Atmospheric emissions of atomic and molecular oxygen have been observed since the middle of the 19th century. In the last decades, it has been shown that emissions of excited oxygen atom O(¹D) and molecular oxygen in electronically-vibrationally excited states O₂(b¹Σ⁺_g, v) and O₂(a¹Δ_g, v) are related by a unified photochemical mechanism in the mesosphere and lower thermosphere (MLT). The current study is performed in the framework of the state-of-the-art model of ozone and molecular oxygen photodissociation in the daytime MLT. In particular, the study includes a detailed description of the formation mechanism for excited oxygen components in the daytime MLT and presents the comparison of widely used photochemical models. The study also demonstrates new results such as i) new suggestions about possible products of collisional reactions of electronically-vibrationally excited oxygen molecules with atomic oxygen and ii) new estimates of O₂(b¹Σ⁺_g, v = 0 – 10) radiative lifetimes which are necessary for solving inverse problems in the lower thermosphere. Moreover, special attention is given to the Barth’s mechanism in order to demonstrate that its contribution to O₂(b¹Σ⁺_g, v) and O₂(a¹Δ_g, v) populations is neglectable in daytime conditions regardless of fitting coefficients. In addition, possible applications of the daytime oxygen emissions are presented, e.g., the altitude profiles O(³P), O₃ and CO₂ can be retrieved by solving inverse photochemical problems where emissions from electronically vibrationally excited states of O₂ are used as proxies. The funding of V.Y., R.M. and I.M. was partly provided by the Russian Fund for Basic Research (grant RFBR No. 20-05-00450).

How to cite: Yankovsky, V., Vorobeva, E., Manuilova, R., and Mironova, I.: Model of Daytime Oxygen Emissions in the Mesopause Region and Above: New Results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2656, https://doi.org/10.5194/egusphere-egu21-2656, 2021.

The airglow emission of the mesopause region comprises molecular bands and atomic lines in the near-ultraviolet to the near-infrared wavelength range, e.g. the prominent roto-vibrational OH bands, a weak FeO/NiO continuum, the green OI line, the NaD doublet and some others. Since ground-based astronomical facilites observe through the Earth's atmosphere, the fingerprint of these emissions is visible in astronomical spectra taken with a telescope.
We have assembled a comprehensive data set of about 100,000 spectra in total taken between 1st of October 2009 and 30th of September 2019 with the X-shooter spectrograph, which is mounted at the Very Large Telescope in the Chilean Atacama desert (24.6°S, 70.4°W). This instrument provides medium-resolution spectra covering the entire wavelength range from 0.3 to 2.5μm simultaneously by incorporating three spectral subranges (UVB: 0.3-0.56μm; VIS: 0.56-1.02μm; NIR: 1.02-2.5μm).

The X-shooter instrument was continuously in operation during the covered period and frequently used by astronomers. Thus, the temporal coverage of the available observations is very dense for astronomical data allowing various airglow studies on time scales from minutes to a full decade. Due to the simultaneously observed wide wavelength range, individual airglow emitters as well as correlations between them can be investigated in detail (cf. Noll et al. 2021, this session, for more information).

In this presentation we describe the properties and the calibration of this unique data set.

How to cite: Kausch, W., Noll, S., Kimeswenger, S., and Moehler, S.: A full decade (2009-2019) of continuous nightglow observations from the NUV to the NIR, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5682, https://doi.org/10.5194/egusphere-egu21-5682, 2021.

Chemiluminescent emission from the mesopause region between 75 and 105 km dominates the Earth's low-to-mid-latitude nocturnal radiation in the wavelength domain from the near-UV to the near-IR. This nightglow consists of various roto-vibrational bands of molecules such as hydroxyl and molecular oxygen as well as individual lines from atoms such as oxygen and sodium. In principle, each line shows an individual vertical emission profile with a characteristic mean peak height and a typical full width at half maximum of less than 10 km. The total emission rate, peak height, and shape of the different profiles depend on the temperature, density, and the concentrations of different chemical species, especially of atomic oxygen. As the state of the mesopause region is strongly affected by the solar activity (especially via the rate of hard UV photons that produce highly reactive radicals) and different kinds of passing waves such as tides and gravity waves that mainly originate in the lower atmosphere, nightglow is also highly variable and can, thus, be used to trace the different processes. Various ground- and space-based observing strategies have already been applied. However, recording the variations of many different (and especially weak) emission lines in parallel with good temporal coverage for perturbations with time scales from minutes to years is challenging. 

