SA53A-01
Important Considerations When Using Hydroxyl Airglow Measurements to Determine Climate Trends of the Mesopause Region.
Spectral calibrations, airglow and possibly auroral contaminations, solar and telluric absorption features and the
selection of transition probabilities can all influence rotational temperatures derived from measurements of
hydroxyl airglow intensities. Consideration and examples are given of these influences. Measurements and
analyses are outlined for data checking that should be undertaken if a hydroxyl airglow data set is to be used to
determine climate trends.
Multiple spectral calibrations should be conducted throughout the observing period, with regular inter-
comparisons to other calibration sources also required. Uncertainties in spectral calibrations should be
expressed as a temperature equivalent.
Sufficient spectral scans at maximum resolution should be obtained under all extreme observing conditions (at
the lowest solar depression angle operated both morning and night, moon and cloud both separately and
combined, aurora and under conditions of enhanced atomic oxygen airglow, and under clear sky conditions but
with high atmospheric water vapour content) so that an uncertainty for the derived rotational temperatures can be
determined for the established data selection criteria.
Once the varying emission and absorption features for the hydroxyl region of interest at your site are understood
for the observing site, then the spectral resolution of the observing instrument can be reduced to increase
temporal resolution with reasonable confidence. This confidence should be tested by investigating the average
rotational temperatures derived from all possible line intensity ratios under the extreme observing conditions
noted. If a spectral-fitting rotational temperature determination is used, the residuals from the fit should be
summed and similarly examined.
Hydroxyl measurements provide a cost effective means of monitoring the temperature of the climate-sensitive
mesopause region on an almost nightly basis. If care is taken, they provide a valuable data set for investigating
climate change.
http:www.aad.gov.au
SA53A-02
The Long Term Trend and Solar Cycle Variation Observed in 12 Years of Hydroxyl Temperatures Over Davis, Antarctica.
Hydroxyl (6-2)-band rotational temperature observations have been accumulated at Davis station, Antarctica
(68°S, 78°E) over 12 consecutive years since 1995. Hydroxyl emissions originate in a layer
~8km thick near 87km altitude and the rotational temperatures derived are a proxy for atmospheric
temperature near the mesopause. This region is modelled to be sensitive to increases in CO2and is
expected to cool over the long term as the increased CO2radiates more absorbed energy to space.
Here we examine the seasonal and inter-year variability in hydroxyl temperatures and use a multiple linear
regression analysis to extract solar cycle and long term linear trend coefficients. A total of 3413 nightly average
temperatures are calculated from over 150,000 individual temperature measurements that pass selection criteria
over the 12 year interval. Winter average temperatures, calculated from the nightly averages between day 108 to
258 each year vary between 203 and 210K and show a solar cycle dependence of about 0.05 K/solar flux unit (or
6K per solar cycle). The long term linear trend in these data (-0.11±0.12 K/year) is not statistically different
from zero, in contrast to some published trends of up to -0.7 K/year.
The winter of 2002 was anomalously warm before the unusual southern hemisphere strat-warm and early ozone
hole break-up. Including this year has a considerable effect on the trend coefficients and our estimate of the
number of years required to detect a statistically significant trend.
http:www.aad.gov.au
SA53A-03
Helium 10830 Å airglow emission response to solar cycle
It has been suggested by Gadsden [1967] that the radiance of Helium 10830 Å line can be used to monitor the solar irradiance in the EUV. Conversely, we can use the 10830 Å line to study the response of the upper atmosphere over the course of a solar cycle. In this paper we will examine Helium emission line at 10830 Å to study the correlation between brightness of the emission line with F10.7 solar flux. The Helium data was acquired from Michelson Interferometers located at two stations, South Pole Station (90° S), Antarctica and Resolute Bay (74.68° N, 94.90° W), Canada. The data will be examined to isolate periods with no auroral contamination. Long term HeI data, from 1992 to 2005, will be presented to study correlation between Helium brightness and the F10.7 solar flux.
