Optical/lidar Workshop Report
Introduction
Although generally invisible to the naked eye, the night sky is a rich source of
UV, visible, and infrared emissions that are signatures of the chemistry, physics, and dynamics that are taking place in the upper atmosphere. We are familiar with the remarkable phenomena of the aurorae at high latitudes, and the so-called nightglow bears many similarities but on a sub-visual scale. Thus, progress in studying and utilizing the nightglow depends on advances in technology that permit measuring low-level light quickly and accurately.
The complexity of the nightglow has been revealed over the years, starting with early studies that identified only a few atomic emissions, to the current realization that there are thousands of atomic and molecular lines [ Cosby et al.
, 2006]. There is in fact no clear viewing region through which the nightglow can be avoided, and thus astronomers are well aware of the importance of making corrections for the nightglow when viewing their target objects. This fact has led to the interesting situation that some of the best nightglow spectra are produced by astronomical instruments at large telescopes, where the intent is not to study the nightglow emissions at all, but to learn how to compensate for them. Such data are referred to as sky spectra.
Sky spectra
A sample sky spectrum is shown in Figure 1, where one sees that the intensities are heavily weighted towards the infrared region, where the dominant emission, from the
OH (hydroxyl) Meinel bands, becomes progressively stronger [ Cosby and Slanger ,
2007]. Figure 2 shows an expanded region, and we see that the line density is quite high.
All of these emission lines can be identified, and most of them are due to the excited states of either OH or molecular oxygen, the latter being substantially weaker than the former.

FIGURE 1. Average nightglow spectrum, from ESI/Keck II
FIGURE 2. Section of nightglow, from Fig. 1. The 7-3 OH Meinel band

The two sky spectra in Figures 1 and 2 were obtained with one of the 8-meter VLT telescopes in Chile with relatively long exposures, typically 40 minutes. For instruments dedicated to aeronomical studies, we are of course interested in much higher acquisition rates.
The goals of any given research project dictate the type of instrumentation that is appropriate. The typical question is whether a study focuses on a single atomic feature
(cf. the oxygen red and green lines), or whether the emphasis is on comparison of features seen over a range of wavelengths. In the former case a technique such as Fabry-
Perot interferometry is a possible choice [ Meriwether, 2006], while in the latter instance one needs to use a dispersing instrument, one of which was used to obtain the spectrum in
Figure 1.
The nightglow is an active phenomenon, exhibiting periods of relative stability interspersed with times when wave motion is very prominent. All-sky imaging is often used to show these large scale movements to best advantage [ Taylor et al ., 2001].
CESAR (Compact Echelle Spectrograph for Aeronomic Research)
The extraordinary quality of the sky spectra from large telescopes prompted the aeronomy community to support the construction of a large echelle spectrograph
(CESAR) with many of the same properties, but with the ability to relocate as needed, particularly to higher latitudes. A.L. Broadfoot was instrumental in creating the initial design. CESAR will have the study of aeronomical issues as its primary function.
The spectrograph with grating post-dispersion will be dedicated to nightglow studies at high spectral resolution (R ~ 20000) between 300 and 1000 nm, and will be easily deployable at different sites. The development of CESAR was conducted by SRI
International, and the optical design and integration of the spectrograph camera is based on the camera of the HIRES spectrograph at the Keck I telescope. The detailed optical design is used to calculate the position of the spectral elements on the detector, predict their image quality, and estimate the level of stray light [ Lavigne et al ., 2010]. Figure 3 shows the instrumental components, and Figure 4 shows a calibration spectrum from a nitrogen discharge, which will be similar to an auroral spectrum.

FIGURE 3 Schematic of CESAR light path
FIGURE 4 CESAR configuration

Combined Lidar/Optical Studies
One of the accomplishments of recent sky spectra studies has been to identify a nightglow component originating with emission from the FeO molecule [ Evans et al .,
2010; Saran et al ., 2011]. This presents us with an interesting opportunity to combine lidar and optical instrumentation to study atmospheric iron, since the FeO source is the reaction of atomic iron and ozone. The missing information in a ground-based study of
FeO is the altitude of the emission, and simultaneous iron lidar measurements will at least provide the atomic iron altitude profile.
This line of inquiry can then be extended because reaction with ozone produces other emitting species, in particular the OH emission (Figures 1/2) and the sodium emission at 589 nm (Figure 1). Thus, three distinct emissions are produced, all dependent on ozone, and with an instrument such as CESAR, with broad spectral coverage, it is in principle possible to carry out multi-dimensional analysis related to ozone, iron, sodium, and H-atom profiles. Figure 1 shows the continuum associated with
FeO at 500-650 nm, and Figure 5 shows an expanded version in which the various additional features are identified in the spectral region [ Saran et al ., 2011].
FIGURE 5. FeO quasi-continuum, with identification of nearby nightglow features
OH Nightglow and Temperature