In this context, we have now achieved to process about 100,000 medium-resolution spectra with a wavelength coverage from 0.3 to 2.5 µm that were taken with the astronomical X-shooter spectrograph at the Very Large Telescope of the European Southern Observatory at Cerro Paranal in Chile between 2009 and 2019. This promising data set allows us to study the variability of hundreds of nightglow lines and mutual correlations on time scales from those related to gravity waves to those related to the solar activity cycle. We will show first results. The goal of the project will be a better understanding of the nightglow layering and the sensitivity of the different emissions to different kinds of changes in the atmospheric conditions. 

How to cite: Noll, S. and Kausch, W.: Variability of various nightglow emissions from about 100,000 VLT/X-shooter spectra, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6593, https://doi.org/10.5194/egusphere-egu21-6593, 2021.

Using a combination of different measurement techniques is important to understand the numerous processes happening in the MLT-region. One of those processes is the excitation of atomic sodium by reaction with ozone which leads to emission of electromagnetic radiation: a phenomenon called Airglow. Although the sodium excitation mechanism was already proposed in 1939 by Sidney Chapman and further investigation was done by a great number of scientists, there are still some key parameters that are not well-known today. One of those parameters is the branching ratio f_A which determines the amount of sodium in the excited state. Exact knowledge of this value would offer the opportunity to use Na-nightglow measurements to determine sodium profiles in the MLT-region. In this study we used both, satellite measurements and ground-based Lidar measurements to help approach a more reliable branching ratio f_A. By comparing measurements that were made by the two instruments OSIRIS on Odin (Satellite) and the Lidar of the Colorado State University (ground-based) we found a branching ratio f_A of 0.064 +- 0.028.

How to cite: Koch, J., Bourassa, A., Roth, C., Lloyd, N., Yuan, T., and She, C.-Y.: Combining satellite and lidar measurements to investigate the sodium nightglow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9430, https://doi.org/10.5194/egusphere-egu21-9430, 2021.

Simulations of the solar thermal migrating semidiurnal (SW2) tide in the mesosphere-lower-thermosphere (MLT) are compared against meteor wind observations from a longitudinal chain of high-latitude SuperDARN radars. The simulations span two full years and are performed using a 3D non-linear mechanistic primitive equation model. In our model, the background Middle Atmosphere is specified to daily mean zonal mean winds and temperatures from the Navy Global Environmental Model - High Altitude (NAVGEM-HA) meteorological analysis system. Thermal tides are forced from the surface to the thermosphere using 3-hourly temperature tendency fields from the Specified Dynamics Whole Atmosphere Community Climate Model With Thermosphere and Ionosphere Extension (SD-WACCMX). Our model accurately reproduces the observed seasonal cycle in the SW2 amplitude and phase, with the exception of summertime amplitudes being overestimated. Sensitivity studies reveal the impact of the seasonal variations in the background atmosphere and tidal forcing. The tropospheric forcing response is found to be highly sensitive to the seasonal variations in the background atmosphere, leading to strong amplification during the summer and mid-winter months. In contrast, the stratospheric forcing response is found to be much less sensitive to the background atmosphere, while being similar in magnitude to the tropospheric forcing response. Based on simulations using a zero-wind atmosphere, the impact of seasonal variations in the tidal forcing is found to be very small for both the tropospheric and stratospheric forcing response. Furthermore, the inclusion of an idealized surface friction profile is found to delay the phase of the tropospheric forcing response, which can strongly impact the simulated tide at MLT altitudes. Both the tropospheric forcing response and the surface friction specification are identified as being possible factors contributing to summertime amplitudes being overestimated.

How to cite: van Caspel, W. and Espy, P.: The mid- to high-latitude migrating semidiurnal tide: Results from a mechanistic tide model and SuperDARN observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1769, https://doi.org/10.5194/egusphere-egu21-1769, 2021.

Mesoscale variations of the rotational temperature of excited hydroxyl (OH*) are studied at altitudes 85 – 90  km using the data of spectral measurements of nightglow emission at Russian observatories Zvenigorod (56 ° N, 37°E.) in years 2004  –  2016, Tory (52 ° N, 103°E) in  2012  –  2017 and Maimaga (63° N,  130° E) in  2014 - 2019. The filtering of mesoscale variations was made by calculations of the differences between the measured values of OH* rotational temperature separated with time intervals of dt ~ 0.5 - 2 hr. Comparisons of monthly variances of the temperature differences for various dt allow us to estimate coherent and non-coherent in time components of the mesoscale temperature perturbations. The first component can be associated with mesoscale waves near the mesopause. The non-coherent component may be produced by instrument errors and atmospheric turbulence. The results allow us correcting the observed mesoscale temperature variances at all listed sites for contributions of instrumental and turbulent errors. Seasonal and interannual changes in the coherent component of mesoscale variances of the temperature at the observational sites are studied, which may reflect respective changes in the intensity of mesoscale internal gravity waves in the mesosphere and lower thermosphere region.

