Mulligan, F.J. and Lowe, R. P.
OH-equivalent temperatures derived from ACE-FTS and SABER
temperature profiles – a comparison with OH*(3-1) temperatures
from Maynooth (53.2 N, 6.4 W).
Annales Geophysicae (ANGEO), 26 (4).
OH-equivalent temperatures were derived from all of the temperature profiles retrieved in 2004 and 2005 by the ACE-FTS instrument in a 5 degree band of latitude centred
on a ground-based observing station at Maynooth. A
globally averaged OH volume emission rate (VER) profile
obtained from WINDII data was employed as a weighting
function to compute the equivalent temperatures. The annual
cycle of temperature thus produced was compared with the
annual cycle of temperatures recorded at the ground-based
station more than a decade earlier from the OH*(3-1) Meinel
band. Both data sets showed excellent agreement in the absolute value of the temperature minimum (~162 K) and in
its time of occurrence in the annual cycle at summer solstice.
Away from mid-summer, however, the temperatures diverged
and reach a maximum disagreement of more than 20K in
mid-winter. Comparison of the Maynooth ground-based data
with the corresponding results from two nearby stations in
the same time-period indicated that the Maynooth data are
consistent with other ground stations. The temperature difference between the satellite and ground-based datasets in winter was reduced to 14–15K by lowering the peak altitude of the weighting function to 84 km. An unrealistically low peak altitude would be required, however, to bring temperatures derived from the satellite into agreement with the ground-based data.
OH equivalent temperatures derived from the SABER instrument
using the same weighting function produced results that agreed well with ACE-FTS. When the OH 1.6μm VER profile measured by SABER was used as the weighting function, the OH equivalent temperatures increased in winter as expected but the summer temperatures were reduced resulting in an approximately constant offset of 8.6±0.8K between ground and satellite values with the ground values higher. Variability in both the altitude and width of the OH layer within a discernable seasonal variation were responsible
for the changes introduced. The higher temperatures
in winter were due to primarily to the lower altitude of the
OH layer, while the colder summer temperatures were due
to a thinner summer OH layer. We are not aware of previous
reports of the effect of the layer width on ground-based
Comparison of OH-equivalent temperatures derived from
ACE-FTS and SABER temperature profiles with OH*(3-1)
temperatures from Wuppertal at 51.3 N which were measured
during the same period showed a similar pattern to the Maynooth data from a decade earlier, but the warm offset
of the ground values was lower at 4.5±0.5 K. This discrepancy between temperatures derived from ground-based
instruments recording hydroxyl spectra and satellite borne
instruments has been observed by other observers. Further
work will be required by both the satellite and ground-based
communities to identify the exact cause of this difference.
||Atmospheric composition and structure; Airglow
and aurora; Pressure, density, and temperature; Instruments
||Faculty of Science and Engineering > Experimental Physics
Dr. Frank Mulligan
||29 Sep 2009 15:14
|Journal or Publication Title:
||Annales Geophysicae (ANGEO)
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