Seasonal variation in the correlation of airglow temperature and emission rate

The hydroxyl (OH) rotational temperature and band emission rate have been derived using year-round, ground-based measurements of the infrared OH nightglow from Sweden from 1991 to 2002. Recent work has suggested that, during the winter, all scales of dynamical variations of radiance and temperature arise from vertical motions, implying that the effective source concentrations of atomic oxygen are constant. The present data show correlations between temperature and radiance both during winter and summer that are consistent with those observed in that previous work. However, during the transition to summer there is a rapid decrease in the temperature and its variation that is not reflected in the band radiance, suggesting that only the shorter-scale variations are accompanied by significant vertical motion. This indicates that the shorter-scale dynamical variations occur against an independent, seasonally changing background temperature profile in a way that is consistent with that predicted by gravity-wave models

Airglow Derived Measurements of Q-Branch Transition Probabilities for Several Hydroxyl Meinel Bands

Spectroscopic measurements of the hydroxyl (OH) airglow emissions are often used to infer neutral temperatures near the mesopause. Correct Einstein coefficients for the various transitions in the OH airglow are needed to calculate accurate temperatures. However, studies showed experimentally and theoretically that the most commonly used Einstein spontaneous emission transition probabilities for the Q-branch of the OH Meinel (6,2) transition are overestimated. Extending their work to several Delta v = 2 and 3 transitions from v' = 3 to 9, we have determined Einstein coefficients for the first four Q-branch rotational lines. These have been derived from high resolution, high signal to noise spectroscopic observations of the OH airglow in the night sky from the Nordic Optical Telescope. The Q-branch Einstein coefficients calculated from these spectra show that values currently tabulated in the HITRAN database overestimate many of the Q-branch transition probabilities. The implications for atmospheric temperatures derived from OH Q-branch measurements are discussed

Tidal Modulation of the Gravitywave Momentum Flux in the Antarctic Mesosphere

Airglow imager and dynasonde/IDI radar wind measurements at Halley Station, Antarctica (76S, 27W) have been used to estimate the diurnal variation of the vertical fluxes of horizontal momentum carried by highfrequency atmospheric gravity waves. The cross-correlation coefficients between the vertical and horizontal wind perturbations were calculated from the sodium airglow imager data collected during four consecutive nights of near total darkness during July of 2000. These were combined with wind-velocity variances from coincident radar measurements to estimate the upper limit of the vertical flux of horizontal momentum during three-hour intervals throughout the period. The resulting momentum flux showed a marked semi-diurnal oscillation in the zonal and meridional components. Calculations of the momentum flux through the Na airglow show variations in period and phase consistent with the observations, implying that tidal propagation and modulated gravity-wave forcing may both affect observed wind variations. INDEX TERMS: 3332 Meteorology and Atmospheric Dynamics: Mesospheric dynamics; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); 3384 Meteorology and Atmospheric Dynamics: Waves and tides

The effect of energetic electron precipitation on middle mesospheric night-time ozone during and after a moderate geomagnetic storm

Using a ground-based microwave radiometer at Troll Station, Antarctica (72°S, 2.5°E, L = 4.76), we have observed a decrease of 20–70% in the mesospheric ozone, coincident with increased nitric oxide, between 60 km and 75 km altitude associated with energetic electron precipitation (E > 30 keV) during a moderate geomagnetic storm (minimum Dst of −79 nT) in late July 2009. NOAA satellite data were used to identify the precipitating particles and to characterize their energy, spatial distribution and temporal variation over Antarctica during this isolated storm. Both the ozone decrease and nitric oxide increase initiate with the onset of the storm, and persist for several days after the precipitation ends, descending in the downward flow of the polar vortex. These combined data present a unique case study of the temporal and spatial morphology of chemical changes induced by electron precipitation during moderate geomagnetic storms, indicating that these commonplace events can cause significant effects on the middle mesospheric ozone distribution

ALOHA-93 Measurements of Intrinsic AGW Characteristics Using the Airborne Airglow Imager and Groundbased Na Wind/Temperature Lidar

