Nightglow Emission Research Articles

Spatial correlation of OH Meinel and O 2 infrared atmospheric nightglow emissions observed with VIRTIS-M on board Venus Express

We present the two-dimensional distribution of the O 2 a 1Δ–X 3Σ (0–0) band at 1.27 μm and the OH Δ v = 1 Meinel airglow measured simultaneously with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express. We show that the two emissions present very similar spatial structures. A cross-correlation analysis indicates that the highest level of correlation is reached with only very small relative shifts of the pairs of images. In spite of the strong spatial correlation between the morphology of the bright spots in the two emissions, we also show that their relative intensity is not constant, in agreement with earlier statistical studies of their limb profiles. We conclude that the two emissions have a common precursor that controls the production of both excited species. We argue that atomic oxygen, which produces O 2 ( 1Δ) molecules by three-body recombination and is the precursor of ozone formation, also governs to a large extent the OH airglow morphology through the H + O 3 → OH * + O 2 reaction.

Influence of the solar UV-radiation intensity on the 630-nm nightglow emission in the 23rd solar cycle

It is well known that the 630-nm nightglow emission intensity in midlatitudes increases by more than a factor of 2 during a sunspot maximum. It has been assumed that the phenomenon is caused by variations in solar UV radiation during a solar cycle (Fishkova, 1983).

Long-term trends in Antarctic winter hydroxyl temperatures

[1] Observations of the hydroxyl nightglow emission with a scanning spectrometer at Davis station, Antarctica (68°S, 78°E), have been maintained over each winter season since 1995. Rotational temperatures are derived from the P-branch lines of the OH(6–2) band near λ840 nm and are a layer-weighted proxy for kinetic temperatures near 87 km altitude. The current 16 year record allows tentative estimation of the atmospheric response in the mesopause region to solar cycle forcing and the underlying long-term linear temperature trend. Seven years of new data have been added since the last reported trend assessments using these data. A multivariate regression analysis on seasonally detrended winter mean hydroxyl temperatures yields a solar cycle coefficient of 4.8 ± 1.0 K/100 solar flux units (SFU) and a linear long-term cooling coefficient of −1.2 ± 0.9 K/decade. These coefficients are consistent within uncertainties for nightly, monthly, and annual mean trend evaluations. A distinct seasonal variation in trend coefficients is found in 30 day sliding window or monthly trend analyses. The largest solar activity response (∼7 K/100 SFU) is measured in March, May–June, and September, and there is little or no solar response in April and August. The long-term trend coefficient shows the largest cooling rate (4–5 K/decade) in August–September, through to warming (2–3 K/decade) for the March and May–June periods. Comparisons of trend results are made with other important hydroxyl measurement sites. Variability in the remaining residual temperatures is examined using lag correlation analyses for the influence of planetary waves, the quasi-biennial oscillation, the polar vortex intensity, and the southern annular mode.

Understanding the variability of nightside temperatures, NO UV and O2IR nightglow emissions in the Venus upper atmosphere

[1] Venus Express (VEX) has been monitoring key nightglow emissions and thermal features (O2 IR nightglow, NO UV nightglow, and nightside temperatures) which contribute to a comprehensive understanding of the global dynamics and circulation patterns above ∼90 km. The nightglow emissions serve as effective tracers of Venus' middle and upper atmosphere global wind system due to their variable peak brightness and horizontal distributions. A statistical map has been created utilizing O2 IR nightglow VEX observations, and a statistical map for NO UV is being developed. A nightside warm layer near 100 km has been observed by VEX and ground-based observations. The National Center for Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) has been updated and revised in order to address these key VEX observations and to provide diagnostic interpretation. The VTGCM is first used to capture the statistically averaged mean state of these three key observations. This correspondence implies a weak retrograde superrotating zonal flow (RSZ) from ∼80 km to 110 km and above 110 km the emergence of modest RSZ winds approaching 60 m s−1 above ∼130 km. Subsequently, VTGCM sensitivity tests are performed using two tuneable parameters (the nightside eddy diffusion coefficient and the wave drag term) to examine corresponding variability within the VTGCM. These tests identified a possible mechanism for the observed noncorrelation of the O2 and NO emissions. The dynamical explanation requires the nightglow layers to be at least ∼15 km apart and the retrograde zonal wind to increase dramatically over 110 to 130 km.

