Optimizing hydroxyl airglow retrievals from long-slit astronomical spectroscopic observations

Astronomical spectroscopic observations from ground-based telescopes contain background emission lines from the terrestrial atmosphere's airglow. In the near infrared, this background is composed mainly of emission from Meinel bands of hydroxyl (OH), which is produced in highly excited vibrational states by reduction of ozone near 90km. This emission contains a wealth of information on the chemical and dynamical state of the Earth's atmosphere. However, observation strategies and data reduction processes are usually optimized to minimize the influence of these features on the astronomical spectrum. Here we discuss a measurement technique to optimize the extraction of the OH airglow signal itself from routine J-, H-, and K-band long-slit astronomical spectroscopic observations. As an example, we use data recorded from a point-source observation by the Nordic Optical Telescope's intermediate-resolution spectrograph, which has a spatial resolution of approximately 100m at the airglow layer. Emission spectra from the OH vibrational manifold from v′ = 9 down to v′ = 3, with signal-to-noise ratios up to 280, have been extracted from 10.8s integrations. Rotational temperatures representative of the background atmospheric temperature near 90km, the mesosphere and lower thermosphere region, can be fitted to the OH rotational lines with an accuracy of around 0.7K. Using this measurement and analysis technique, we derive a rotational temperature distribution with v′ that agrees with atmospheric model conditions and the preponderance of previous work. We discuss the derived rotational temperatures from the different vibrational bands and highlight the potential for both the archived and future observations, which are at unprecedented spatial and temporal resolutions, to contribute toward the resolution of long-standing problems in atmospheric physics

The impact of energetic electron precipitation on mesospheric hydroxyl during a year of solar minimum

In 2008 a sequence of geomagnetic storms occurred triggered by high-speed solar wind streams from coronal holes. Improved estimates of precipitating fluxes of energetic electrons are derived from measurements on board the NOAA/POES 18 satellite using a new analysis technique. These fluxes are used to quantify the direct impact of energetic electron precipitation (EEP) during solar minimum on middle atmospheric hydroxyl (OH) measured from the Aura satellite. During winter, localized longitudinal density enhancements in the OH are observed over northern Russia and North America at corrected geomagnetic latitudes poleward of 55°. Although the northern Russia OH enhancement is closely associated with increased EEP at these longitudes, the strength and location of the North America enhancement appear to be unrelated to EEP. This OH density enhancement is likely due to vertical motion induced by atmospheric wave dynamics that transports air rich in atomic oxygen and atomic hydrogen downward into the middle atmosphere, where it plays a role in the formation of OH. In the Southern Hemisphere, localized enhancements of the OH density over West Antarctica can be explained by a combination of enhanced EEP due to the local minimum in Earth's magnetic field strength and atmospheric dynamics. Our findings suggest that even during solar minimum, there is substantial EEP-driven OH production. However, to quantify this effect, a detailed knowledge of where and when the precipitation occurs is required in the context of the background atmospheric dynamics.publishedVersio

Electrojet Estimates From Mesospheric Magnetic Field Measurements

The auroral electrojet is traditionally measured remotely with magnetometers on ground or in low Earth orbit (LEO). The sparse distribution of measurements, combined with a vertical distance of some 100 km to ground and typically >300 km to LEO satellites, means that smaller scale sizes can't be detected. Because of this, our understanding of the spatiotemporal characteristics of the electrojet is incomplete. Recent advances in measurement technology give hope of overcoming these limitations by multi-point remote detections of the magnetic field in the mesosphere, very close to the electrojet. We present a prediction of the magnitude of these disturbances, inferred from the spatiotemporal characteristics of magnetic field-aligned currents. We also discuss how Zeeman magnetic field sensors (Yee et al., 2021) onboard the Electrojet Zeeman Imaging Explorer satellites will be used to essentially image the equivalent current at unprecedented spatial resolution. The electrojet imaging is demonstrated by combining carefully simulated measurements with a spherical elementary current representation using a novel inversion scheme.publishedVersio

Modelled effects of temperature gradients and waves on the hydroxyl rotational distribution in ground-based airglow measurements