     The analysis of nightglows data was supported by the grant #19-35-90130 of the Russian Foundation for Basic Research. Hydroxyl nightglow data at the Tory site were obtained with the equipment of the Center for Common Use «Angara» http://ckp-rf.ru/ckp/3056/ at the ISTP SB RAS within budgetary funding from the Basic Research Program (Project 0278-2021-0003). Data of the “Geomodel” Resource Center of Saint-Petersburg State University were used.

How to cite: Popov, A. A., Gavrilov, N. M., Perminov, V. I., Pertsev, N. N., Medvedeva, I. V., Ammosov, P. P., Gavrilyeva, G. A., and Kaltovskoi, I. I.: Coherent and Non-Coherent Components of Mesoscale Variations of Hydroxyl Rotational Temperature near the Mesopause., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8336, https://doi.org/10.5194/egusphere-egu21-8336, 2021.

The four-dimensional local ensemble transform Kalman filter (4D-LETKF) data assimilation system for the whole
neutral atmosphere is updated to better represent disturbances with wave periods shorter than 1 day in the mesosphere and
10 lower thermosphere (MLT) region. First, incremental analysis update (IAU) filtering is introduced to reduce the generation
of spurious waves arising from the insertion of the analysis updates. The IAU is better than other filtering methods, and also
is commonly used for the middle atmospheric data assimilation. Second, the horizontal diffusion in the forecast model is
modified to reproduce the more realistic tidal amplitudes that were observed by satellites. Third, the Sounding of the
Atmosphere using Broadband Emission Radiometry (SABER) and Special Sensor Microwave Imager/Sounder (SSMIS)
15 observations in the stratosphere and mesosphere also are assimilated. The performance of the resultant analyses is evaluated
by comparing them with the mesospheric winds from meteor radars, which are not assimilated. The representation of
assimilation products is greatly improved not only for the zonal mean field but also for short-period and/or horizontally
small-scale disturbances. 

How to cite: Koshin, D., Sato, K., Kohma, M., and Watanabe, S.: An update on the 4D-LETKF data assimilation system for the wholeneutral atmosphere , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13005, https://doi.org/10.5194/egusphere-egu21-13005, 2021.

After several recent stratospheric sudden warming (SSW) events, the stratopause disappeared and reformed at a higher altitude, forming an elevated stratopause (ES). The relative roles of atmospheric waves in the mechanism of ES formation are still not fully understood. We performed a hindcast of the 2018/19 SSW event using a gravity-wave (GW) permitting general circulation model containing the mesosphere and lower thermosphere (MLT), and analyzed dynamical phenomena throughout the entire middle atmosphere. An ES formed after the major warming on 1 January 2019. There was a marked temperature maximum in the polar upper mesosphere around 28 December 2018 prior to the disappearance of the descending stratopause associated with the SSW. This temperature structure with two maxima in the vertical is referred to as a double stratopause (DS). We showed that adiabatic heating from the residual circulation driven by GW forcing (GWF) causes barotropic and/or baroclinic instability before DS formation, causing in situ generation of planetary waves (PWs). These PWs propagate into the MLT and exert negative forcing, which contributes to DS formation. Both negative GWF and PWF above the recovered eastward jet play crucial roles in ES formation. The altitude of the recovered eastward jet, which regulates GWF and PWF height, is likely affected by the DS structure. Simple vertical propagation from the lower atmosphere is insufficient to explain the presence of the GWs observed in this event.

How to cite: Okui, H., Sato, K., Koshin, D., and Watanabe, S.: Formation of the double stratopause and elevated stratopause associated with the major stratospheric sudden warming in 2018/19, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9322, https://doi.org/10.5194/egusphere-egu21-9322, 2021.

The quasi-two-day wave (Q2DW) is the strongest and most widely-studied planetary wave occurring in the mesosphere. Existing observational analyses are based on either single-satellite or -station approaches, which suffer from temporal and spatial aliasing, respectively. The current work implements and develops dual-station approaches to investigate the mesospheric Q2DWs  and their nonlinear interactions with tides using winds from two longitudinal sectors at 53°N latitude.  An 8-year composite analysis reveals seasonal and altitude variations of Q2DWs and their secondary waves (SWs) from nonlinear interactions with tides.   The Q2DWs maximize in local summer, whereas their 16hr and 9.6hr SWs appear more in winter.

How to cite: He, M., Forbes, J. M., Li, G., Jacobi, C., and Hoffmann, P.: Quasi-two-day waves at 53°N latitude, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14578, https://doi.org/10.5194/egusphere-egu21-14578, 2021.