Nightglow imaging of different types of events, including a mesospheric bore observed on the night of February 15, 2007 from Tirunelveli (8.7°N)

All-sky imaging observations on the night of February 15, 2007, from Tirunelveli (8.7°N), revealed a variety of short-period small-scale dynamical features comprising of atmospheric gravity waves, ripples and a mesospheric bore. A total of 11 wave events other than the convective ripple patches were identified. All the ripple features of wavelengths less than 10km were found to propagate nearly in the same direction and they were frequent in the early part of the night. The presence of a dynamical ripple in all-sky images was also inferred. One of the identified events displayed steepening of the leading front and subsequent evolution into a mesospheric bore. Apart from these events, there were several ‘quasi-coherent’ features that were not clearly discernable for deduction of propagation velocities or wavelengths. In order to have a better understanding of these events, we used winds measured by co-located MF radar to derive intrinsic wave parameters. The MF radar winds and average of zenith intensity values from the imager indicate the presence of medium frequency waves of time periods in the range 0.75–3h. In addition, two sets of temperature measurements made by SABER instrument onboard TIMED mission were also used. The temperature profiles implied strong tidal control on this night. An attempt was also made to infer the source of the wave activity observed at mesospheric altitudes and the effects of wind filtering at altitudes below nightglow emission altitudes.

Effects of solar cycle variations on oxygen green line emission rate over Kiso, Japan

A sixteen year long dataset of mesospheric OI 557.7 nm green line nightglow emission rate, measured over Kiso (35.79°N, 137.63°E), Japan using ground-based photometers is spectrally investigated using the Hilbert-Huang Transform (HHT). The spectrograms reveal the presence of semi-annual, annual and quasi-biennial oscillations in consonance with the results obtained from wavelet analysis in an earlier study. In addition, due to the use of the HHT, we have been able to investigate the very low frequency solar cycle variation in the emission rate. It is found that there is a significant solar cycle effect on the oxygen green line emission rate. The mean amplitude of variation is approximately 20% and it is also found that it is maximum at midnight. A correlation study between the means of the emission rate and the solar radio flux at 10.7 cm also shows that the effect of solar activity on the oxygen green line emission rate is maximum at midnight.

Nighttime nitric oxide densities in the Southern Hemisphere mesosphere–lower thermosphere

Observations of NO2 continuum nightglow emissions from OSIRIS, on board the Odin satellite, have been used to derive an eight‐year time series of Southern Hemispheric nitric oxide densities, [NO]. OSIRIS is one of the very few current satellite instruments deriving ground state [NO] during the polar winter. The production of NO in this region is strongly dependent on energetic particle precipitation (EPP), and densities are therefore observed to vary with solar activity. Between 2003 and 2009, mean Antarctic winter [NO] in the mesosphere–lower thermosphere decreased by a factor of 3.7, and now, after the prolonged solar minimum that spanned 2008–2009, [NO] in this region is on the rise. As solar activity increases in the next few years, the production of NO is expected to increase. As downward advection readily transports NOx into the lower mesosphere and stratosphere, such an increase will lead to a greater potential for stratospheric ozone loss.

Characteristics of planetary-scale waves simulated by a new venusian mesosphere and thermosphere general circulation model

We have developed a new general circulation model (GCM) for the venusian mesosphere and thermosphere (80-about 180 km). Our GCM simulations show that winds in the subsolar-to-antisolar direction (SS–AS) are predominant above about 90 km. A weak return flow of the SS–AS is seen below about 90 km. We performed GCM simulations imposing the planetary-scale waves (thermal tides, Rossby wave, and Kelvin wave) at the lower boundary. Although the diurnal and semidiurnal tides are damped below 95 km, the Rossby wave propagates up to around 130 km. However, the amplitude of the Rossby wave is too small (<1 m/s) to affect the general circulation. On the other hand, the Kelvin wave propagates up to about 130 km with a maximum zonal wind fluctuation of approximately 5.9 m/s on average. The amplitude of the Kelvin wave sometimes exceeds 10 m/s around the terminator. The Kelvin wave causes a temporal variation in the wind velocity at the altitude of the O 2-1.27 μm nightglow emission (about 95 km). Using a newly developed 1-D nightglow model and the composition distribution calculated from our GCM, we investigated the impact of the Kelvin wave on the nightglow distribution. Our results suggest that the Kelvin wave would cause temporal variations in the nightglow emission in the 23:50–00:20 LT region with an intensity of 1.1–1.3 MR and a period of approximately 4 days.