The OH molecule has long been a target of atmospheric research, for a variety of reasons. The most evident is that it provides an easily accessible and intense emission, as seen in Figure 1. Because it is produced by the reaction of H-atoms with ozone, the intensity is related to the concentrations of each. The initial OH product is highly vibrationally excited, and the numerous bands appearing in Figure 1 originate with levels from v = 3 to v = 9 in the OH ground state. A further important property is that since the radiator is a molecule, the distribution of rotational transitions (Figure 2, a portion of the
OH 7-3 band) is considered a measure of the atmospheric temperature at the altitude of emission.
If this last point is verified, then measurement of the intensity distribution in an
OH band is a direct temperature probe. However, there are still numerous caveats, and it is hoped that the envisioned lidar/optical facility will contribute to clarifying the situation. These issues include questions of rotational equilibration [ Dodd et al ., 1994], the distribution with altitude of the vibrationally-excited molecules [ McDade , 1991], the specific pathways of vibrational relaxation [Llewellyn], and the effects of collisional quenching by relevant air particles, in particular atomic and molecular oxygen [???].
Twilight/Dayglow Investigations
Astronomical sky spectra are generally limited to times when the sky is sufficiently dark for observations, between the hours of evening and morning astronomical twilight. That is not a necessary restriction for an aeronomical instrument.
Therefore an optical facility at intermediate and low latitudes can be used to observe what is called the twilight glow but is actually high-altitude dayglow. The dayglow has many of the characteristics of aurorae, so extending sky spectra in this manner will prove to be a valuable tool for atmospheric studies. An example of a twilight glow spectrum and a simulation is shown in Figure 6, where early evening data acquisition showed an O
2
band
( b-X 1-1) that is only produced at high altitude, verified by the fact that the associated temperature from the rotational distribution is 900 K, far higher than the 200 K normally found for this oxygen band system [ Slanger et al.
, 2003].

FIGURE 6. Twilight glow spectrum of the O
2
( b-X ) 1-1 band and a 900 K simulation, demonstrating that the band originates at an altitude well above the mesosphere
Atmospheric Helium
Another example of a quasi-dayglow spectrum is shown in Figure 7, where the helium line at 388.9 nm appearsas a feature caused by the solar pumping of metastable triplet He [
Sharpee, O’Neill, and Slanger
, 2008]. It should be noted that this same transition can be accessed via lidar pumping in the absence of solar illumination, also the case for the helium line at 1.083 μm [ Waldrop et al ., 2005]. Prominent in the figure is emission from the N
2
+ ( B-X ) 0-0 band.

FIGURE 7. UVES/VLT twilight glow spectrum, showing solar pumped helium and the N
2
+
1NG 0-0 band.
A further investigation of interest related to the twilight/dayglow makes it possible to study the effects of solar flares and coronal mass ejections on the atmosphere.
New emissions can be produced when high energy photons slam into the upper atmosphere, and it is likely that such enhanced excitation will even be reflected in the nightglow.
Relationships to Extra-solar Planets
The in-depth study of the terrestrial atmosphere that will be afforded by the envisioned lidar/optical facility can be viewed, in the long term, as a contribution to the burgeoning field of extra-solar planet investigations. Studies with the Kepler telescope are making it clear that there are enormous numbers of planets in the universe, and that our solar system can be viewed as a not uncommon example of a star surrounded by a number of diverse planets. It seems probable that many of the planets will be found to have oxygen/nitrogen atmospheres, and, from our local sampling, atmospheres of CO
2
. It is therefore to be expected that much of what we learn from our sun’s planets will be directly transferable to the investigation of extra-solar planet atmospheres within the next couple of decades. In a sense, it is not necessary to reinvent the wheel for each atmosphere.

References
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Llewellyn ???
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