Tides are crucially important to the dynamics of the MLT. Therefore, models which aim to span the whole atmosphere must be capable of reproducing these tides, making observations of tides vital to constrain model development. Here, we present a novel climatology of 12- and 24-hour tides, measured at heights of 80–100 km by a meteor radar over the Rothera Station, Antarctica (68°S, 68°W). We use these observations to test two GCMs: WACCM and eCMAM (the latter 24-hr only). Our observations reveal large-amplitude tides with strong seasonal variability. The 12-hour tide maximises around the equinoxes and the smaller-amplitude 24-hour tide maximises in summer.  WACCM reproduces 12-hour tidal amplitudes at 80 km well, but not their increase with height or equinoctial maxima, and reproduces the observed small variation in 24-hr tidal amplitude with height well but with anomalously-large amplitudes. eCMAM reproduces observed 24-hr tidal amplitudes and their small variation with height. Our observations also reveal sizeable day-to-day variability in tidal amplitude at planetary wave periods, which we suggest originates from non-linear tidal/planetary-wave coupling. Furthermore, we see notable differences between observed and model background winds which are not reproduced in the models; we propose these differences may arise from the lack of in-situ gravity-wave sources in the models.

How to cite: Dempsey, S. M., Hindley, N., Moffat-Griffin, T., Wright, C., Smith, A., Du, J., and Mitchell, N.: Winds and tides of the Antarctic mesosphere and lower thermosphere: One year of meteor-radar observations over Rothera (68°S, 68°W) and comparisons with WACCM and eCMAM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15631, https://doi.org/10.5194/egusphere-egu21-15631, 2021.

Specular meteor radars (SMRs) are a major ground-based instrument to study the mesosphere and the lower thermosphere (MLT) dynamics. The recently developed multi-static approach of SMRs allows maximising the number of measurements from different viewing angles, hence enabling the estimation of horizontal wind fields and their second-order statistics (power spectrum, momentum fluxes). We have installed the operational versions of these techniques in Germany, Peru and Argentina, called SIMONe (Spread-spectrum Interferometric Multistatic meteor radar Observing Network) systems. Here, we present a validation study of multi-static meteor radar analysis by using virtual radar systems on the upper-atmosphere extension of the ICOsahedral Non-hydrostatic (UA-ICON) general circulation model with a horizontal grid spacing of 5 km. This particular study is focusing on the estimates of gradients and vertical velocities with these multi-static systems.

How to cite: Charuvil Asokan, H., Luis Chau, J., Conte, J. F., Baumgarten, G., Vierinen, J., and Borchert, S.: Validation of multi-static meteor radar analysis using realistic mesospheric dynamics from UA-ICON model: Reliability of gradients and vertical velocities, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14635, https://doi.org/10.5194/egusphere-egu21-14635, 2021.

How to cite: Guarino, M. V., Feng, W., Gardner, C., Marsh, D., and Plane, J.: Wave-induced constituent transport in the middle and upper atmosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9679, https://doi.org/10.5194/egusphere-egu21-9679, 2021.

Noctilucent clouds are often cited as potential indicators of climate change in the middle
atmosphere. They owe their existence to the very cold summer mesopause region (~130K) at mid
and high latitudes. We analyze trends derived from the Leibniz-Institute Middle Atmosphere
Model (LIMA) and the MIMAS ice particle model (Mesospheric Ice Microphysics And tranSport model)
for the years 1871-2008 and for middle, high and arctic latitudes, respectively.
Model runs with and without an increase of carbon dioxide and water vapor (from methane oxidation)
concentration are performed. Trends are most prominent after ~1960 when the increase of both
carbon dioxide and water vapor accelerates. Negative trends of (geometric) NLC altitudes are primarily
due to cooling below NLC altitudes caused by carbon dioxide increase. Increases of ice particle
radii and NLC brightness with time are mainly caused by an enhancement of water vapor.
Several ice layer and background parameter trends are similar at high and arctic latitudes but are
substantially different at middle latitudes. This concerns, for example, occurrence rates, ice water
content (IWC), and number of ice particles in a column. Considering the time period after 1960,
geometric altitudes of NLC decrease by approximately 260m per decade, and brightness increases by
roughly 50% (1960-2008), independent of latitude. NLC altitudes decrease by approximately 15-20m
per increase of carbon dioxide by 1ppmv. The number of ice particles in a column and also at the
altitude of maximum backscatter is nearly constant with time. At all latitudes, yearly mean NLC
appear at altitudes where temperatures are close to 145+/-1K. Ice particles are present nearly
all the time at high and arctic latitudes, but are much less common at middle latitudes. Ice water
content and maximum backscatter are highly correlated, where the slope depends on latitude. This
allows to combine data sets from satellites and lidars. Furthermore, IWC and the concentration of
water vapor at the altitude of maximum backscatter are also strongly correlated. Results from
LIMA/MIMAS agree nicely with observations.

