ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-36-13-2018New insights for mesospheric OH: multi-quantum vibrational relaxation as a
driver for non-local thermodynamic equilibriumKalogerakisKonstantinos S.ksk@sri.comMatsievDanielCosbyPhilip C.https://orcid.org/0000-0002-5139-018XDoddJames A.FalcinelliStefanohttps://orcid.org/0000-0002-5301-6730HedinJonasKutepovAlexander A.NollStefanhttps://orcid.org/0000-0003-1957-4170PankaPeter A.https://orcid.org/0000-0001-9801-8249RomanescuConstantinThiebaudJérôme E.Center for Geospace Studies, SRI International, Menlo Park,
California, USAformerly at: Molecular Physics Laboratory, SRI International, Menlo Park, California, USAAir Force Research Laboratory (AFRL), Space Vehicles Directorate,
Kirtland Air Force Base, New Mexico, USADepartment of Civil and Environmental Engineering, University of
Perugia, Perugia, ItalyDepartment of Meteorology (MISU), Stockholm University, Stockholm,
Swedenformerly at: Physical Sciences Division, SRI International, Menlo Park, California, USAThe Catholic University of America, Washington DC, USANASA Goddard Space Flight Center, Greenbelt, Maryland, USAInstitute for Astro- and Particle Physics, University of Innsbruck,
Innsbruck, AustriaInstitute of Physics, University of Augsburg, Augsburg, GermanyGerman Remote Sensing Data Center (DFD), German Aerospace Center
(DLR), Oberpfaffenhofen, GermanyAeris Technologies, Redwood City, California, USAretiredKonstantinos S. Kalogerakis (ksk@sri.com)9January2018361132415June20174October201715November2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://angeo.copernicus.org/articles/36/13/2018/angeo-36-13-2018.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/36/13/2018/angeo-36-13-2018.pdf
The question of whether mesospheric OH(v) rotational population
distributions are in equilibrium with the local kinetic temperature has been
debated over several decades. Despite several indications for the existence
of non-equilibrium effects, the general consensus has been that emissions
originating from low rotational levels are thermalized. Sky spectra
simultaneously observing several vibrational levels demonstrated reproducible
trends in the extracted OH(v) rotational temperatures as a function of
vibrational excitation. Laboratory experiments provided information on
rotational energy transfer and direct evidence for fast multi-quantum
OH(high-v) vibrational relaxation by O atoms. We examine the relationship
of the new relaxation pathways with the behavior exhibited by OH(v)
rotational population distributions. Rapid OH(high-v) + O multi-quantum
vibrational relaxation connects high and low vibrational levels and enhances the hot tail
of the OH(low-v) rotational distributions. The effective rotational
temperatures of mesospheric OH(v) are found to deviate from local
thermodynamic equilibrium for all observed vibrational levels. Dedicated to
Tom G. Slanger in celebration of his 5 decades of research in
aeronomy.
Atmospheric composition and structure (airglow and aurora; middle atmosphere composition and chemistry); history of geophysics (atmospheric sciences)Introduction
The emission of radiation from vibrationally excited OH is an important
observable in the Earth's upper atmosphere and has been the topic of numerous
studies over the past several decades. This emission dominates the visible
and infrared emissions from the atmosphere in this altitude region and has
been used to investigate atmospheric density changes, temperature
fluctuations, waves, tides, and species concentrations. Only a fraction of
OH(v) radiates; collisional energy transfer between OH(v) and other
atmospheric constituents significantly influences this emission and the
mesospheric heat budget. Laboratory studies and theoretical calculations have
investigated the relevant collisional energy transfer processes because they
play a key role in determining the observed vibrational population
distributions and their relative emission intensities. Modeling calculations
of these emissions have also attracted considerable interest and are an
essential part of the synergistic interplay between observations, laboratory
experiments, and theoretical calculations.
In 1948, Aden Meinel reported complex near-infrared emissions in night sky
spectra recorded using a grating spectrograph at the Lick Observatory, but
these features remained unidentified at first (Meinel, 1948, 1950a). Meinel
observed similar intense emissions at the Yerkes Observatory and, following
the suggestion of Gerhard Herzberg (Herzberg, 1951), attributed them to OH
rovibrational transitions within the electronic ground state (Meinel, 1950b,
c, d). These emissions, known as the OH Meinel bands, represent some of the
most prominent features in the visible and infrared regions of the nightglow.
The overall emission exhibits an intensity peak near 87 km with a full width
at half maximum of ∼ 8 km (Sivjee, 1992; McDade, 1991; Meriwether,
1989; Baker and Stair, 1988; Evans et al., 1973). The peaks of the emission
profiles of the individual vibrational levels exhibit modest altitude
dependence with an upward shift of ∼ 0.5 km for each increasing
vibrational quantum number. This altitude dependence has been the topic of
several investigations (e.g., von Savigny and Lednytskyy, 2013; von Savigny
et al., 2012; Liu and Shepherd, 2006; McDade, 1991; López-Moreno et al.,
1987).