Atomic oxygen distributions in the Venus thermosphere: Comparisons between Venus Express observations and global model simulations

Nightglow emissions provide insight into the global thermospheric circulation, specifically in the transition region (∼70–120 km). The O 2 IR nightglow statistical map created from Venus Express (VEx) Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) observations has been used to deduce a three-dimensional atomic oxygen density map. In this study, the National Center of Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) is utilized to provide a self-consistent global view of the atomic oxygen density distribution. More specifically, the VTGCM reproduces a 2D nightside atomic oxygen density map and vertical profiles across the nightside, which are compared to the VEx atomic oxygen density map. Both the simulated map and vertical profiles are in close agreement with VEx observations within a ∼30° contour of the anti-solar point. The quality of agreement decreases past ∼30°. This discrepancy implies the employment of Rayleigh friction within the VTGCM may be an over-simplification for representing wave drag effects on the local time variation of global winds. Nevertheless, the simulated atomic oxygen vertical profiles are comparable with the VEx profiles above 90 km, which is consistent with similar O 2 ( 1Δ) IR nightglow intensities. The VTGCM simulations demonstrate the importance of low altitude trace species as a loss for atomic oxygen below 95 km. The agreement between simulations and observations provides confidence in the validity of the simulated mean global thermospheric circulation pattern in the lower thermosphere.

Electronic kinetics of molecular oxygen in the nightglow

Altitude profiles of O2 nightglow emission intensities give information about composition of the atmosphere. Remote measurements of intensities of the emissions from satellites or rockets allow estimation of concentrations of atomic oxygen at the altitudes of the nightglow. Peculiarities of O2 electronic-state kinetics in the nightglow of the atmosphere are studied here. A model of the kinetics based on calculated quenching-rate coefficients for Herzberg states of O2 is suggested. The model considers intramolecular and intermolecular electron-energy transfer processes in molecular collisions as production and loss mechanisms of Herzberg states. The quenching-rate coefficients are applied in the calculation of vibrational populations of O2(A3Σu +), O2(A′3Δu) and O2(c1Σu −) in the nightglow. The function of the distribution of electronically excited molecular oxygen after the process of three-body recombination is estimated using experimental data and a least-squares fitting program. Calculated vibrational populations of A3Σu + and A′3Δu Herzberg states at an altitude of 95 km are compared with results of experimental measurements.

Global distributions of OH and O2 (1.27 μm) nightglow emissions observed by TIMED satellite

In order to investigate the global distributions of temporal variations of OH and O-2 nightglow emissions, we statistically analyzed their variations with altitude, local time, and season, using the OH and O-2 airglow emission rate data observed by the TIMED satellite between 2002 and 2009. The results indicated that the OH nightglow emission was stronger than dayglow emission and the O-2 nightglow emission was weaker than dayglow emission. In the tropics, the OH nightglow intensity reached its maximum near midnight; at higher latitudes, the OH nightglow intensities after sunset and before sunrise were much strong. At the equinoxes, the O-2 nightglow intensity in the tropics decreased with local time; at the solstices, the local time-latitude distribution of the O-2 nightglow intensity had a valley (with weak emission). As for the altitude-latitude distributions of nightglow emission rates, the distribution for OH nightglow at the equinoxes had one peak (with strong emission) at the equator, with a peak height around 85 km; the peak for the March equinox was stronger than that for the September equinox. The distribution for O-2 nightglow at the equinoxes had three peaks, lying at 30A degrees in the spring and autumn hemispheres and at the equator, and the peak height at the equator was the lowest. The distributions for both OH and O-2 nightglow emissions at the solstices had three peaks. Both nightglow intensities in the tropics had obvious annual and semi-annual variations, the peaks and valleys for semi-annual variations appeared near the equinoxes and solstices, respectively, and the peak at the March equinox was larger than that at the September equinox. The distributions of both OH and O-2 nightglow intensities showed a hemispheric asymmetry.