How to cite: Lübken, F.-J. and Baumgarten, G.: Trends in noctilucent clouds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4607, https://doi.org/10.5194/egusphere-egu21-4607, 2021.

Polar Mesospheric Summer Echoes (PMSE) are regions of enhanced radar backscatter at 80 to 90 km that are assumed to form in the presence of neutral air turbulence and charged ice particles as a result of spatial variations in the electron density. Changes in the electron temperature, as can be generated by the EISCAT heater, influence the electron diffusivity as well as the charging of the ice particles and both are parameters that influence the radar scattering. In many cases, an overshoot effect [1] can be observed when the backscattered power is reduced during heater-on and rises above the initial signal during heater-off. We present observations made on the 11-12 and 15-16 of August 2018 with the EISCAT VHF radar during PMSE conditions. The EISCAT heating facility, operated at 5.423 MHz, was run in identical cycles where the heater was on for 48 seconds and off for 168 seconds. The observations clearly show the overshoot effect, caused by the cyclic heating of PMSE.  The surface charge of the ice particles increases during the heater-on intervals because of the higher electron temperature. As the heater is turned off the electrons are quickly cooled. The dust particles, however, still carry a higher charge, i.e. more electrons, so that the electrons cannot immediately obtain the initial density distribution. The typical result is that the electron density gradients are increased, which in turn lead to increased radar scattering, an overshoot. During the heater off phase, dust and plasma conditions are expected to relax back to undisturbed conditions. A theory was developed by Havnes [1] to explain the overshoot and we use a dusty plasma code [2] based on this theory to calculate the overshoot curves. They agree well with the average of the observational data. There is clear indication that during high precipitation the PMSE cloud is not affected by the heater and accordingly does not show an overshoot effect. 

1.     Havnes, O. (2004). Polar Mesospheric Summer Echoes (PMSE) overshoot effect due to cycling of artificial electron heating. Journal of Geophysical Research: Space Physics, 109(A2).

2.     Biebricher, A., Havnes, O., Hartquist, T. W., & LaHoz, C. (2006). On the influence of plasma absorption by dust on the PMSE overshoot effect. Advances in Space Research, 38(11), 2541-2550.

How to cite: Gunnarsdottir, T., Poggenpohl, A., Havnes, O., and Mann, I.: Polar Mesospheric Summer Echoes (PMSE) during artificial heating, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7237, https://doi.org/10.5194/egusphere-egu21-7237, 2021.

Noctilucent Clouds (NLC) are observed since 1997 by a RMR lidar at a mid-latitude site at Kühlungsborn/Germany (54°N, 12°E). In June 2019, we detected the brightest NLC so far, having a backscatter coefficient at 532 nm of ~50^-10 /m/sr, while 2.5^-10 /m/sr is a typical value at this location. Another three NLC in that period reached a backscatter coefficient of more than 20^-10 /m/sr. These strong NLC allow, e.g., for high-resolved studies with temporal resolution of 10 seconds and vertical resolution of 45 m. We will show examples of high-frequency oscillations in our data that cannot be found with typical integration times of several minutes. The period in June 2019 was not only unique in terms of NLC brightness, but also regarding NLC occurrence. While the all-year average is ~6 %, the occurrence rate in 2019 was 13 % and, and 20% if we consider June only. In the past, we found an anti-correlation between solar activity and NLC occurrence: Increasing solar UV radiation results in enhanced radiative heating and photolytic water vapor destruction. However, the high number of NLC in 2019 can only partly be explained by solar activity, even if the Lyman-alpha flux was slightly lower compared to previous years. TIMED/SABER monthly averaged temperature profiles showed an unusual low mesopause in June 2019, related to lower-than-average temperatures below 83 km. We claim that this as the main reason for the comparatively frequent and bright NLC. At the same time, meridional wind data of our nearby meteor radar show only weak southward winds and even a wind reversal at 93 km, which is not typical for the season. We will discuss potential reasons for the strange dynamical situation. We note that the weather dependent lidar observations are in good agreement with the radar observations of ice particles, so-called Mesospheric Summer Echoes (MSE). Co-located radar observations also showed unusually large occurrence rates of MSE in June 2019 as well as the occasion of many MSE below 83 km altitude.