The mesosphere represents a transition region between viscous flow and
turbulent transport in the layers below and free molecular flow and
diffusion above it. In the rarified environment of the mesosphere, the
collision frequency is relatively low and it is not always possible for
excited species to attain thermodynamic equilibrium, especially at the
higher altitudes. For OH in particular, the question of whether the
rotational temperature determined by observations is equivalent to the local
kinetic temperature has been debated since the discovery of the Meinel band
emission in the 1950s. This question is a matter of profound fundamental
importance for our understanding of mesospheric chemistry and dynamics
and for the interpretation of the variability of OH emissions. If
thermodynamic equilibrium is attained, the observed emissions can be
considered a reliable proxy that monitors temperature variability and
mesospheric heat deposition. If not, then the determined OH rotational
quasi-temperatures and their changes are controlled by the relevant
altitude-dependent production, removal, and energy transfer processes.
The structure of the paper is as follows. First, we introduce the sources and
sinks for the mesospheric vibrationally excited hydroxyl radical OH(v). Then,
we briefly review the history of the debate on the equivalence of the OH(v)
rotational temperatures and local kinetic temperature and the relevant
atmospheric observations deriving rotational temperatures in the second half
of the last century. The next two sections present significant relevant
developments during the past decade based on ground-based observations of the
OH Meinel band emission using astronomical telescopes and laboratory studies
investigating the multi-quantum relaxation of OH(v) by O atoms. We then
present a comparison of results from observations at three different
telescope sites, which provide unequivocal evidence that thermalization is
incomplete for all observed OH vibrational levels. The available
laboratory studies on OH rotational relaxation corroborate this conclusion.
Moreover, we consider multi-quantum vibrational relaxation processes with
emphasis on the recent developments for OH(v) + O and examine the role
of these processes in generating low vibrational levels with rotational
excitation. Finally, we briefly discuss the implications of the new insights
for our understanding of the mesosphere and future needs for relevant
observations, laboratory measurements, and modeling calculations.
Mesospheric OH sources and sinksProduction by H + O3
The main source of mesospheric OH(v) is the H + O3 reaction, as
initially suggested by Bates and Nicolet (1950a, b) and Herzberg (1951). This
reaction has a large exothermicity of 3.38 ev and is characterized by a
strongly inverted internal state distribution of product OH. The majority of
the available energy is channeled into OH internal degrees of freedom, with
significant rotational excitation and more than ∼ 90 % of the
nascent product OH appearing in vibrational levels v=7–9 (Klenerman and
Smith, 1987; Ohoyama et al., 1985; Charters et al., 1971). The most probable
vibrational level is v=9, which is also the highest energetically allowed
level. The main source of the O3 molecules is three-body recombination
O + O2+M (M= N2, O2). Photodissociation of water
is the main source of hydrogen atoms that form a thin layer in the
mesosphere, as originally pointed out by Bates and Nicolet (1950a, b).
Removal by radiative decay and rotational–vibrational relaxation
The OH Meinel band emission originates from radiative transitions between
different rovibrational levels of the OH ground state. These transitions
redistribute the vibrational and rotational level populations. The relevant
radiative transition probabilities have evolved during the past decades and
do not represent as significant a source of uncertainty as they did in the
early years after the discovery of the OH emission. The most recent
calculations (Brooke et al., 2016; van der Loo and Groenenboom, 2007, 2008)
appear to be converging (Slanger, 2016). A thorough critical evaluation
considering comparisons of all the recent sets of radiative transition
probabilities is still needed to provide relevant guidance for future studies.
Aside from radiative transitions, reactions and collisional relaxation are
responsible for the removal or redistribution of the nascent OH vibrational and
rotational population distributions. Vibrational relaxation by molecular
oxygen in the mesosphere is the dominant loss process for most altitudes and
a significant fraction proceeds via multi-quantum pathways (Adler-Golden,
1997; Shalashilin et al., 1995; McDade, 1991). Laboratory experiments and
theoretical calculations are needed to elucidate the details of multi-quantum
vibrational relaxation pathways involving OH(v).
In contrast to O2, vibrational relaxation by N2 is extremely
inefficient and is dominated by single-quantum vibrational relaxation
(Adler-Golden, 1997). Loss of vibrationally excited OH by atomic oxygen has
been rather poorly understood and represents a major source of uncertainty
for understanding and modeling mesospheric nightglow. Because of the
importance of O atoms, we discuss in more detail recent relevant developments
from laboratory experiments in Sect. 4.
Regarding the rotational equilibration of excited OH, the role of both major
atmospheric constituents N2 and O2 is crucially important, with
N2 dominating because of its larger abundance. In laboratory
experimental studies, Holtzclaw et al. (1997) investigated the collisional
relaxation of rotationally excited OH(v=1–3) by O2 at 100 K. They
found that their observations could be reproduced by a model in which only
transitions with ΔN=± 1 were considered. Based on the
state-to-state rotational energy transfer rate constants determined for
rotational levels N=8–25, a bottleneck in the population flow was
established for N=14. Kliner and Farrow (1999) performed experiments
studying OH(v=0) in rotational levels N=1–12 and determined that
rotational relaxation by O2 and N2 is more efficient for lower
rotational levels than for higher ones. They also found that an exponential
gap model successfully reproduced their measurements and that translational
relaxation of the nascent OH occurred much faster than rotational relaxation.