SA53A-04
1997 and 2006 Solar Minimum Comparisons of Geocoronal Hydrogen Data
Ground-based Fabry-Perot observations of the hydrogen Balmer-alpha emission have been used since the late 1970s to investigate hydrogen in the geocorona, spanning the upper thermosphere and exosphere. Atomic hydrogen in this region is a byproduct of hydrogen-containing species below such as methane and water vapor. Models have predicted 50-75 % increases in upper atmospheric hydrogen as a consequence of a doubling of tropospheric concentrations of methane, a primary greenhouse gas. The 11-year solar cycle is a dominant source of natural variability in the upper atmosphere and its effect on hydrogen distributions and emissions must be understood to investigate possible signs of longer-term climatic trends. We will discuss data from the present near-solar minimum winter compared with those from the previous near- solar minimum period of 1997, all taken with the same instrument, the Wisconsin H-alpha Mapper Fabry-Perot (Kitt Peak, AZ), and using the same nebular calibration source for absolute intensity calibration. The newer data are consistent with observations over the rise of the solar cycle with lower intensities observed during solar minimum compared with solar maximum conditions. We will also discuss preliminary comparisons with earlier data and the extra challenges associated with comparing data taken with different, though similarly designed instruments. The geocoronal hydrogen column emission observed by the Fabry-Perot is a function of the hydrogen density profile, the solar excitation flux, and radiative transfer including the contribution of multiple scattering below the Earth's shadow. We will discuss work in progress to use forward modeling to retrieve the hydrogen column abundance from the emission observations.
SA53A-05
Gravity wave seasonal variations and their influences on polar mesospheric clouds in Antarctica
Gravity waves (GWs) play an important role in the dynamics of global middle and upper atmosphere. Quantitatively characterizing GW in the upper stratosphere and mesosphere are still rare in Antarctica. In this paper we present a study of GWs using the lidar data obtained at South Pole (90°S) from December 1999 to January 2001 and at Rothera (67.5°S, 68.0°W) from December 2002 to March 2005 with the University of Illinois Fe Boltzmann/Rayleigh lidar. The root-mean-square (RMS) relative density perturbation in 30- 45 km derived from the Rayleigh lidar data is used to characterize the stratospheric GW strength. The obtained GW characteristics include vertical wavelength of 2-10 km, vertical phase velocity of 2-6 km/h, and the period of 0.5-3.5 hours at both Rothera and South Pole. Seasonal variations of GW strength at Rothera are observed to be larger than those at the South Pole. Averaged RMS relative density perturbation at Rothera is 1.19+/-0.38 in winter and 0.46+/-0.24 in summer, while averaged RMS relative density perturbation at South Pole is 0.81+/-0.22 in winter and 0.40+/-0.25 in summer. The observed larger difference in winter but nearly no difference in summer in the GW strength between Rothera and South Pole may be explained by the GW source difference and wind filtering effect. Topography of Rothera and South Pole is very different - featureless area at South Pole but mountain and coast area near Rothera. Thus, GW sources are expected to be stronger at Rothera than at South Pole, which results in larger stratospheric GW strength at Rothera than at the South Pole in winter, when wind filtering effect is very weak. However, due to the strong wind filtering effect in summer, the GWs that can reach stratosphere are similar at these two sites. It has been suggested that GW may influence the formation or occurrence of polar mesospheric clouds (PMC). An initial study using Rayleigh lidar at Sondrestrom (67.0°N, 50.9°W) indicates that the strengths of GW and PMC are negatively correlated. However, such a correlation has not been examined in Antarctica until now. The total backscatter coefficients (TBC) measured by the Fe lidar are used to represent PMC brightness in the mesopause region. We investigate the correlation between daily (instead of hourly) averaged TBC and RMS density perturbation. The derived linear correlation coefficient (LCC) is -0.37 with confidence level of 92% at Rothera and -0.09 with confidence level of 40% at South Pole. Although the obtained LCC is not statistically significant (confidence level higher than 95% is considered as significant), our data indicate a negative correlation between GW and PMC at Rothera. The data also indicate that there is no correlation between GW and PMC at the South Pole. This may be explained by the large temperature difference between Rothera and South Pole in summer mesopause region. As the Rothera temperature is close to the PMC formation threshold (~150 K) the temperature perturbation induced by GW can significantly alter the PMC formation or disappearance. Meanwhile, the temperature perturbation induced by GW may not drive the South Pole temperature above the threshold, thus, PMC brightness is not affected much.