How to cite: Gerding, M., Baumgarten, G., Lübken, F.-J., Clahsen, M., and Zecha, M.: On the observation of very bright and abundant Noctilucent Clouds at Kühlungsborn/Germany (54°N, 12°E) in June 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8904, https://doi.org/10.5194/egusphere-egu21-8904, 2021.

The polar summer mesosphere is the Earth’s coldest region, allowing the formation of mesospheric ice clouds, potentially linked to climate change. These clouds produce strong radar echoes that are used as tracers of mesospheric dynamics. Here we report the first observations of extreme vertical drafts in the mesosphere, characterized by velocities larger than 40 m/s, i.e., more than five standard deviations larger than the observed wind variability. The morphology seems to resemble mesospheric bores, however the scales observed are much larger. Powerful vertical drafts, intermittent in space and time, emerge also in direct numerical simulations of stratified flows, predicting non-Gaussian statistics of vertical velocities. This evidence suggests that mesospheric bores might result from the interplay of gravity waves and turbulent motions. Our extreme event is interpreted as a mesospheric "super-bore", impacting mesospheric mixing and ice-formation, and would potentially impact planning of sub-orbital flights, and the investigation of biological material in the near space.

How to cite: Chau, J. L., Marino, R., Feraco, F., Urco, J. M., Baumgarten, G., Luebken, F.-J., Hocking, W. K., Schult, C., Renkwitz, T., and Latteck, R.: Extreme vertical drafts in the polar summer mesosphere: A super mesospheric bore?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9055, https://doi.org/10.5194/egusphere-egu21-9055, 2021.

Atmospheric structures due to gravity waves, turbulence, Kelvin Helmholtz instabilities, etc. in the mesosphere are being studied with a varying of ground-based and satellite-based instruments. At scales less than 100 km, they are mainly studied with airglow imagers, lidars, and radars. Typical radar observations have not been able to resolve spatial and temporal ambiguities due to the strength of radar echoes, the size of the system, and/or the nature of the atmospheric irregularities. In this work we observed spatially and temporally resolved structures of PMSE with unprecedented horizontal resolution, using the improved radar imaging accuracy of the Middle Atmosphere Alomar Radar System (MAARSY) with the aid of a multiple-input multiple output (MIMO) technique. The studies are performed in both the brightness of the mesospheric echoes and their Doppler velocities. The resolutions achieved are less than 1 km in the horizontal direction, less than 300m in altitude, and less than 1 minute in time, in an area of ~15km x 15km around 85km of altitude. We present a couple of wavelike monochromatic events, one drifting with the background neutral wind, and one propagating against the neutral wind. Horizontal wavelengths, periods, and vertical and temporal coverage of the events are described and discussed. A theory of stratified turbulence is employed in the present study. In particular, it is shown that the structure that propagates with the background wind is a large-scale turbulent KHI event.  Some important turbulence characteristics, such as a turbulent dissipation rate, buoyancy Reynolds number, and Froude number, support our conclusion.

How to cite: Latteck, R., Chau, J., Urco, M., Vierinen, J., and Avsarkisov, V.: High spatiotemporal radar observation of the polar summer mesosphere using MAARSY in a MIMO configuration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12948, https://doi.org/10.5194/egusphere-egu21-12948, 2021.

Ozone in the polar middle atmosphere is known to be affected by charged energetic particles precipitating into the atmosphere from the magnetosphere. In recent years there has been increased interest in the sources and consequences of electron precipitation into the atmosphere. Substorms are an important source of electron precipitation. They occur hundreds of times a year and drive processes which cause electrons to be lost into our atmosphere. The electrons ionise neutrals in the atmosphere resulting in the production of HO_x and NO_x, which catalytically destroy ozone. Simulations have examined substorm driven ozone loss and shown it is likely to be significant. However, this has not previously been verified from observations. Here we use polar mesospheric ozone observations from the Global Ozone Monitoring by Occultation of Stars (GOMOS) and Microwave Limb Sounder (MLS) instruments to investigate the impact of substorms. Using the superposed epoch technique we find consistent 10-20% reduction in mesospheric ozone in both data sets. This provides the first observational evidence that substorms are important to the ozone balance within the atmosphere. 

How to cite: Chapman-Smith, K., Seppälä, A., Rodger, C., and Hendry, A.: Observational evidence of polar mesospheric ozone loss following substorm events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-524, https://doi.org/10.5194/egusphere-egu21-524, 2021.