No information is available on how collisions of OH with atomic oxygen may
affect the hydroxyl rotational excitation, for example in the case that reactive or
energy transfer processes have cross sections that vary as a function of the
initial OH rotational level.
The long-standing debate on whether OH rotational temperatures are in
LTE
Meinel (1950b) recognized that his observations of resolved OH spectral
features lent themselves favorably to the determination of spectroscopic
temperatures. He explored a variety of approaches, including analysis of
transitions involving low rotational levels of the resolved P branch and
the total relative intensities of the P, Q, and R branches of
individual vibrational bands. He also found the rotational temperature of v=9 to be similar to that of v=4 and interpreted this finding as an
indication that the rotational and ambient kinetic gas temperatures were
equivalent (Meinel, 1950d). Nevertheless, an important conclusion from
Meinel's early work was that accurate results required quantitative
knowledge of the OH excitation mechanism and the OH Meinel band absolute
transition probabilities.
Other notable early studies (Wallace, 1961, 1962; McPherson and
Vallance-Jones, 1960; Kvifte, 1959) supported the notion that consistent
rotational temperatures in local thermodynamic equilibrium (LTE) could be
obtained from the steady-state OH population. A key argument was that near
the altitude of the emission layer each excited OH radical undergoes numerous
collisions before radiating, and thus one can assume that the OH rotational
temperature is in LTE. Chamberlain (1995) summarized the situation as follows: “…although this
conclusion is not definitely established, it is reasonable to suppose that
the rotational temperatures are indicative of the gas-kinetic
temperatures.”
A vigorous debate ensued over several decades, with studies occasionally
indicating discrepancies in the extracted rotational temperatures. Some
researchers reported that the OH(v) rotational temperatures appeared to
increase with the vibrational level (Khomich et al., 2008, and references
therein; Perminov et al., 2007, and references therein; Suzuki and Tohmatsu,
1976; Krasovskij and Šefov, 1965; Shefov, 1961). Shefov (1961) suggested
that there are significant deviations from LTE for OH(high-v) rotational
levels with quantum number N > 5. However, the relatively
poor signal, limitations in the available Einstein transition probabilities,
active parallel debates on the effects of intensity, latitude, season of the
year, and possibly the widespread consensus in favor of LTE limited the
confidence in these results (Shefov, 1972a, b).
Other studies presented additional examples in which rotational features
could be explained by non-equilibrium conditions (Gattinger and Vallance
Jones, 1973; Harrison et al., 1971). Nichols et al. (1972) considered the
effect of collisional relaxation and the possibility of emission from a
non-thermalized rotational population distribution. Vallance Jones (1973)
reviewed the available arguments from both sides, but could not reach a
definitive conclusion. He suggested that dynamical effects might also
influence mesospheric OH. Suzuki and Tohmatsu (1976) considered a collection
of measurements from the literature and claimed that the rotational
temperatures exhibit a dependence on the OH vibrational level from which the
emission originates. This conclusion raised doubts because the comparisons of
Suzuki and Tohmatsu involved selective data sets from different times and
locations and also because different data collections did not appear to
corroborate similar trends (Dick, 1977; Krassovsky et al., 1977). The main
argument refuting the claim of Suzuki and Tohmatsu emphasized the fact that
temperature differences can be reliably established only by
simultaneous measurements. Otherwise, the temporal and spatial variations of
the emissions and the uncertainties and possible systematic bias of
different techniques hinder meaningful conclusions.
Reports indicating non-equilibrium conditions for OH continued appearing in
the literature until the end of the last century. Pendleton et al. (1989)
reported the observation of OH Meinel (7,4) band emission from rotational
level N=13. The column emission rates determined were estimated to be
approximately 4 orders of magnitude larger than what would be expected under
LTE. That work was later expanded with observations from vibrational levels v=3–7, providing additional evidence for incomplete thermalization of
OH(v) rotational excitation (Pendleton et al., 1993). Perminov and
Semenov (1992) reported non-equilibrium intensities for transitions involving
rotational levels N=6–9 of the (7,3) band.
Finally, a series of studies by Dodd and coworkers using data from the
Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS) aboard Space
Shuttle mission STS-39 provided evidence for extremely high rotational
excitation, up to ∼ 2.3 eV in rotational energy. These studies
reported results for several rotational levels of OH(v=0–9). Quite
remarkably, spectra of pure rotational transitions for the four lowest
vibrational levels, v=0–3, indicated population in rotational levels up
to N=33 (Dodd et al., 1993; Smith et al., 1992). Dodd et al. (1994)
developed a model for the observed OH(v, N) column number densities and
found that additional production of OH(low-v) was required
to match the observations. This work also concluded that, based on the
ΔN= 0, ±1 selection rule for dipole-allowed transitions, the
radiative cascade from nascent rovibrational levels cannot account
for the observed emissions from the highest rotational levels of
OH(v= 0–3). The authors suggested two possible
explanations for the enhanced rotational excitation: direct excitation by the
H + O3 reaction or resonant vibrational-to-rotational energy
transfer from nascent OH(v) following collisions with O atoms. In a
subsequent experimental study, Dodd and coworkers were able to detect
rotationally excited OH(v=0, 1) from the reaction of O3 with fast H
atoms. Nevertheless, the laboratory evidence led to the conclusion that the
OH(v=0, 1) yield is very small compared to that of the high vibrational
levels v=7–9 (Dodd et al., 1999).