Spectroscopy of the hydroxyl (OH) airglow has been a commonly used way to remotely sense temperatures in the mesopause region for many decades. This technique relies on the OH rotational state populations to be thermalized through collisions with the surrounding gas into a Boltzmann distribution characterized by the local temperature. However, deviations of the rotational populations from a Boltzmann distribution characterized by a single temperature have been observed and attributed to an incomplete thermalization of the OH from its initial, non-thermodynamic equilibrium distribution. Here we address an additional cause for the apparent amount of excess population in the higher rotational levels of the OH airglow brought about by integrating these OH emissions through vertical gradients in the atmospheric temperature. We find that up to 40% of the apparent excess population, currently attributed to incomplete thermalization, can be due to the vertical temperature gradients created by waves. Additionally, we find that the populations of the different upper vibrational levels are affected differently. These effects need to be taken into account in order to assess the true extent of non-thermodynamic-equilibrium effects on the OH rotational populations

The climatology of zonal wave numbers 1 and 2 planetary wave structure in the MLT using a chain of Northern Hemisphere SuperDARN radars

The zonal wave components 1 and 2 were extracted from the meridional wind along the latitude band of 51–66°N for the years 2000–2008 using eight Super Dual Auroral Radar Network (SuperDARN) radars spanning longitudes from 25°E to 150°W. Each extracted zonal component represents the superposition of all temporal periods with that zonal structure and indicates the total planetary wave energy available with that wave number. The Hovmöller diagrams show stationary as well as eastward and westward traveling planetary waves propagating in the background wind. The method used to detect the zonal planetary wave components in the SuperDARN data are detailed and validated using UK Meteorological Office data, which allows the evolution of S1 and S2 planetary wave energy between the stratosphere and mesosphere to be assessed. The climatology of zonal wave number 1 and 2 planetary wave activity in the mesosphere-lower thermosphere (MLT) is presented and compared to the activity in the stratosphere. The MLT climatology of the mean wind anomalies shows stronger planetary wave activity during winter and weaker activity during summer with enhancement around midsummer and autumn equinox. The climatology of the mean wind displays similar amplitudes apart from very strong S1 planetary wave amplitudes during summer. In addition planetary wave activity during winters with major and minor stratospheric warming events are examined and contrasted

Observations of planetary waves in the mesosphere-lower thermosphere during stratospheric warming events

This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW) activity in the mesosphere–lower thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain of eight SuperDARN radars at high northern latitudes that span longitudes from 150° W to 25° E and latitudes from 51 to 66° N. Zonal wave number 1 and 2 components were extracted from the meridional wind for the years 2000–2008. The observed wintertime PW activity shows common features associated with the stratospheric wind reversals and the accompanying stratospheric warming events. Onset dates for seven SSW events accompanied by an elevated stratopause (ES) were identified during this time period using the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). For the seven events, a significant enhancement in wave number 1 and 2 PW amplitudes near 95 km was found to occur after the wind reversed at 50 km, with amplitudes maximizing approximately 5 days after the onset of the wind reversal. This PW enhancement in the MLT after the event was confirmed using SD-WACCM. When all cases of polar cap wind reversals at 50 km were considered, a significant, albeit moderate, correlation of 0.4 was found between PW amplitudes near 95 km and westward polar-cap stratospheric winds at 50 km, with the maximum correlation occurring ∼ 3 days after the maximum westward wind. These results indicate that the enhancement of PW amplitudes near 95 km is a common feature of SSWs irrespective of the strength of the wind reversal

Optimizing hydroxyl airglow retrievals from long-slit astronomical spectroscopic observations