Monochromatic Acoustic Gravity Waves (AGWs) with periods \u3c 1 hour are a prevalent feature in the mesospheric airglow layers. These waves are important dynamically and energetically to the region where their temporal and spatial morphology are not well established. The purpose of this study is establish the intrinsic AGW characteristics over an extended region (as flown by the NCAR Electra aircraft) and to present the data in terms of the predicted spectral domain defined by the Brunt‐Vaisala frequency and the diffusive filtering limit proposed by Gardner [1994]. On October 21, 1993, observations were made from the NCAR Electra aircraft during a 6 hour flight in a large triangle N and W of Maui, for a integral distance of ∼3000 km. The entire area observed [∼1 M km²] had a monochromatic AGW propagating toward the NW and the western half had a SW propagating wave superimposed. These waves were also observed with the Michelson interferometer on the aircraft and an airglow imager at the Haleakala location during this time. Intrinsic phase velocities were computed where the Na Wind/Temperature (W/T) lidar at Haleakala provided a measure of the mean wind to compensate phase velocities observed with the imager. The data were tabulated and plotted in an AGW spectral reference frame and compared to cutoff conditions predicted by diffusive filtering theory

Dynamic and Chemical Aspects of the Mesospheric Na ‘Wall’ Event on 9 October 1993 During the ALOHA Campaign

On October 9, 1993, observations were made from the National Center for Atmospheric Research Electra aircraft during a flight from Maui, Hawaii, toward a low-pressure system NW of the island, a flight of 7 hours in total. The leading edge (wall) of a bright airglow layer was observed 900 km NW of Maui at 0815 UT, which was traveling at 75 m s−1 toward the SE, reaching Haleakala, Maui, about 3.25 hours later [see Swenson and Espy, 1995]. An intriguing feature associated with the event was the large increase in the mesospheric Na column density at the wall (∼180%). The enhancement was distributed over a broad region of altitude and was accompanied by significant perturbations in the Meinel (OH) and Na D line airglow emission intensities, as well as the temperature. This paper describes an investigation of the combined measurements from the aircraft and at Haleakala, including an analysis of the event using a gravity wave dynamic model. The modeled atmospheric variations associated with the leading edge of the “wall” wave are then applied to models of the neutral and ionic chemistry of sodium in order to establish whether the enhancement was caused by the release of atomic Na from a local reservoir species, as opposed to redistribution by horizontal convection. The most likely explanation for the Na release was the neutralization of Na+ ions in a sporadic E layer that was first transported downward by a large amplitude (≈10%) atmospheric gravity wave and then vertically mixed as the wave pushed the atmosphere into a super adiabatic state with associated convective instabilities and overturning

Mesospheric nitric oxide model from SCIAMACHY data

We present an empirical model for nitric oxide (NO) in the mesosphere (≈60–90&thinsp;km) derived from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartoghraphY) limb scan data. This work complements and extends the NOEM (Nitric Oxide Empirical Model; Marsh et al., 2004) and SANOMA (SMR Acquired Nitric Oxide Model Atmosphere; Kiviranta et al., 2018) empirical models in the lower thermosphere. The regression ansatz builds on the heritage of studies by Hendrickx et al. (2017) and the superposed epoch analysis by Sinnhuber et al. (2016) which estimate NO production from particle precipitation. Our model relates the daily (longitudinally) averaged NO number densities from SCIAMACHY (Bender et al., 2017b, a) as a function of geomagnetic latitude to the solar Lyman-α and the geomagnetic AE (auroral electrojet) indices. We use a non-linear regression model, incorporating a finite and seasonally varying lifetime for the geomagnetically induced NO. We estimate the parameters by finding the maximum posterior probability and calculate the parameter uncertainties using Markov chain Monte Carlo sampling. In addition to providing an estimate of the NO content in the mesosphere, the regression coefficients indicate regions where certain processes dominate.</p

Measurement of the 3He mass diffusion coefficient in superfluid 4He over the 0.45-0.95 K temperature range

We have measured the mass diffusion coefficient D of 3He in superfluid 4He at temperatures lower than were previously possible. The experimental technique utilizes scintillation light produced when neutron react with 3He nuclei, and allows measurement of the 3He density integrated along the trajectory of a well-defined neutron beam. By measuring the change in 3He density near a heater as a function of applied heat current, we are able to infer values of D with 20% accuracy. At temperatures below 0.7 K and for concentrations of order 10^{-4} we find D=(2.0+2.4-1.2)T^-(6.5 -/+ 1.2) cm^2/s, in agreement with a theoretical approximation.Comment: 8 pages, 5 figures. Submitted to Europhysics Letters and prepared in that journal's forma