| It is customary to determine local temperatures in the mesosphere and MLT by using
Boltzmann plots based on the rotational distributions of the bands of the OH Meinel system,
assuming that populations in these levels are in local thermodynamic equilibrium
(LTE) with the kinetic temperature [Beig et al., Rev. Geophys., 2003; Turnbull and
Lowe, PSS, 1989; von Savigny and Lednyts’kyy, GRL, 2013]. It has long been
known that the higher rotational levels are not in LTE [Dodd et al., JGR,1994],
so that a conventional Boltzmann plot cannot be used to obtain a temperature –
only the lowest rotational levels are used, in the hope that LTE for such levels is
appropriate.
Because the atmosphere is dynamically active, it is important that the OH bands be
observed simultaneously, particularly if the intent is to compare apparent temperatures from
different vibrational levels. Using sky spectra from the Keck II telescope and the ESI echelle
spectrograph, it has been shown that the LTE assumption seems to be invalid even for low
rotational levels, based on earlier observations that show a reproducible pattern of apparent
temperature vs OH vibrational level, with a general upward trend of temperature with
increasing vibrational level, averaging 15-20 K [Cosby and Slanger, Can. J. Phys,
2007].
This work has now been repeated with a much larger database. using the X-shooter
telescope and echelle spectrograph at the VLT (Very Large Telescope) in Chile [Noll et al.,
ACPD, 2015]. The results are in close accord with the earlier work, showing the same general
pattern, with a marked temperature maximum at OH(v = 8), and an upward “temperature”
trend from v = 2 to v = 9.
As the OH layer lies below the mesopause, kinetic temperatures should fall from that
layer ( 87 km) to the mesopause, near 95 km. Typically the modeled temperature in the OH
layer is 17 K higher than that in the O2(b,v=0) layer [NRLMSIS00]. Rocket and satellite
experiments indicate that there is a trend in altitude of the OH levels – the higher levels lie
above the lower [von Savigny and Lednyts’kyy, GRL, 2013], and thus the higher levels
should be colder than the lower. These conclusions are not met by the experimental
results.
The two sets of data, taken in different decades, with different investigators, in different
hemispheres, and with different instruments, are remarkably similar. The point in
common is that broadband echelle spectrographs were used to collect the data. Thus,
it is important to realize that there are problems with the standard ground-based
mesospheric temperature measurement procedure, most likely due to the details of
the relaxation processes of OH(v) involving the most important colliders, O2 and
O(3P). |