Probing the Analytical Cancelation Factor of Short Scale Gravity Waves Using Na Lidar and Nightglow Data from the Andes Lidar Observatory

The cancelation factor (CF) is a model for the ratio between gravity wave perturbations in the airglow intensity to that in the ambient temperature. The CF model allows to estimate the momentum and energy flux of gravity waves seen in nightglow images as well as the divergence of these fluxes due to waves propagating through the mesosphere and lower thermosphere region, where the nightglow and the Na layers are located. This study uses a set of T/W Na Lidar data and zenith nightglow image observations of the OH and O(1S) emissions to test and validate the CF model from the experimental perspective. The dataset analyzed was obtained during campaigns carried out at the Andes Lidar Observatory (ALO), Chile in 2015, 2016, and 2017. The CF modeled function was compared with observed points from an empirical method for vertically propagating waves that calculates directly the ratio of the gravity wave amplitude seen in nightglow images to the wave amplitude seen in lidar temperatures. We show that the CF analytical relationship underestimates the observed results generally. However, the O(1S) emission line has better agreement respect to the theoretical value due to simpler nightglow photochemistry. In contrast, the observed CF ratio from the OH emission deviates by a factor of two from the modeled asymptotic value.

Titan’s Ultraviolet Airglow Variability with Solar Cycle and Saturn Local Time

<p>The Cassini spacecraft observed Titan’s upper atmosphere and its airglow emissions from 2005 to 2017. It is now established that the solar XUV radiation is the main source of dayglow, while magnetospheric particle precipitation principally acts on the nightside of the satellite. Nevertheless, one of the questions remaining unanswered after the end of the Cassini mission concerns the role and quantification of the magnetospheric particle precipitation and other minor sources such as micrometeorite precipitation and cosmic galactic ray at Titan. We report here on enhancements observed in Ultraviolet (UV) observations of Titan airglow made with the Cassini-Ultraviolet Imaging Spectrograph (UVIS). Enhancements are correlated with magnetospheric changing conditions occurring while the spacecraft, and thus Titan, are known to have crossed Saturn’s magnetopause and have been exposed to the magnetosheath environment. The processing and interpretation of 13+ years of airglow observations at Titan allows now for global studies of the upper atmosphere as a function of the Saturn Local Time (SLT) and the solar cycle.</p><p>Nitrogen airglow occur at about 1100 km of altitude in Titan’s upper atmosphere. Observations by the Cassini-UVIS instrument revealed the emission of the LBH band system, VK band system as well as Nitrogen atomic emission lines at 1085Å and 1493Å, as the prominent features of airglow emissions at Titan, as shown in Figures 1 and 2. Measurements were made at a wide range of solar incidence angles and Saturn Local Time (SLT), during the entire Cassini mission, allowing for the investigation of the upper atmosphere response to the magnetospheric environment and energetic particle precipitation. Additionally, observations were taken in a variety of solar condition, from solar maximum to minimum. UVIS observations of Titan around 12PM SLT (near Saturn’s magnetopause) present evidence of Titan’s upper atmosphere response to a fluctuating magnetospheric environment.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.9617eca672fe56938492951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=975f92d7d9d43faa47cacd77ad47438f&ct=x&pn=gnp.elif" alt=""></p><p><strong>Figure 1.</strong> Airglow intensity as a function of the saturn Local Time (SLT), for observation taken close the Saturn’s magnetopause (12PM SLT, labelled ‘12h’) and observations taken around miadnight SLT (labelled ‘24h’). Dayglow spectra exhibit higher averaged airglow intensity than Nightglow spectra.</p><p>We present here comparisons of the spectral emissions from the dayglow (Solar incidence angle <110°) and nightglow (Solar incidence angle ≥110°) between a rayheight of 900-1200 km around noon (±1 h) and around midnight (±1 h) SLT, during solar minima and maxima conditions (Fig. 2). Results show an enhancement of the airglow brightness with increasing particle precipitation, especially at SLT close to noon (i.e. close to the magnetopause), during solar maximum and minimum. Correlation between the ratio of the V-K, LBH, and NI-1493Å emission peaks are also presented.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.2357e48772fe52168492951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=2c6d843782e300fc27ec3db3de320caf&ct=x&pn=gnp.elif" alt=""></p><p><strong>Figure 2.</strong> Dayglow intensity as a function of the saturn Local Time (SLT) and solar cycle. Observations have been dispatched in four groups as a function of Titan’s orbital position within Saturn’s magnetosphere and maximum oe minimum stage of the solar cycle. Results suggest that solar maximum conditions around midgnight SLT favor the apparition of the brightest dayglow.</p><p>In the past decade, results from the Cassini-UVIS instrument greatly improved our understanding of airglow production at Titan. However, combining remote-sensing datasets, such as Cassini-UVIS data, with in-situ measurements taken by the Cassini Plasma Spectrometer (CAPS) instrument can provide us with a more rigorous assessment of the airglow contribution and correlations between data from simultaneous observations of in-situ Cassini instruments (CAPS, RPWS and MIMI) has been possible on few occasions. UVIS results present here will be put in context with results from in-situ simultaneous observations.</p><!-- COMO-HTML-CONTENT-END --> <p class="co_mto_htmlabstract-citationHeader"> <strong class="co_mto_htmlabstract-citationHeader-intro">How to cite:</strong> Royer, E., Cooper, M., Ajello, J., Esposito, L., and Crary, F.: Titan’s Ultraviolet Airglow Variability with Solar Cycle and Saturn Local Time, Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-415, 2020 </p>

