Mid-Latitude Mesospheric Zonal Wave 1 and Wave 2 in Recent Boreal Winters

Planetary waves in the mesosphere are studied using observational data and models to establish their origin, as there are indications of their generation independently of waves in the stratosphere. The quantitative relationships between zonal wave 1 and wave 2 were studied with a focus on the mid-latitude mesosphere at 50°N latitude. Aura Microwave Limb Sounder measurements were used to estimate wave amplitudes in geopotential height during sudden stratospheric warmings in recent boreal winters. The moving correlation between the wave amplitudes shows that, in comparison with the anticorrelation in the stratosphere, wave 2 positively correlates with wave 1 and propagates ahead of it in the mesosphere. A positive correlation r = 0.5–0.6, statistically significant at the 95% confidence level, is observed at 1–5-day time lag and in the 75–91 km altitude range, which is the upper mesosphere–mesopause region. Wavelet analysis shows a clear 8-day period in waves 1 and 2 in the mesosphere at 0.01 hPa (80 km), while in the stratosphere–lower mesosphere, the period is twice as long at 16 days; this is statistically significant only in wave 2. Possible sources of mesospheric planetary waves associated with zonal flow instabilities and breaking or dissipation of gravity waves are discussed.

On the relationship of energetic particle precipitation and mesopause temperature

Energetic particle precipitation (EPP) has the potential to change the neutral atmospheric temperature in the mesopause region. However, recent results are inconsistent, leaving the mechanism and the actual effect still unresolved. In this study we have searched for electron precipitation events and investigated a possible correlation between D-region electron density enhancements and simultaneous neutral temperature changes. The rotational temperature of the excited hydroxyl (OH) molecules is retrieved from the infrared spectrum of the OH airglow. The electron density is monitored by the European Incoherent Scatter Scientific Association (EISCAT) Svalbard Radar. We use all available experiments from the International Polar Year (IPY) in 2007–2008 until February 2019. Particle precipitation events are characterized by rapid increases in electron density by a factor of 4 at an altitude range of 80–95 km, which overlaps with the nominal altitude of the infrared OH airglow layer. The OH airglow measurements and the electron density measurements are co-located. Six of the 10 analysed electron precipitation events are associated with a temperature decrease of 10–20 K. Four events were related to a temperature change of less than 10 K. We interpret the results in terms of the change in the chemical composition in the mesosphere. Due to EPP ionization the population of excited OH at the top of the airglow layer may decrease. As a consequence, the airglow peak height changes and the temperatures are probed at lower altitudes. The observed change in temperature thus depends on the behaviour of the vertical temperature profile within the airglow layer. This is in agreement with conclusions of earlier studies but is, for the first time, constructed from electron precipitation measurements as opposed to proxies. The EPP-related temperature change recovers very fast, typically within less than 60 min. We therefore further conclude that this type of EPP event reaching the mesopause region would only have a significant impact on the longer-term heat balance in the mesosphere if the lifetime of the precipitation was much longer than that of an EPP event (30–60 min) found in this study.

GRIPS-HI, a novel spectral imager for ground based measurements of mesopause temperatures

The NDMC (Network for the Detection of Mesospheric Change) is a global network of measurement sites dedicated to the surveillance of the mesopause region. One main objective of the network is the early identification of climate signals. A key parameter is the mesopause temperature which can be derived from the emission spectrum of a layer of vibrationally excited hydroxyl (OH) at an altitude of approximately 87 km 87\hspace{0.1667em}\text{km} . Foremost, emission lines in the SWIR regime between 1520 nm 1520\hspace{0.1667em}\text{nm} and 1550 nm 1550\hspace{0.1667em}\text{nm} are of interest for remote temperature sensing. This report deals with the development of a new generation of GRIPS instruments, which are commonly employed for the observation of mesopause temperatures. The new prototype demonstrates how the application of so called Spatial Heterodyne Interferometers (SHI) can overcome the limitations of currently used grating spectrometers, in terms of spectral resolution and optical throughput. The presented prototype proposes improvements in optical throughput and spectral resolution of about one order of magnitude, significantly reducing the uncertainties of the measured mesopause temperatures. Furthermore, an SHI can be built in monolithic configurations which are aligned and characterized once during assembly without the need of realignment at the measurement site. This makes SHI based instruments ideal for mobile applications.

Mid-Latitude Mesospheric Zonal Wave 1 and Wave 2 in Winter 2020–2021

Planetary waves in the mesosphere are studied using observational data and models to establish their origin, as there are indications of their generation independently of waves in the stratosphere. The quantitative relationships between zonal wave numbers m = 1 (wave 1) and m = 2 (wave 2) were studied with a focus on the mid-latitude mesosphere at 50N latitude. Aura Microwave Limb Sounder measurements were used to estimate wave amplitudes in geopotential height during the 2020–2021 winter major sudden stratospheric warming. The moving correlation between the wave amplitudes shows that, in comparison with the anticorrelation in the stratosphere, wave 2 positively correlates with wave 1 and propagates ahead of it in the mesosphere. A positive correlation r = 0.5–0.6, statistically significant at the 95% confidence level, is observed at 1–5-day time lag and in the 75–91 km altitude range, which is the upper mesosphere–mesopause region. Wavelet analysis shows a clear 8-day period in waves 1 and 2 in the mesosphere at 0.01 hPa (80 km), while in the stratosphere–lower mesosphere the period is twice as long at 16-days; this is statistically significant only in wave 2. Possible sources of mesospheric planetary waves are discussed.

A full decade (2009-2019) of continuous nightglow observations from the NUV to the NIR

<p>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.<br>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).</p><p>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).</p><p>In this presentation we describe the properties and the calibration of this unique data set.</p>

Variability of various nightglow emissions from about 100,000 VLT/X-shooter spectra

<p>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. </p><p>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. </p>