Initial ground-based thermospheric wind measurements using Doppler asymmetric spatial heterodyne spectroscopy (DASH)

We present the first thermospheric wind measurements using a Doppler Asymmetric Spatial Heterodyne (DASH) spectrometer and the oxygen red-line nightglow emission. The ground-based observations were made from Washington, DC and include simultaneous calibration measurements to track and correct instrument drifts. Even though the measurements were made under challenging thermal and light pollution conditions, they are of good quality with photon statistics uncertainties between about three and twenty-nine meters per second, depending on the nightglow intensity. The wind data are commensurate with a representative set of Millstone Hill Fabry-Perot wind measurements selected for similar geomagnetic and solar cycle conditions.

Two-dimensional time-dependent model of the transport of minor species in the Venus night side upper atmosphere

We present a numerical tool developed to quantify the role of processes controlling the spatio-temporal distribution of the NO ultraviolet and O 2 ( Δ g 1 ) infrared nightglows in the Venus night side upper atmosphere, observed with the VIRTIS and SPICAV instruments on board Venus Express. This numerical tool consists in a two-dimensional chemical-transport time-dependent model which computes in a hypothetical rectangular solving domain the spatio-temporal distributions of the number densities of the four minor species at play in these two nightglow emissions. The coupled nonlinear system of the four partial differential equations, describing the spatio-temporal variations of the minor species, has been solved using a finite volume method with a forward Euler method for the time integration scheme. As an application, we have first simulated a time-constant supply of atoms through the upper boundary of the solving domain. The fluxes are inhomogeneous relative to its horizontal direction, in order to simulate regions of enhanced downward flow of oxygen and nitrogen giving rise to NO and O 2 brightening. Given that these two emissions show large time variations, we have also simulated a time-dependent downward flux of O and N atoms. It results from these simulations that the lack of correlation between the NO and O 2 ( Δ g 1 ) nightglows largely result from to the coupling between horizontal and vertical transport processes and the very different chemical lifetimes of the two species. In particular, we have quantified the role of each process generating spatio-temporal de-correlations between the NO and O 2 ( Δ g 1 ) nightglows.

Atmospheric airglow fluctuations due to a tsunami‐driven gravity wave disturbance

A spectral full‐wave model is used to study the upward propagation of a gravity wave disturbance and its effect on atmospheric nightglow emissions. Gravity waves are generated by a surface displacement that mimics a tsunami having a maximum amplitude of 0.5 m, a characteristic horizontal wavelength of 400 km, and a horizontal phase speed of 200 m/s. The gravity wave disturbance can reach F region altitudes before significant viscous dissipation occurs. The response of the OH Meinel nightglow in the mesopause region (∼87 km altitude) produces relative brightness fluctuations, which are ∼1% of the mean for overhead viewing. The wave amplitudes grow as the wave disturbance propagates upward, which causes the thermospheric nightglow emission responses to be large. For overhead viewing, the brightness fluctuations are ∼50% and 43% of the mean for the OI 6300 Å and O 1356 Å emissions, respectively. The total electron content fluctuation is ∼33% of the mean for overhead viewing. For oblique viewing, the relative brightness fluctuations are slightly smaller than those obtained for overhead viewing. In spite of this, the thermospheric nightglow brightness fluctuations are large enough that oblique viewing could provide early warning of an approaching tsunami. Thus, the monitoring of thermospheric nightglow emissions may be a useful augmentation to other observational techniques of tsunami effects in the thermosphere/ionosphere system.