Despite the aforementioned indications of non-LTE behavior in the observed
OH(v) rotational distributions, a common practice in the aeronomy community
has been to assume that the rotational temperatures of isolated vibrational
levels, obtained using a couple of selected spectral lines from low
rotational quantum numbers, are in LTE and reflect the local kinetic
temperature.
Recent developments on the vibrational relaxation of OH(high-v) by O
atoms
The removal of OH(high-v) by O involves several pathways, including
reaction to produce H + O2, single-quantum vibrational relaxation,
and multi-quantum vibrational relaxation.
OH(v=9)+O→H+O2→OH(v=8)+O→OH(v<8)+O
Laboratory measurements showed that the deactivation of OH(high-v) by
O(3P) atoms is surprisingly fast, with a total removal rate constant of
(4 ± 1) × 10-10 cm3 s-1 for OH(v= 9) + O at room temperature (Kalogerakis et al., 2011). For the lower
temperatures relevant to the mesosphere, we can estimate a value of
(3 ± 1) × 10-10 cm3 s-1 if we assume that
the temperature dependence of OH(v=9) + O is the same as that
reported for experiments studying OH(v=7) + O (Thiebaud et al.,
2010). These large values for the total removal rate constant approach the
gas kinetic limit and defied an explanation until recently.
In 2015, Sharma et al. (2015) proposed that the interaction of OH(v) with O
atoms also involves a fast, spin-allowed, multi-quantum
vibration-to-electronic (V–E) energy transfer pathway:
OH(v≥5)+O(3P)→OH(0≤v′≤v-5)+O(1D).
Recent experiments (Kalogerakis et al., 2016) investigated this new pathway
for OH(v=9) and provided laboratory evidence for rapid interconversion of
the type
OH(v=9)+O(3P)⇄OH(v=3)+O(1D).
In those experiments, an ultraviolet laser was used to photodissociate
O3 in mixtures containing a small amount of H2 in Ar bath gas.
Under the conditions employed, deactivation of O(1D) was relatively
inefficient and OH(v=3) was produced in significant amounts from the
photon-initiated reaction of O(1D) with H2. At the high laser
energy used, the dissociation of O3 was practically complete and
therefore minimized the importance of the H + O3 reaction as a
source of OH(v=9). A second tunable dye laser pulse was used to detect
transient production of OH(v=9) arising from electronic-to-vibrational
(E–V) energy transfer from O(1D) to OH(v=3).
The new multi-quantum V–E relaxation pathway was found to be the most
efficient process for the deactivation of OH(v=9) by O atoms and provides an
explanation for the surprisingly large increase in the rate constant of
OH(v) + O by more than 1 order of magnitude between v= 0
(Burkholder et al., 2015, and references therein) and v=9 (Kalogerakis et
al., 2011). Moreover, this relaxation pathway explains why previous
theoretical calculations (Caridade et al., 2013; Varandas, 2004) yielded
substantially smaller total removal rate constants for OH(high-v) + O
than the experimentally measured ones. Based on the calculations by
Varandas (2004) for the reactive pathway (1) and for single-quantum
relaxation, the experimentally measured value for the OH(v=9) + O
total removal rate constant is more than 4 times larger than the
theoretical rate constant for reaction and approximately 1 order of
magnitude larger than the calculated value for single-quantum vibrational
relaxation. A new generation of theoretical calculations involving excited
potential energy surfaces will be necessary for experimental and theoretical
results to converge. Laboratory measurements are also needed to quantify the
relative importance of single-quantum and multi-quantum relaxation for atomic
and molecular oxygen.
Finally, we note that Sharma et al. (2015) showed that the OH(v) + O
multi-quantum vibrational relaxation of Eq. (2) ultimately results in
enhanced CO2 4.3 µm emission. This enhancement involves energy
transfer from O(1D) to N2, deactivation of the resulting N2(v≥2) by N2(v=0) to produce N2(v=1), and vibrational
energy transfer from N2(v=1) to the v3 mode of CO2, which
promptly emits a 4.3 µm photon. Just as important, Panka et
al. (2016, 2017a, b) recently implemented the Sharma mechanism in model
calculations that resulted in very good agreement with observations of the
nighttime CO2(ν3) 4.3 µm and the OH Meinel band
emissions from the Sounding of the Atmosphere using Broadband Emission
Radiometry (SABER) instrument aboard the NASA Thermosphere, Ionosphere,
Mesosphere Energetics and Dynamics (TIMED) satellite.
Recent developments from observations by astronomical telescopes
and evidence for non-LTE
During the last decade, renewed interest in exploiting the capabilities of
astronomical telescopes has provided the most recent relevant information on
OH(v) rotational distributions (Noll et al., 2015, 2016; Oliva et al.,
2015; Cosby and Slanger, 2007). The seminal studies of Cosby and
Slanger (2007) and Noll et al. (2015) examined mesospheric OH(v) in great
detail using high-resolution sky spectra. Most important, these studies
fulfill the crucial requirements of simultaneous, quantum-state-resolved measurements for all available vibrational levels.
