The motivation here is to probe the intra-annual variability of the eddy diffusion coefficient (k$_{zz}$) and eddy velocity in the MLT based on the climatology of the region, initially accomplished by \citeA{GarciaandSolomon1985a}. In the current study, the intra-annual cycle was divided into 26 two-week periods for each of three zones: the northern hemisphere (NH), southern hemisphere (SH), and equatorial (EQ). Sixteen years of SABER (2002-2018) and 10 years of SCIAMACHY (2002-2012) O density measurements, along with NRLMSIS\textsuperscript{\textregistered} 2.0 were used for calculation of atomic oxygen eddy diffusion velocities and fluxes. Our prominent findings include a dominant annual oscillation below 87 km in the NH and SH zones, with a factor of 3-4 variation between winter and summer at 83 km, and a dominant semiannual oscillation at all altitudes in the EQ zone. The measured global average k$_{zz}$ at 96 km lacks the intra-annual variability of upper atmosphere density data deduced by \citeA{Qian2009}. The very large seasonal (and hemispherical) variations in k$_{zz}$ and O densities are important to separate and isolate in satellite analysis and to incorporate in MLT models.