Seasonal and QBO variations in the OH nightglow emission observed by TIMED/SABER

Using TIMED/SABER observations, we present global distribution of the semiannual oscillation (SAO), annual oscillation (AO), and quasi‐biennial oscillation (QBO) in the OH nightglow peak emission rate and height as well as the intensity. The latitudinal variations of the SAO, AO, and QBO in the peak emission rate are similar to those in the intensity. For the peak emission rate and the intensity, the SAO and QBO amplitudes have three peaks (one at the equator and others at about 35°S and 35°N). The AO amplitude peaks at about 20°S and 20°N, respectively. The SAO phase is delayed poleward from the equinoxes at the equator to the solstices at 50°S/N; in addition, the phases of the AO are delayed poleward from 30°S. For the peak height, the SAO and QBO amplitudes have three peaks (around the equator, 40°S, and 40°N). Its AO amplitudes at 50°S and 50°N are larger than those at other latitudes; the phase of the SAO shifts from the solstice at the equator to near the equinoxes at 50°S/N. The airglow QBO is stronger in tropics than midlatitude and is likely the real QBO oscillation at the equator. In addition, the emission in the Southern Hemisphere is weaker than that in the Northern Hemisphere. The SAO and QBO are hemispherically symmetrical, and the AO is hemispherically antisymmetrical at some latitudes. The peak emission rate and peak height SAOs are generally in antiphase. The peak emission rate and intensity SAOs are generally in phase.

Hydroxyl airglow on Venus in comparison with Earth

Hydroxyl nightglow is intensively studied in the Earth atmosphere, due to its coupling to the ozone cycle. Recently, it was detected for the first time also in the Venus atmosphere, thanks to the VIRTIS-Venus Express observations. The main Δ ν=1, 2 emissions in the infrared spectral range, centred, respectively, at 2.81 and 1.46 μm (which correspond to the (1-0) and (2-0) transitions, respectively), were observed in limb geometry ( Piccioni et al., 2008) with a mean emission rate of 880±90 and 100±40 kR (1R=10 6 photon cm −2 s −1 (4 πster) −1), respectively, integrated along the line of sight. In this investigation, the Bates–Nicolet chemical reaction is reported to be the most probable mechanism for OH production on Venus, as in the case of Earth, but HO 2 and O may still be not negligible as mechanism of production for OH, differently than Earth. The nightglow emission from OH provides a method to quantify O 3, HO 2, H and O, and to infer the mechanism of transport of the key species involved in the production. Very recently, an ozone layer was detected in the upper atmosphere of Venus by the SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) instrument onboard Venus Express ( Montmessin et al., 2009); this discovery enhances the importance of ozone to the OH production in the upper atmosphere of Venus through the Bates–Nicolet mechanism. On Venus, OH airglow is observed only in the night side and no evidence has been found whether a similar emission exists also in the day side. On Mars it is expected to exist both on the day and night sides of the planet, because of the presence of ozone, though OH airglow has not yet been detected. In this paper, we review and compare the OH nightglow on Venus and Earth. The case of Mars is also briefly discussed for the sake of completeness. Similarities from a chemical and a dynamical point of view are listed, though visible OH emissions on Earth and IR OH emissions on Venus are compared.

The distributions of the OH Meinel and [formula omitted] nightglow emissions in the Venus mesosphere based on VIRTIS observations

O 2 ( a 1 Δ ) and recently discovered OH Meinel nightglow emissions have been observed at the limb with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M) instrument on board the Venus Express satellite. Hydroxyl bands belonging to Δ v = 1 sequence between 2.60 and 3.14 μ m and to Δ v = 2 sequence at 1.40 – 1.46 μ m have been unambiguously identified. In this study, we analyze the statistical distribution of the Δ v = 1 OH Meinel band sequence and the a 1 Δ g - X 3 Σ (0–0) band of the O 2 Infrared Atmospheric bands at 1.27 μm. We also present an analysis of the correlation between the two emissions. From a statistical point of view, we find that the limb intensity of both emissions reach their maximum value near the antisolar point, while they are significantly dimmer in the vicinity of the terminator. The average altitude of the limb emissions peaks are 95.3 ± 3 km and 96 ± 2.7 km, respectively for the OH Δ v = 1 sequence and O 2 ( a 1 Δ ) emissions. The average intensities are 0.41 ± 0.37 MR and 28 ± 22 MR, respectively, corresponding to a mean ratio of about 70. The altitude of the OH nightglow layer is closely related to that of the O 2 ( a 1 Δ ) emission and some level of co-variation of the maximum intensity along the line of sight is observed. It is suggested that the global subsolar to antisolar circulation plays a key in the control of both airglows by carrying oxygen atoms from the day to the night side of the planet. The O atoms recombine to produce O 2 ( a 1 Δ ) molecules and they also act as precursors of ozone whose reaction with H produces excited hydroxyl.