Atmospheric effects of solar proton events (SPEs) have been studied for decades, because their drastic impact can be used to test our understanding of upper stratospheric and mesospheric chemistry in the polar cap regions. For example, odd hydrogen and odd nitrogen are produced during SPEs, which leads to depletion of ozone in catalytic reactions, such that the effects are easily observed from satellites during the strongest events. Until recently, the complexity of the ion chemistry in the lower ionosphere (i.e., in the D region) has restricted global models to simplified parameterizations of chemical impacts induced by energetic particle precipitation (EPP). Because of this restriction, global models have been unable to correctly reproduce some important effects, such as the increase in mesospheric HNO₃ or the changes in chlorine species. Here we use simulations from the WACCM-D model, a variant of the Whole Atmosphere Community Climate Model, to study the statistical response of the atmosphere to the 66 strongest SPEs which occurred in the years 1989–2012. Our model includes a set of D-region ion chemistry, designed for a detailed representation of the atmospheric effects of SPEs and EPP in general. We use superposed epoch analysis to study changes in O₃, HO_x (OH + HO₂), Cl_x (Cl + ClO), HNO₃, NO_x (NO + NO₂) and H₂O. Compared to the standard WACCM which uses an ion chemistry parameterization, WACCM-D produces a larger response in O₃ and NO_x and a weaker response in HO_x and introduces changes in HNO₃ and Cl_x. These differences between WACCM and WACCM-D highlight the importance of including ion chemistry reactions in models used to study EPP. 

How to cite: Kalakoski, N., Verronen, P. T., Seppälä, A., Szeląg, M. E., Kero, A., and Marsh, D. R.: Statistical response of middle atmosphere composition to solar proton events in WACCM-D simulations: the importance of lower ionospheric chemistry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14579, https://doi.org/10.5194/egusphere-egu21-14579, 2021.

Energetic electron precipitation (EEP) ionizes the Earth's atmosphere and leads to production of nitric oxide (NO) throughout the polar Mesosphere and Lower Thermosphere (MLT). In this study we investigate the direct and indirect NO response to the EEP using the Whole Atmosphere Community Climate Model (WACCM) version 6. In comparison to observations from SOFIE / AIM (Solar Occultation For Ice Experiment / Aeronomy of Ice in the Mesosphere), we find that EEP production of NO in the D-region is well simulated when both medium energy electron precipitation and negative and cluster ion chemistry are included in the model. However, the main EEP production of NO occurs in the E-region, and there the observed and modeled production differ. This discrepancy impacts also the D-region due to downward transport of long lived NO. The transport across the mesopause is seasonally dependent, and WACCM’s underestimate of D-region NO is highest during winter when downwelling from above is strong. The drivers of this transport are further investigated by a sensitivity study of WACCM’s gravity wave forcing.

How to cite: Smith-Johnsen, C., Nesse Tyssøy, H., Marsh, D. R., Smith, A., and Maliniemi, V.: The role of energetic electron precipitation and background dynamics on the seasonal NO variability in the MLT region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4525, https://doi.org/10.5194/egusphere-egu21-4525, 2021.

Precipitating auroral and radiation belt electrons are considered an important part of the natural forcing of the climate system.  Recent studies suggest that this forcing is underestimated in current chemistry-climate models. The HEPPA III intercomparison experiment is a collective effort to address this point. Here, eight different estimates of medium energy electron (MEE) (>30 keV) ionization rates are assessed during a geomagnetic active period in April 2010.  The objective is to understand the potential uncertainty related to the MEE energy input. The ionization rates are all based on the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA/POES and EUMETSAT/MetOp spacecraft series. However, different data handling, ionization rate calculations, and background atmospheres result in a wide range of mesospheric electron ionization rates. Although the eight data sets agree well in terms of the temporal variability, they differ by about an order of magnitude in ionization rate strength both during geomagnetic quiet and disturbed periods. The largest spread is found in the aftermath of the geomagnetic activity.  Furthermore, governed by different energy limits, the atmospheric penetration depth varies, and some differences related to latitudinal coverage are also evident. The mesospheric NO densities simulated with the Whole Atmospheric Community Climate Model driven by highest and lowest ionization rates differ by more than a factor of eight. In a follow-up study, the atmospheric responses are simulated in four chemistry-climate models and compared to satellite observations, considering both the model structure and the ionization forcing.

How to cite: Nesse Tyssøy, H., Sinnhuber, M., Asikainen, T., Bender, S., Clilverd, M. A., Funke, B., van de Kamp, M., Pettit, J., Randall, C., Reddmann, T., Rodger, C. J., Rozanov, E., Smith-Johnsen, C., Sukhodolov, T., Verronen, P. T., Wissing, J. M., and Yakovchuk, O.: HEPPA III intercomparison experiment on electron precipitation impacts:  Estimated ionization rates during a geomagnetic active period in April 2010, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5287, https://doi.org/10.5194/egusphere-egu21-5287, 2021.