Trends in the Airglow Temperatures in the MLT Region—Part 1: Model Simulations

Airglow intensity-weighted temperature variations induced by the CO2 increase, solar cycle variation (F10.7 as a proxy) and geomagnetic activity (Ap index as a proxy) in the Mesosphere and Lower Thermosphere (MLT) region were simulated to quantitatively assess their influences on airglow temperatures. Two airglow models, MACD-00 and OHCD-00, were used to simulate the O(1S) greenline, O2(0,1) atmospheric band, and OH(8,3) airglow temperature variations induced by these influences to deduce the trends. Our results show that all three airglow temperatures display a linear trend of ~−0.5 K/decade, in response to the increase of CO2 gas concentration. The airglow temperatures were found to be highly correlated with Ap index, and moderately correlated with F10.7, with the OH temperature showing an anti-correlation. The F10.7 and Ap index trends were found to be ~−0.7 ± 0.28 K/100SFU and ~−0.1 ± 0.02 K/nT in the OH temperature, 4.1 ± 0.7 K/100SFU and ~0.6 ± 0.03 K/nT in the O2 temperature and ~2.0 ± 0.6 K/100SFU and ~0.4 ± 0.03 K/nT in the O1S temperature. These results indicate that geomagnetic activity can have a rather significant effect on the temperatures that had not been looked at previously.

Mesospheric fronts in airglow images and the variation of the bottomside sodium layer densities measured by a sodium lidar at Tromsø, Norway

<p>Mesospheric frontal systems are waves extending to hundreds of kilometers along their phase fronts and appear like a boundary. They are observed in the upper mesospheric airglow imaging observations of OH, sodium and OI greenline nightglow emissions. It is believed that the fronts result from gravity wave dynamics associated with favorable background conditions like thermal ducting. Many of the frontal systems are identified as mesospheric bores when they are accompanied with sudden airglow intensity changes across the frontal boundary. Most of the frontal systems propagate with phase locked undulations following the leading front, while some induce turbulence behind the front. Though the existence of the frontal systems in the mesosphere is known for more than two decades, their role and importance is not understood properly. In this work, we use airglow data from an all-sky imager located at Tromsø to identify the frontal systems, particularly using OH images. Collocated five-beam sodium lidar measurements are used to identify the structuring in sodium densities around time of passage of the frontal systems. The sodium lidar at Tromsø is a versatile system capable of measuring sodium densities, temperatures and winds in the upper mesospshere region. Hence, we obtain the wind and temperature information to study the background conditions during passage of the intense frontal systems. Though, mostly we focus on OH airglow images as they are observed with broad pass band resulting in higher signal strength, we also utilize images from other emissions like OI greenline and sodium whenever they are available and free from auroral features. Interestingly, we find formation of some unusual structuring in the bottomside sodium layer around the passage of the frontal systems. We show different cases during winter months of the years 2013-14 and 2014-15 and investigate the relationship between unusual bottomside structuring in the sodium layer and passage of the frontal systems.</p>

Mesospheric OH layer altitude at midlatitudes: variability over the Sierra Nevada Observatory in Granada, Spain (37° N, 3° W)

The mesospheric OH layer varies on several timescales, primarily driven by variations in atomic oxygen, temperature, density and transport (advection). Vibrationally excited OH airglow intensity, rotational temperature and altitude are closely interrelated and thus accompany each other through these changes. A correct interpretation of the OH layer variability from airglow measurements requires the study of the three variables simultaneously. Ground-based instruments measure excited OH intensities and temperatures with high temporal resolution, but they do not generally observe altitude directly. Information on the layer height is crucial in order to identify the sources of its variability and the causes of discrepancies in measurements and models. We have used SABER space-based 2002–2015 data to infer an empirical function for predicting the altitude of the layer at midlatitudes from ground-based measurements of OH intensity and rotational temperature. In the course of the analysis, we found that the SABER altitude (weighted by the OH volume emission rate) at midlatitudes decreases at a rate of 40 m decade−1, accompanying an increase of 0.7 % decade−1 in OH intensity and a decrease of 0.6 K decade−1 in OH equivalent temperature. SABER OH altitude barely changes with the solar cycle, whereas OH intensity and temperature vary by 7.8 % per 100 s.f.u. and 3.9 K per 100 s.f.u., respectively. For application of the empirical function to Sierra Nevada Observatory SATI data, we have calculated OH intensity and temperature SATI-to-SABER transfer functions, which point to relative instrumental drifts of −1.3 % yr−1 and 0.8 K yr−1, respectively, and a temperature bias of 5.6 K. The SATI predicted altitude using the empirical function shows significant short-term variability caused by overlapping waves, which often produce changes of more than 3–4 km in a few hours, going along with 100 % and 40 K changes in intensity and temperature, respectively. SATI OH layer wave effects are smallest in summer and largest around New Year's Day. Moreover, those waves vary significantly from day to day. Our estimations suggest that peak-to-peak OH nocturnal variability, mainly due to wave variability, changes within 60 days at least 0.8 km for altitude in autumn, 45 % for intensity in early winter and 6 K for temperature in midwinter. Plausible upper limit ranges of those variabilities are 0.3–0.9 km, 40–55 % and 4–7 K, with the exact values depending on the season.