Cosby and Slanger (2007) and Noll et al. (2015) examined the variability of
mesospheric OH(v) using data from astronomical telescopes. The former work
used data from the High-Resolution Echelle Spectrograph (HIRES) on the Keck I
telescope, the Echelle Spectrograph and Imager (ESI) on the Keck II
telescope, and the UV-visual Echelle Spectrograph (UVES) of the Very Large
Telescope (VLT) in Paranal, Chile, while the latter used data from the
X-Shooter echelle spectrograph of the VLT. Both groups reported that
the rotational temperatures determined from transitions involving the
lowest rotational levels exhibit a clear vibrational level
dependence, with the rotational temperature increasing by approximately
15 K as the OH vibrational quantum number increases from v=2 to v=8
(Noll et al., 2015; Cosby and Slanger, 2007). They also found that OH(v=8) has a significantly higher rotational temperature than OH(v=9).
The two aforementioned high-resolution data sets were obtained by different
groups and instruments and at different locations and times. The trend in the
overall behavior of the OH(v) rotational temperatures as a function of the
vibrational level persists even when different sets of transition
probabilities are used in the analysis. This behavior can be considered an
indication of non-LTE behavior because the mesopause and its associated
temperature minimum occur several kilometers higher (∼ 5–10 km) than the OH
layer. Because the altitude of each OH(v) sublayer is thought to gradually
increase with vibrational level (Noll et al., 2016; von Savigny and
Lednytskyy, 2013; von Savigny et al., 2012; Liu and Shepherd, 2006; McDade,
1991; López-Moreno et al., 1987), the exact opposite trend for
Tcold, i.e., a temperature decrease with increasing altitude,
would be expected if the OH(v) rotational temperatures were indeed in
equilibrium with the local kinetic temperature.
In the literature, three often used OH Meinel bands for determining
rotational temperatures are the (8–3), (6–2), and (3–1) bands. When
temperatures of the aforementioned or other OH Meinel bands are reported in
the literature, the observed differences are usually attributed to
uncertainties associated with the employed techniques or the natural
variability of mesospheric OH emissions; considering the possible role of
non-LTE effects is a relatively rare occurrence (von Savigny et al., 2012;
Dyrland et al., 2010).
Another set of relevant near-infrared observations highlighting the
persistent high rotational excitation of mesospheric OH(v) was reported by
Oliva et al. (2015), who used the GIANO high-resolution spectrograph at the
La Palma Observatory to obtain sky spectra in the wavelength range
0.97–2.4 µm. This group averaged data for 2 h at a
resolution of ∼ 36 000 in an attempt to provide a better
characterization of the nightglow continuum and “sky suppression” for
astronomical investigations. The results demonstrate high rotational
excitation even for the lowest OH vibrational levels, in excellent agreement
with the observations of Cosby and Slanger (2007). Figure 1 shows the
rotational population distributions for levels v=2, 3, 8, 9 from the
data set of Oliva et al. (2015). The H + O3 nominal energetic limit
is indicated by dashed lines for v=8, 9. These two rotational population
distributions are markedly different than those for v=2, 3, with the
latter two displaying persistent tails of highly rotationally excited levels.
We note that yet higher rotational levels in OH(v=2, 3) have been
reported by Dodd and coworkers with rotational energies of 10 000 cm-1
and 12 000 cm-1, respectively (Dodd et al., 1993, 1994; Smith et al.,
1992). The significantly larger radiative rates for the pure
rotation–rotation transitions and the limb viewing geometry enabled Dodd and
coworkers to sensitively detect signals for rotational transitions
originating from levels as high as N=33 (for vibrational levels v=0–3). As mentioned above, such high rotational excitation cannot be
accounted for with dipole-allowed transition selection rules and requires an
alternate source.
OH(v) rotational temperatures reported in studies of
high-resolution astronomical sky spectra and information relevant to
collisional relaxation near 90 km.
a Cosby and Slanger (2007); single observation 3MAR00 05:39 UT;
transition probabilities from Goldman et al. (1998); single-temperature fit
using low rotational lines.
b Noll et al. (2015); averaged results from 343 spectra, each containing
25 OH bands; transition probabilities from the HITRAN2012 database; Rothman
et al. (2013); single-temperature fit using low rotational lines.
c Oliva et al. (2015); averaged data for 2 h; transition
probabilities from van der Loo and Groenenboom (2008); two-temperature fit of
all rotational lines with Tcold fixed at 200 K.
d Brooke et al. (2016).
e Based on a collision frequency of 104 Hz, estimated from typical
total number densities encountered at 90 km (NRLMSISE-00 model; Picone et
al., 2002).
f Based on a pressure estimate for 90 km with a value of 0.2 Pa
(NRLMSISE-00 model; Picone et al., 2002).