Inferring hydroxyl layer peak heights from ground-based measurements of OH(6-2) band integrated emission rate at Longyearbyen (78° N, 16° E)

Abstract. Measurements of hydroxyl nightglow emissions over Longyearbyen (78° N, 16° E) recorded simultaneously by the SABER instrument onboard the TIMED satellite and a ground-based Ebert-Fastie spectrometer have been used to derive an empirical formula for the height of the OH layer as a function of the integrated emission rate (IER). Altitude profiles of the OH volume emission rate (VER) derived from SABER observations over a period of more than six years provided a relation between the height of the OH layer peak and the integrated emission rate following the procedure described by Liu and Shepherd (2006). An extended period of overlap of SABER and ground-based spectrometer measurements of OH(6-2) IER during the 2003–2004 winter season allowed us to express ground-based IER values in terms of their satellite equivalents. The combination of these two formulae provided a method for inferring an altitude of the OH emission layer over Longyearbyen from ground-based measurements alone. Such a method is required when SABER is in a southward looking yaw cycle. In the SABER data for the period 2002–2008, the peak altitude of the OH layer ranged from a minimum near 76 km to a maximum near 90 km. The uncertainty in the inferred altitude of the peak emission, which includes a contribution for atmospheric extinction, was estimated to be ±2.7 km and is comparable with the ±2.6 km value quoted for the nominal altitude (87 km) of the OH layer. Longer periods of overlap of satellite and ground-based measurements together with simultaneous on-site measurements of atmospheric extinction could reduce the uncertainty to approximately 2 km.

Concurrent observations of the ultraviolet nitric oxide and infrared O2 nightglow emissions with Venus Express

Two prominent features of the Venus nightside airglow are the nitric oxide δ and γ bands produced by radiative association of O and N atoms in the lower thermosphere and the O2 infrared emission generated by three‐body recombination of oxygen atoms in the upper mesosphere. The O2 airglow has been observed from the ground, during the Cassini flyby, and with VIRTIS on board Venus Express. It now appears that the global structure of the two emissions shows some similarities, but the statistical location of the region of strongest emission is not coincident. The Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) ultraviolet spectrograph has collected a large number of spectra of the Venus nitric oxide nightside airglow. Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) images have been obtained at the limb and in the nadir‐viewing mode and have provided new information on the horizontal and vertical distribution of the emission. We present the first concurrent observations of the two emissions observed with Venus Express. We show that nadir observations generally indicate a low degree of correlation between the two emissions observed quasi‐simultaneously at a common location. A statistical study of limb profiles indicates that the altitude and the brightness of the two airglow layers generally do not covary. We suggest that this lack of correlation is explained by the presence of strong horizontal winds in the mesosphere‐thermosphere transition region. They carry the downflowing atoms over large distances in such a way that regions of enhanced NO emission generally do not coincide with zones of bright O2 airglow.

Evidence of mesospheric gravity‐waves generated by orographic forcing in the troposphere

We report on the first imaging observations of stationary mesospheric gravity‐waves in the three nightglow emissions of OH (695–1050 nm), Na (589.3 nm) and O(1S) (557.7 nm). The waves were observed in all‐sky images at the El Leoncito Observatory (ELO), Argentina (31.8°S, 69.3°W), during the period of 1–8 July 2008. Over the course of each night, the band‐like wave features exhibited a zero or near‐zero observed horizontal phase speed and had lifetimes of several hours. These factors, coupled with the winds measured locally, and the orientation of the waves to the Andes mountain range located nearby, suggested that the waves were mountain waves generated by ∼70 ms−1 eastward winds flowing over the Andes.