P. J. Espy

Trends and variability of mesospheric temperature at high-latitudes

Abstract Using ground-based measurements of the hydroxyl (OH) Meinel (3,1) band nightglow near 1500 nm, nightly means of mesospheric temperature and OH radiance from 1991 to 1998 have been derived over Stockholm (59.5°N, 18.2°E). Time-series analysis techniques applied both to the eight-year data set as well as to an annual superposed epoch revealed several statistically significant periodic components. A trend analysis that included these periodic components revealed a small positive trend over the eight-year temperature time series. However, examining the trends on a month-to-month basis revealed positive trends during winter, small negative trends during equinox, and no significant trend during summer. This seasonal variability indicates that dynamic feedbacks, rather than radiative forcing of the mesosphere by infrared active gases, may dominate the response of the mesosphere to greenhouse gas emissions. In support of this an examination of the variability in the superposed epoch of OH temperature and radiance showed strong impulses near equinox. A simple gravity-wave transmission and dissipation model indicates that these are due in part to seasonal increases in the gravity-wave transmission of the lower atmosphere, and enhanced wave heating and mixing in the mesosphere.

Seasonal Variations of the Gravity Wave Momentum Flux in the Antarctic Mesosphere and Lower Thermosphere

Airglow imager and dynasonde/imaging Doppler interferometer (IDI) radar wind measurements at Halley Station, Antarctica (75.6degreesS, 26.6degreesW) have been used to estimate the seasonal variation of the vertical fluxes of horizontal momentum carried by high-frequency atmospheric gravity waves. The cross-correlation coefficients between the vertical and horizontal wind perturbations were calculated from sodium (Na) airglow imager data collected during the austral winter seasons of 2000 and 2001. These were combined with wind velocity variances from coincident radar measurements to estimate the daily averaged upper limit of the vertical flux of horizontal momentum due to gravity waves. The resulting momentum flux at the Na airglow altitudes, while displaying a large day-to-day variability, showed a marked rotation from the northwest to the southeast throughout the winter season. Calculations show that this rotation is consistent with seasonal changes in the wind field filtering of gravity waves below the Na airglow region. The calculations also indicate that while the magnitude of the meridional wind is small, this filtering leads to the observed seasonal changes in the meridional momentum flux.

Tidal modulation of the gravity‐wave momentum flux in the Antarctic mesosphere

Airglow imager and dynasonde/IDI radar wind measurements at Halley Station, Antarctica (76degreesS, 27degreesW) have been used to estimate the diurnal variation of the vertical fluxes of horizontal momentum carried by high-frequency 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.

Springtime transition in upper mesospheric temperature in the northern hemisphere

Abstract Airglow emission observations by the wind imaging interferometer (WINDII) on the Upper Atmosphere Research Satellite and three optical ground-based stations previously revealed a “springtime transition” in atomic oxygen. The transition is characterized by a rapid 2-day rise in the night-time oxygen nightglow emission rate by a factor of 2 to 3 followed by a subsequent decrease by a factor of 10 in the same period of time indicating a depletion of atomic oxygen that persists for days. The current study examines signatures in the upper mesosphere temperature field (70– 95 km height range), derived from the WINDII Rayleigh scattering observations, which may be associated with this springtime depletion of the atomic oxygen. Comparisons with ground-based OH airglow rotational temperatures, Na lidar and Rayleigh scattering lidar temperatures, and meteor radar temperatures at middle and high latitudes in the Northern Hemisphere are presented and discussed. Data from the northern springtimes in 1992 and 1993 are reported upon in detail. It was found that all datasets used in the study agree well with each other taking account of the day/night time mean differences. A rapid temperature enhancement was observed at spring equinox at northern midlatitudes followed by a period of mean temperature colder than the one observed prior to the enhancement event, a pattern similar to that associated with the “springtime transition” observed in the oxygen emissions. The enhancement was also revealed in the average annual temperatures at 87 km , obtained by combining observations from 1992 to 1996, and in more recent temperature data from 1998 and 1999 at mid- and high northern latitudes. The results suggest that the temperature enhancement is associated with the last stratospheric warming event, observed at the end of March and early April.

Quasi‐biennial modulation of the semidiurnal tide in the upper mesosphere above Halley, Antarctica

We present an analysis of a long-term archive of horizontal wind data derived from meteor wind observations from a SuperDARN radar at Halley, Antarctica (76S, 27W). Systematic differences between the 12-hour component in the meridional wind and the climatological mean are observed showing evidence of a quasi-biennial modulation of the high-latitude semidiurnal tide in the upper mesosphere. The amplitude of the observed tides is enhanced when the equatorial stratospheric quasibiennial oscillation above 10 hPa is westerly. This enhancement is greatest in the summertime tidal amplitudes when the zonal wavenumber one (S = 1) nonmigrating component dominates the semidiurnal wind field, and is coincident with an enhancement of the summertime planetary wave activity in the upper mesosphere. These observations strongly support the hypothesis that the S = 1 component of the semidiurnal tide observed at high latitudes is due to a non-linear interaction between the migrating S = 2 semidiurnal tide and quasi-stationary S = 1 planetary waves.