Noll et al. (2016, 2015) and Cosby and Slanger (2007) determined OH(v)
rotational temperatures by considering lines from a few low rotational
quantum numbers. They determined Tcold by performing a fit to
their truncated data set using a Boltzmann distribution for a single
temperature. In contrast, Oliva et al. (2015) performed a two-temperature fit
to all observed rotational lines regardless of quantum number and fixed the
value of Tcold at a nominal mesospheric temperature of 200 K. A
summary of the results from the previous studies of Cosby and Slanger (2007),
Noll et al. (2015), and Oliva et al. (2015) is presented in Table 1. This
table also includes additional information relevant to rotational relaxation
near 90 km that will be discussed below.
A remarkable trend is evident from the rotational temperatures corresponding
to high rotational levels, Thot, shown in the fifth column of
Table 1, as determined by Oliva et al. (2015). Thot rises
dramatically as the vibrational level decreases, with values from
approximately 1000 K for v=9 to ∼ 12 000 K for v=2. We also
highlight the different behavior of Thot for vibrational levels
v=7–9, in which more than 90 % of nascent OH(v) is produced
following the H + O3 reaction when compared to the behavior of the
lowest vibrational levels that exhibit the most extreme Thot
values. The sixth column of Table 1 shows the radiative lifetime of OH(v),
τrad, based on the most recent study by Brooke et al. (2016).
The seventh column presents the calculated number of collisions during
τrad, and the last one shows estimates for the product of
τrad multiplied by an estimate of the pressure near
90 km (e.g., NRLMSISE-00 model; Picone et al., 2002).
In their experimental study, Kliner and Farrow (1999) found that OH
translational relaxation in N2 and O2 occurred very rapidly within
a value of (P×τrad) of
∼ 27 Pa µs. In stark contrast, they found that complete
rotational equilibration of OH(v=0, N≤12) requires a value of
(P×τrad) that is approximately 70 times larger,
∼ 1.9 kPa µs. Even after correction for the temperature
difference between the experiments of Kliner and Farrow and the mesosphere,
the (P×τrad) values shown in Table 1 are
significantly smaller than 1.9 kPa µs for all listed OH(v)
vibrational levels. We note here that the higher the rotational excitation, the
slower the rate of rotational relaxation, and much higher rotational levels
than N=12, up to N=33, have been observed for mesospheric OH(v=0–3) (Smith et al., 1992). Therefore, the calculated values of
P×τrad in Table 1 provide another clear
indication of non-local equilibrium conditions for OH(v).
Mesospheric OH(v=2, 3, 8, and 9) rotational
population distributions adapted from Fig. 2 of Oliva et al. (2015).
Vibrational levels v=2 and 3 are the most near-resonant
multi-quantum relaxation pathways for v=8 and 9, respectively,
according to Eq. (2). The dashed lines show the nominal energetic limit for
reaction H + O3.
From Table 1, we also note that the number of collisions experienced by
OH(low-v) before emission is significantly larger than that experienced by
OH(high-v). This is consistent with the expectation that the lowest OH
vibrational levels are closer to thermal equilibrium, especially for the
lower parts of the mesosphere. For low-N and low-v levels, the radiative
relaxation rate by pure rotational transitions is also significantly smaller
than that for rovibrational transitions, and both rates are smaller than the
rate of collisional relaxation (Dodd et al., 1994). However, because of the
existence of multi-quantum relaxation pathways that connect high and low
vibrational levels, a fraction of the rotational population distribution is
always in non-LTE, as demonstrated in Fig. 1.
Another important piece of evidence for the characterization of the OH
rotational temperatures as non-LTE for all observed levels stems from the
fact that in the studies by Noll et al. (2016) and Cosby and Slanger (2007),
consideration of additional rotational levels beyond the lowest three
resulted in gradually different results for the OH rotational temperature.
For example, Noll et al. (2016, 2015) reported that the determined OH
rotational temperature increased by 1 K on average when the analysis
considered the P1 (N=4) line together with the first three
P1-branch lines. For the analysis of the P2-branch lines, use of
the first four rotational lines of this branch resulted in an average
increase in the rotational temperatures by 11 K when compared to the
reference set of the first three P1-branch lines (Noll et al., 2016).
OH(v) rotational temperatures Tcold and Thot for
low and high rotational quantum numbers, respectively, from our reanalysis
of the data reported by Oliva et al. (2015). Fit results for a single
temperature (Tcold) and for two temperatures (Tcold and Thot)
are shown using the most recent set of transition probabilities by Brooke et
al. (2016) together with the difference between the values determined by
the two different fit types.
To further support the conclusions of this report, we reanalyzed the data set
of Oliva et al. in two different ways. First, we followed a similar approach
to that of Cosby and Slanger (2007) and truncated the Oliva et al. data set
to only include low OH rotational lines originating from levels with
rotational energy less than 500 cm-1. We then performed fits to a
simple Boltzmann distribution, which will heretofore be referred to as
single-temperature fits. Second, we slightly varied the approach of Oliva et
al. by performing two-temperature fits with both Tcold and
Thot as unconstrained adjustable parameters. For the results
reported here we used the most recent set of transition probabilities
available in the literature (Brooke et al., 2016; abbreviated as BBW16).
Analysis of a data subset using the transition probabilities of van der Loo
and Groenenboom (2008, 2007) yielded results that are very similar for the
bands reported by Oliva et al. The results of our analysis using BBW16 are
summarized in Table 2.