Astronomical spectroscopic observations from ground-based telescopes contain background emission lines from the terrestrial atmosphere's airglow. In the near infrared, this background is composed mainly of emission from Meinel bands of hydroxyl (OH), which is produced in highly excited vibrational states by reduction of ozone near 90km. This emission contains a wealth of information on the chemical and dynamical state of the Earth's atmosphere. However, observation strategies and data reduction processes are usually optimized to minimize the influence of these features on the astronomical spectrum. Here we discuss a measurement technique to optimize the extraction of the OH airglow signal itself from routine J-, H-, and K-band long-slit astronomical spectroscopic observations. As an example, we use data recorded from a point-source observation by the Nordic Optical Telescope's intermediate-resolution spectrograph, which has a spatial resolution of approximately 100m at the airglow layer. Emission spectra from the OH vibrational manifold from v′ = 9 down to v′ = 3, with signal-to-noise ratios up to 280, have been extracted from 10.8s integrations. Rotational temperatures representative of the background atmospheric temperature near 90km, the mesosphere and lower thermosphere region, can be fitted to the OH rotational lines with an accuracy of around 0.7K. Using this measurement and analysis technique, we derive a rotational temperature distribution with v′ that agrees with atmospheric model conditions and the preponderance of previous work. We discuss the derived rotational temperatures from the different vibrational bands and highlight the potential for both the archived and future observations, which are at unprecedented spatial and temporal resolutions, to contribute toward the resolution of long-standing problems in atmospheric physics

Observation of Quasiperiodic Structures in the Hydroxyl Airglow on Scales Below 100 m

Gravity waves are known to transport energy and momentum to the middle atmosphere. The breaking processes associated with divergence of fluxes of energy and momentum into the atmosphere occur on scales that cannot be resolved in models and therefore have to be parameterized. The question remains as to whether it is possible to use a turbulence model on scales below 100 m and what kind of turbulence model should that be. Here we use high spatial resolution observations of the OH nightglow located near 90‐km altitude from the Nordic Optical Telescope to observe quasiperiodic structures down to horizontal scales of 4.5 m. These results indicate that a Kolmogorov type of energy cascade model of turbulence with a −5/3 power law appears to be satisfied down to these short, 4.5‐m scales

Migrating tide climatologies measured by a high-latitude array of SuperDARN HF radars

This study uses hourly meteor wind measurements from a longitudinal array of 10 high-latitude SuperDARN high-frequency (HF) radars to isolate the migrating diurnal, semidiurnal, and terdiurnal tides at mesosphere–lower-thermosphere (MLT) altitudes. The planetary-scale array of radars covers 180∘ of longitude, with 8 out of 10 radars being in near-continuous operation since the year 2000. Time series spanning 16 years of tidal amplitudes and phases in both zonal and meridional wind are presented, along with their respective annual climatologies. The method to isolate the migrating tides from SuperDARN meteor winds is validated using 2 years of winds from a high-altitude meteorological analysis system. The validation steps demonstrate that, given the geographical spread of the radar stations, the derived tidal modes are most closely representative of the migrating tides at 60∘ N. Some of the main characteristics of the observed migrating tides are that the semidiurnal tide shows sharp phase jumps around the equinoxes and peak amplitudes during early fall and that the terdiurnal tide shows a pronounced secondary amplitude peak around day of year (DOY) 265. In addition, the diurnal tide is found to show a bi-modal circular polarization phase relation between summer and winter

SuperDARN Observations of Semidiurnal Tidal Variability in the MLT and the Response to Sudden Stratospheric Warming Events

Using meteor wind data from the Super Dual Auroral Radar Network (SuperDARN) in the Northern Hemisphere, we (1) demonstrate that the migrating (Sun‐synchronous) tides can be separated from the nonmigrating components in the mesosphere and lower thermosphere (MLT) region and (2) use this to determine the response of the different components of the semidiurnal tide (SDT) to sudden stratospheric warming (SSW) conditions. The radars span a limited range of latitudes around 60°N and are located over nearly 180° of longitude. The migrating tide is extracted from the nonmigrating components observed in the meridional wind recorded from meteor ablation drift velocities around 95‐km altitude, and a 20‐year climatology of the different components is presented. The well‐documented late summer and wintertime maxima in the semidiurnal winds are shown to be due primarily to the migrating SDT, whereas during late autumn and spring the nonmigrating components are at least as strong as the migrating SDT. The robust behavior of the SDT components during SSWs is then examined by compositing 13 SSW events associated with an elevated stratopause recorded between 1995 and 2013. The migrating SDT is seen to reduce in amplitude immediately after SSW onset and then return anomalously strongly around 10–17 days after the SSW onset. We conclude that changes in the underlying wind direction play a role in modulating the tidal amplitude during the evolution of SSWs and that the enhancement in the midlatitude migrating SDT (previously reported in modeling studies) is observed in the MLT at least up to 60°N