Odd oxygen measurements during the Noctilucent Cloud 93 rocket campaign

The NLC-93 campaign at Esrange, Sweden, addressed the chemical, dynamical and electrical environment of the summer mesopause in the presence of noctilucent clouds (NLC). A major topic was the detailed investigation of odd oxygen abundances and their structure in the vicinity of the cloud layer. Applying independent in situ measurements, we have obtained consistent profiles of atomic oxygen, ozone, and mesospheric total density. Atomic oxygen was measured on two rocket flights by means of a newly developed probe utilizing the 1304 A resonance fluorescence / absorption technique. Detailed Monte Carlo simulations of the radiative transfer and the instrument aerodynamics are included in the data analysis. We find relatively low atomic oxygen densities with a peak of 2 × 1011 cm−3 at 96 km. An airglow radiometer recorded the O2 IR-atmospheric band emission, allowing the retrieval of ozone densities by means of an inversion model that considers both direct and indirect sources of O2 (a1Δg). The odd oxygen measurements are compared to a steady state photochemical model. While good agreement is found in the mesosphere, the results deviate close to the cold mesopause. Significant vertical structures are found in the atomic oxygen profiles; sharp gradients and distinct minima coincide with the observed NLC layer. Possible influences of heterogeneous chemistry on the odd oxygen abundance are discussed.

Equilibrium temperature of water-ice aerosols in the high-latitude summer mesosphere

Abstract Previous models of the equilibrium temperature and existence regions of mesospheric aerosols have shown significant radiative heating of the aerosols and, consequently, a substantially reduced existence region. We have developed an iterative model that extends this previous work by incorporating a complete collisional energy transfer algorithm, including the effects of vertical winds and particle fall velocity, that is appropriate for the free molecular flow conditions found in the mesosphere. We have also updated the ice refractive index used in the model and accounted for the dependence of the radiative heating and collisional cooling terms on particle temperature. Finally, a radiation model has been used to calculate the solar, terrestrial and atmospheric radiative inputs including the effects of multiple scattering and atmospheric absorption. As with the previous models, the particle temperature is calculated under steady-state conditions, assuming the background gas temperature remains constant and the aerosol does not change size, state or altitude. Under these conditions, the largest differences from previous models occur as a result of the updated ice index of refraction, particularly in the visible, which produces significantly less aerosol heating. These temperatures are combined with the observed properties of mesospheric aerosols to place limits on the water vapour mixing ratio, vertical-wind speeds, and maximum particle sizes. It is found that H2O mixing ratios of 10 ppmv and vertical winds of order 0.02 m s −1 are consistent with observed particle distributions, and these lead to a radiative limit on the maximum particle radius of 250 nm .

Polar cap observations of mesospheric and lower thermospheric 4-hour waves in temperature

1] Lower thermospheric and upper mesospheric rotational temperatures have been derived from ground-based measurements of the O-2(0-1) and OH(6-2) nightglow emissions over Resolute Bay, Canada ( 74.68degreesN, 94.90degreesW) and OH(4-2) nightglow emission over Kiruna, Sweden (67.90degreesN, 21.10degreesE). From measurements taken during two nights in November, 2001, we have observed a dominant and coherent 4-hr oscillation in both the O-2 and OH airglow emission rates and rotational temperatures. The phases for the two oscillation events remain almost constant at each location, indicating that these oscillations may not be caused by transient passage of instantaneous gravity waves. More importantly, there is little phase difference in universal time between the 4-hr oscillations in temperature at Resolute and Kiruna. The small phase difference gives a strong indication that the oscillation maybe related to a zonally symmetric tide, since any tidal waves in temperature with a nonzero zonal wavenumber are very weak at high latitudes. Waves in both events have large vertical wavelengths (76 and 175 km). The 4-hr wave shows almost no latitudinal variation from Resolute to Kiruna.

Observing the vertical branch of the mesospheric circulation at latitude 60°N using ground‐based measurements of CO and H2O

In this report daily ground-based measurements of vertical profiles of CO and H2O from 2002 are used to trace the vertical movement of air caused by the seasonally varying mean meridional circulation. In the spring (days 100-130) the estimated ascending velocity is found to vary with time and altitude from about between 250 m/day at 65 km altitude on day 100 to a maximum of similar to 450 m/day at 85-90 km altitude on day 130. In the late summer and fall the descending velocity goes from 0 (when the downwelling starts) to similar to 250 m/day at 75 km day 280. The mesospheric adiabatic temperature changes caused by the vertical movement of air are compared to mesopause temperature as estimated by the radiance of the Meinel band OH airglow.

Simultaneous measurements of the O2(¹Δ) and O2(¹Σ) Airglows and ozone in the daytime mesosphere

We report simultaneous measurements of the O2(¹Δ) and O2(¹Σ) airglow volume emission rate profiles in the daytime mesosphere. We use these measurements to derive ozone concentrations separately from each airglow. The measurements were made as part of the Mesosphere-Thermosphere Emissions for Ozone Remote Sensing (METEORS) sounding rocket project launched from White Sands Missile Range, New Mexico. These data offer the opportunity to assess the consistency of ozone profiles derived from measurements of the oxygen airglows and thus provide a fundamental test of our understanding of dayglow physics and solar energy deposition. In both airglow-derived profiles the ozone decreases with altitude throughout the middle mesosphere and a pronounced secondary maximum exists near 90 km. The derived ozone profiles are in agreement to within measurement uncertainty. This agreement confirms that the major processes responsible for the generation of mesospheric oxygen dayglow are well understood via measurement.