Results for OH(v) Tcold (solid circles)
and Thot (slanted
crosses) rotational temperatures obtained from simultaneous
two-temperature fits of the data set reported by Oliva et al. (2015).
Figure 2 shows the results for Tcold and Thot from
our analysis of the Oliva et al. (2015) data set. The most striking finding
is that the two-temperature fit generates significantly different results for
the values of Tcold. Postulating a Boltzmann distribution at an
elevated temperature Thot has an effect on the population
distribution with low rotational excitation that determines the value of
Tcold. In the case of the single-temperature fits, the
contributions from the non-LTE rotational population distribution are not
subtracted from the observed population at low rotational quantum levels. As
shown in Table 2 for the data set by Oliva et al., this results in values of
Tcold from two- and single-temperature fits that differ by as
much as ΔTcold=-28 K for OH(v=8). Therefore, to
determine accurate rotational temperatures it is essential to take into
account the non-LTE contributions for each OH(v) vibrational level.
Clearly, the fraction of the population that is in highly rotationally
excited levels and the “temperature” of this non-LTE distribution will
influence the extent to which the determination of Tcold will be
affected. The present results suggest that OH temperature measurements
observing one of the lowest vibrational levels might be least affected by the
non-LTE effects on Tcold under certain atmospheric conditions,
for example the altitude and distribution of the atomic oxygen layer. Regardless,
this cannot be assumed to be the case without a detailed understanding of the
processes that give rise to OH rotational excitation as the nascent
rovibrational population distribution relaxes and of the most adequate form
to describe the effects of that relaxation.
It follows that a reassessment of OH rotational temperature measurements
reported to date is warranted. Detailed analysis of existing or future
high-resolution data sets that encompass a wide range of rotational levels is
needed, including systematic checks of the extent to which a two-temperature
Boltzmann distribution is an adequate representation of the rotational
population distribution for all OH(v) vibrational levels, a detailed
consideration of the selection of individual rotational lines and their
effect on estimated uncertainties, and a critical evaluation of all available
sets of OH transition probabilities.
In summary, the most recent high-resolution studies of OH(v) nightglow
involving simultaneous OH(v) observations and using different
astronomical telescopes consistently demonstrate that the rotational
temperatures determined from the lowest rotational levels, Tcold,
are affected by non-LTE effects. The rotational population distributions have
extremely hot tails and the rotational temperatures determined for high
rotational levels, Thot, increase rapidly as the vibrational
quantum number decreases. The available laboratory evidence from studies on
OH rotational energy transfer also indicates that, under mesospheric
conditions, thermalization of the OH rotational population distributions for
all vibrational levels is not complete. Therefore, the
rotational temperatures routinely determined from mesospheric OH(v)
observations cannot be generally assumed to reflect the local kinetic
temperature.
OH rotational temperatures and multi-quantum relaxation
We will now consider the role of OH(v) multi-quantum vibrational relaxation
as a possible driver of non-LTE conditions. The multi-quantum vibrational
relaxation process of Eq. (2) provides an efficient means that directly
connects high and low OH vibrational levels in the presence of O atoms. This
finding addresses a long-standing discrepancy between OH(v) atmospheric
observations and model calculations, with the models underestimating the
number density of low vibrational levels unless additional processes that
produce OH(low-v) are invoked. One such example, mentioned above, is the
deficit that Dodd et al. (1994) encountered in their “chemical production”
model. Other models have also encountered similar difficulties (Grygalashvyly
et al., 2014; Grygalashvyly, 2015, and references therein).
Just as important, the multi-quantum vibrational relaxation of Eq. (2) may be
a source of rotational excitation for OH(low-v). The process of Eq. (3)
involving the OH(v= 9) / OH(v= 3) pair is exothermic by
ΔE=-118 cm-1, assuming the O(3P2) ground state is
involved. This is the most near-thermoneutral vibrational level combination
for this process in the OH(v) vibrational manifold, followed by the
OH(v= 5) / OH(v= 0) pair (ΔE=-343 cm-1).
The OH(v= 8) / OH(v= 2) combination is exothermic by
ΔE=-1122 cm-1, the OH(v= 7) / OH(v= 1) by
ΔE=-2111 cm-1, and the OH(v=6) / OH(v=0)
pair by ΔE=-3095 cm-1. If the final vibrational quantum
number is smaller than the most near-resonant paths shown above, the
resulting exothermicity values are far greater. The relative propensity of
these processes has not yet been investigated. In all cases, however, the
energy released in the relaxation process is partitioned between
translational energy and rotational excitation. Thus, as the multi-quantum
relaxation OH(high-v) + O generates lower vibrational levels, the
result will be additional kinetic and rotational excitation, an observation
consistent with the behavior seen for Thot in Table 1. Because of
this process and other possible multi-quantum relaxation pathways
involving O2 and N2 (McCaffery et al., 2011; Adler-Golden, 1997;
Dodd et al., 1994) that could lead to the production of rotationally excited
OH(low-v), it is insufficient to think of the observed OH(low-v)
rotational levels only as nascent rovibrationally excited OH that has
gradually relaxed and equilibrated through radiation and several collisions.