We present the initial results from investigation of polar mesospheric summer echoes (PMSE) spectra at 224 MHz observed by EISCAT VHF radar operated from Ramfjordmoen near Tromsø during July 2019. Since EISCAT UHF measurements were not available, we used the sudden enhancements in electron densities derived from the VHF observations above 90 km as indicators of particle precipitation. We note that the altitude extent of the PMSE increased along with an enhancement of the strength of the pre-existing PMSE. However, a closer examination reveals that the PMSE strengths vary significantly between different heights in the region of 80 to 90 km. Interestingly, the spectral widths show well separated regimes between the top and the bottom part of the PMSE layers following particle precipitation. In the altitudes where the maximum enhancement in PMSE backscatter occurred, there is no corresponding enhancement in the spectral widths. The frequency Doppler shifts showed alternating upward and downward motions without much difference before and after the particle precipitation. This indicates that the moderate levels of particle precipitation observed herein did not affect the vertical winds considerably. Further, after the particle precipitation subsided, the PMSE intensities continued to be stronger for a while.

How to cite: Narayanan, V. L., Mann, I., and Häggström, I.: Observations of spectra of polar mesospheric summer echoes at 224 MHz using EISCAT radar during particle precipitation events – Some case studies from July 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6382, https://doi.org/10.5194/egusphere-egu21-6382, 2021.

Energetic particle precipitation (EPP) impact on the middle atmospheric ozone chemistry plays potentially an important role in the connection between space weather and Earth's climate system. A variant of the Whole Atmosphere Community Climate Model (WACCM-D) implements a detailed set of ionospheric D-region chemistry instead of a simple parameterization used in the earlier WACCM versions, allowing to capture the impact of EPP in more detail, thus improving the model for long-term climate studies. Here, we verify experimentally the ion chemistry of the WACCM-D by analysing the middle atmospheric ozone response to the EPP forcing during well-known solar proton events (SPEs). We use a multi-satellite approach to derive the middle atmospheric sensitivity for the SPE forcing as a statistical relation between the solar proton flux and the consequent ozone change. An identical sensitivity analysis is carried out for the WACCM-D model results, enabling one-to-one comparison with the results derived from the satellite observations. Our results show a good agreement in the sensitivity between satellites and the WACCM-D for nighttime conditions. For daytime conditions, we find a good agreement for the satellite data sets that include the largest SPEs (max proton flux >10^4 pfu). However, for those satellite data-sets with only minor and moderate SPEs, WACCM-D tends to underestimate the sensitivity in daytime conditions. In summary, the comparisons WACCM-D ion chemistry, combined with the transportation, demonstrates a realistic representation of the SPE sensitivity of ozone, and thus provides a conservative platform for long-term EPP impact studies.

How to cite: Nilsen, K., Kero, A., Verronen, P., Szelag, M., Kalakoski, N., and Jia, J.: sensitivity of middle atmospheric ozone to solar proton events: comparison between climate model and satellites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10818, https://doi.org/10.5194/egusphere-egu21-10818, 2021.

How to cite: Verronen, P. T., Marsh, D. R., Szeląg, M. E., and Kalakoski, N.: Magnetic-local-time dependency of radiation belt electron precipitation: impact on ozone in the polar middle atmosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14120, https://doi.org/10.5194/egusphere-egu21-14120, 2021.

Polar Mesospheric Winter Echoes (PMWE) have been observed by VHF radars for quite some years. Until now, most of the studies were focussed on either major events, that occurred during solar and geomagnetic severely distorted conditions or statistical parameters like their seasonal and interannual occurrence rates as well as altitude distributions were investigated. However, especially the origin of PMWE and underlying processes are still under debate and further observations aim to contribute to this question. Recent PMWE observations with the MAARSY VHF radar included experiments using multiple beam directions to investigate the spatial structure and evolution of PMWE. Within this study we present results of MAARSY radar observations of PMWE layers complemented by simultaneous measurements by the Saura HF radar, located less than 20km apart. Major products of the Saura radar are horizontal winds and electron density within the D region. These parameters are important for both the formation and visibility of PMWE. The spectral width and localization of VHF and HF radar echoes for the presence of PMWE are analyzed and compared in the context of turbulence. Furthermore, observations during the solar minimum for the season 2019/2020 appear to be a suitable period to deepen the investigation of background conditions, excluding intensive geomagnetic disturbances.

How to cite: Renkwitz, T., Latteck, R., Strelnikova, I., Chau, J. L., and Strelnikov, B.: D region observations by VHF and HF radars to investigate Polar Mesospheric Winter Echoes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14463, https://doi.org/10.5194/egusphere-egu21-14463, 2021.