In recent times it has become increasingly clear that
releases of trace gases from human activity have a potential
for causing change in the upper atmosphere. However,
our knowledge of systematic changes and trends in
the temperature of the mesosphere and lower thermosphere
is relatively limited compared to the Earths lower
atmosphere, and not much effort has been made to synthesize
these results so far. In this article, a comprehensive
review of long-term trends in the temperature of the region
from 50 to 100 km is made on the basis of the available
up-to-date understanding of measurements and model calculations.
An objective evaluation of the available data
sets is attempted, and important uncertainly factors are
discussed. Some natural variability factors, which are
likely to play a role in modulating temperature trends,
are also briefly touched upon. There are a growing number
of experimental results centered on, or consistent with,
zero temperature trend in the mesopause region (80–100
km). The most reliable data sets show no significant trend
but an uncertainty of at least 2 K/decade. On the other
hand, a majority of studies indicate negative trends in
the lower and middle mesosphere with an amplitude of
a few degrees (2–3 K) per decade. In tropical latitudes
the cooling trend increases in the upper mesosphere.
The most recent general circulation models indicate
increased cooling closer to both poles in the middle
mesosphere and a decrease in cooling toward the summer
pole in the upper mesosphere. Quantitatively, the
simulated cooling trend in the middle mesosphere produced
only by CO 2 increase is usually below the observed
level. However, including other greenhouse gases
and taking into account a “thermal shrinking” of the
upper atmosphere result in a cooling of a few degrees
per decade. This is close to the lower limit of the observed
nonzero trends. In the mesopause region, recent
model simulations produce trends, usually below 1 K/decade,
that appear to be consistent with most observations
in this region

/pdf/review-of-mesospheric-temperature-trends-8sjapsrnjq.pdf

Review of mesospheric temperature trends

A new line list including positions and absolute transition strengths (in the form of Einstein A values and oscillator strengths) has been produced for the OH ground X 2 Π state rovibrational (Meinel system) and pure rotational transitions. All possible transitions are included with v ′ and v ″ up to 13, and J up to between 9.5 and 59.5, depending on the band. An updated fit to determine molecular constants has been performed, which includes some new rotational data and a simultaneous fitting of all molecular constants. The absolute transition strengths are based on a new dipole moment function, which is a combination of two high level ab initio calculations. The calculations show good agreement with an experimental v = 1 lifetime, experimental μ v values, and Δv=2 line intensity ratios from an observed spectrum. To achieve this good agreement, an alteration in the method of converting matrix elements from Hund׳s case (b) to (a) was made. Partitions sums have been calculated using the new energy levels, for the temperature range 5–6000 K, which extends the previously available (in HITRAN) 70–3000 K range. The resulting absolute transition strengths have been used to calculate O abundances in the Sun, Arcturus, and two red giants in the Galactic open and globular clusters M67 and M71. Literature data based mainly on [O I] lines are available for the Sun and Arcturus, and excellent agreement is found.

/pdf/line-strengths-of-rovibrational-and-rotational-transitions-9p6livhf78.pdf

Line strengths of rovibrational and rotational transitions in the X2Π ground state of OH

New hydroxyl transition probabilities and their importance in airglow studies

Vibrational relaxation rates for the v=2 level of the X 2Πi state of the OH radical have been measured in a low pressure flow system, using a novel two‐laser pump‐and‐probe technique. The OH is prepared in the v=2 level by overtone pumping (2←0) and monitored by ultraviolet laser‐induced fluorescence in the (1,2) band of the A–X system. Scanning the time delay between the lasers at a given collider pressure produces exponential decay whose rate as a function of collider pressure yields the rate constant. We determine values (all cm3 s−1 units) for NH3: (1.20±0.15)×10−10; CH4: (2.3±0.2)×10−12; CO2: (6.7±1.1)×10−13; N2O: (4.6±0.6)×10−13; O2: (2.6±0.54)×10−13; N2 and H2: ≤10−14. Except for ammonia, these are two to three orders of magnitude smaller than those measured for relaxation of v=1 in the A 2Σ+ excited state of OH, where attractive forces appear to play a role.

Vibrational relaxation of OH (X 2Πi, v=2)

A theoretical study of the sensitivity of the dipole moment function (DMF) of the X2Pi ground state of OH to basis-set saturation and to refinements in the correlation treatment is presented. Emphasis is placed on determining the slope of the DMF at r(e) and the r value at which the maximum occurs. Consideration is given to the effect of oxygen polarization functions up through h type, expansion of the active orbital space to include the O 3d-delta orbital, the effect of higher excitations using the averaged coupled-pair functional method, and the effect of evaluating the dipole moment as an energy derivative rather than as an expectation value. The theoretical DMFs obtained here, which should be the most accurate to date, differ markedly from an empirical DMF of Turnbull and Lowe that is based on experimentally derived intensity ratios. The theoretical DMFs agree better with a recently published DMF of Nelson et al., but suggest that this empirical DMF is also inaccurate for r greater than 2.3 a0.

Theoretical study of the dipole moment function of OH(X 2Π)

Theoretical transition probabilities for the OH Meinel system

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Theoretical transition probabilities for the OH Meinel system