Discussion
We begin with some remarks relevant to the interpretation of mesospheric OH
rotational temperatures. Because not only high rotational levels, but also
the lower ones, are not in complete LTE it is essential to conduct
simultaneous measurements that resolve transitions from low and high
vibrational and rotational levels to develop a deeper understanding of the
observed non-LTE behavior. The fine details of the variability and
vibrational level dependence exhibited by Tcold and
Thot for mesospheric OH can be attributed to the complicated
reaction and collisional relaxation dynamics of the relevant OH(v)
production and removal mechanisms. These are not yet fully understood,
notwithstanding the fact that the recently established fast OH(v) + O
multi-quantum V–E pathway represents an important new insight. Additional
studies of this type are required to probe the short- and long-term
variability and better understand the dynamics of non-LTE conditions in the
mesosphere.
The next point to highlight is that the steep gradient in the number density
of mesospheric atomic oxygen guarantees strong altitude dependence for these
multi-quantum vibrational relaxation pathways. For example, the number
density of O atoms is approximately 1 % of that of the O2 near 88 km
and increases by about a factor of 3 to approximately 10 % of the
O2 number density near 95 km (e.g., NRLMSISE-00 model; Picone et al.,
2002). It is also well established that atomic oxygen exhibits rich
variability that, combined with the very different OH(v) removal rate
constants by O and O2, is expected to lead to highly complex behavior.
Cosby and Slanger (2007) reported that the temperature difference ΔT9,3=T(v= 9) -T(v= 3) in one of the studied data
sets increases as the [OH(v= 3)] / [OH(v= 9)] intensity
ratio decreases during the night; i.e., ΔT9,3 increases as the
relative efficiency of collisional relaxation from v= 9 to v= 3
decreases. It is tempting to attempt an explanation for this behavior
invoking a possible role for the new multi-quantum OH(v) + O relaxation
pathway and variations in the O atom number density in the mesosphere during
the night (e.g., Smith et al., 2010). As the airglow layer conditions change
during the night, a number of multi-quantum relaxation processes vary in
importance and contribute accordingly to the observed ΔT9,3
values. These phenomena deserve further exploration and underscore the
importance of understanding the observations in terms of the detailed atomic
and molecular processes involved.
From the discussion above it is evident that the collisional relaxation
dynamics of mesospheric OH are complex and variable. The recent developments
regarding the OH(high-v) + O fast, multi-quantum vibrational relaxation
that is coupled to the CO2 4.3 µm emission represent
significant advances (Panka et al., 2017b; Kalogerakis et al., 2016; Sharma
et al., 2015). At the same time, it is also clear that our understanding of
the relevant details requires further refinement. Considering the large
amount of extant data from mesospheric ground- and space-based observations,
it is essential that synergistic theoretical, modeling, and laboratory
studies continue to address the remaining gaps in our knowledge. Recent
measurements demonstrate that the OH(v) rotational temperatures determined
from observations are partially equilibrated effective rotational
temperatures, even for the lowest rotational levels studied. Most important,
the emerging vision for the future is that by developing a detailed
understanding at the atomic and molecular level of the mechanisms that
control mesospheric non-LTE processes it will be possible to determine O atom
densities and altitude profiles from simultaneous, high-resolution,
ground-based observations. The new multi-quantum OH(v) + O vibrational
relaxation processes open exciting new research opportunities to probe and
understand the variability of mesospheric OH(v) non-LTE conditions.
Conclusions
The available evidence from laboratory experiments and recent observations
involving simultaneous OH(v) measurements at high resolution demonstrates
that mesospheric OH(v) rotational temperatures cannot be generally assumed
to correspond to the local kinetic temperature regardless of the vibrational
or rotational level. The observed steady-state rotational population
distributions of mesospheric OH(v) exhibit pronounced hot tails that are
not fully thermalized. The recently established fast multi-quantum
vibrational relaxation of OH(high-v) by O atoms efficiently populates
OH(low-v) levels and enhances rotational excitation. The quantitative
details of the processes involved require additional synergistic
investigations by observations, modeling and theoretical calculations,
and laboratory experiments. The multi-quantum vibrational relaxation of
OH(v) opens a new window into the details of mesospheric non-LTE conditions
and the possibility to develop novel diagnostic tools for monitoring O atoms
in this region of the atmosphere.
The data set from Oliva et al. (2015) presented here and
information relevant to the analysis is available on the Open Science
Framework website https://doi.org/10.17605/OSF.IO/NKWPJ.
The authors declare that they have no conflict of
interest.
Acknowledgements
This paper is dedicated to Tom G. Slanger in celebration of his 5 decades of
pioneering research in aeronomy at SRI International. The material presented
here is based in part on work supported by the US National Science Foundation
(NSF) awards AGS-1441896 and AST-1410297. Stefan Noll received funding from
project P26130 of the Austrian Science Fund (FWF) and is now funded by
project no. 1328/1-1 of the German Research Foundation (DFG). The seeds for
this work were planted by a series of laboratory studies at SRI International
over several years with support from the NASA Geospace Science Program. This
effort is currently supported by NASA grant 80NSSC17K0638. Data and specific
information used in the analysis are included in the paper. The topical editor, Petr Pisoft, thanks two anonymous
referees for help in evaluating this paper.
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