Abstract
The Einstein A coefficients are considered to be a significant source of uncertainty in the measurement of OH rotational temperatures. Using simultaneous ground and spaced-based observations of OH emission, five sets of Einstein A coefficients were examined for their impact upon rotational temperature calculations. The ground-based observations are taken from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) instrument which is a high resolution, r = λ / Δ λ ≥ 20 , 000 , spectrograph operating in the H-band from approximately 1.5 to 1.7 μ m. APOGEE collected over one-hundred-and-fifty-thousand spectra of the night sky over a period from June 2011 to June 2013. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite has made simultaneous atmospheric measurements with the APOGEE spectrograph. SABER observes the OH volume emission rate (VER) around 1.6 μ m, providing measurements coincident with those of the OH emission in the APOGEE sky spectra. Four of the five sets of Einstein A coefficients tested yielded statistically identical mean rotational temperatures of approximately 195 K for the OH ( 4 − 2 ) transition. The Einstein A coefficients were found to have a significant impact upon the measured OH ( v ′ = 4 ) vibrational populations with some sets of coefficients yielding populations over 50% greater. Simultaneous SABER observations were used to determine which set of Einstein A coefficients best reflected atmospheric temperatures, and four of the five tested coefficients yielded nearly identical results. The difference between OH rotational temperatures and SABER temperatures was on average 1 K.
Highlights
The measurement of OH rotational temperatures is used as a proxy for atmospheric temperatures near the mesopause, but significant problems exist in their measurement. von Savigny et al [1]demonstrated that the altitude of peak OH emission varied with vibrational level, and Cosby and Slanger [2], Noll et al [3], and Hart [4] all showed that measured OH rotational temperatures were strongly dependent upon the upper vibrational level
Demonstrated that the altitude of peak OH emission varied with vibrational level, and Cosby and Slanger [2], Noll et al [3], and Hart [4] all showed that measured OH rotational temperatures were strongly dependent upon the upper vibrational level
In this work we only examined the OH(4 − 2) transition as it matches the emission sampled by the SABER instrument
Summary
The measurement of OH rotational temperatures is used as a proxy for atmospheric temperatures near the mesopause, but significant problems exist in their measurement. von Savigny et al [1]. Liu et al [10] calculated the OH(9 − 4), OH(8 − 3), OH(6 − 2), OH(5 − 1), and OH(3 − 0) rotational temperatures using the transition probabilities from Mies [6], Turnbull and Lowe [8], Rothman et al [17], van der Loo and Groenenboom [16], and Langhoff [7]. These rotational temperatures were compared to the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) volume emission rate (VER). 2 years, and the complimentary observations allow for a deeper investigation into the effects that the Einstein A coefficients have upon ground-based OH rotational temperature measurements
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