A summary of our recent work using terrestrial nightglow spectra, obtained from astronomical instrumentation, to directly measure, or evaluate theoretical values for fundamental parameters of astrophysically important atomic lines

Cosby, P. C.

Huestis, D. L.

Sharpee, B. D.

Slanger, T. G.

NASA Technical Reports Server

NASA LAW, February 14-16, 2006, UNLV, Las Vegas
Studying Atomic Physics Using the Nighttime Atmosphere as a
Laboratory
B. D. Sharpee, T. G. Slanger, D. L. Huestis, & P. C. Cosby
Molecular Physics Laboratory, SRI International, Menlo Park, CA 94025
brian.sharpee@sri.com
ABSTRACT
A summary of our recent work using terrestrial nightglow spectra, obtained
from astronomical instrumentation, to directly measure, or evaluate theoretical
values for fundamental parameters of astrophysically important atomic lines.
1. Introduction
While the terrestrial nightglow is dominated both in total emission intensity and num-
bers of emission lines by molecular emitters such as OH and O2, emission lines corresponding
to atomic transitions are also prevalent. The optically forbidden [O I] 2p4 1D2-
1S0 λ5577
“green line” and [O I] 2p3 3P2,1-
1D2 λλ6300,6364 “red” lines are strong and well-known com-
ponents of the nightglow. The high sensitivity of astronomical instrumentation also reveals
much weaker features such as [N I] 2p4 4So
3/2-
2Do
3/2,5/2 λλ5198,5200, the [O II] 2p
3 2Do
3/2,5/2–
2Po
1/2,3/2 λλ7320,7330 doublets, and permitted transitions of neutral O arising from electron
radiative recombination of O+.
Many of these lines are present in low-ionization astrophysical plasmas such as those
found in planetary nebulae and H II regions, and are used as diagnostics of their kinetics,
physical conditions, and abundances. The use of these lines as diagnostics assumes accurate
knowledge of their fundamental atomic parameters, namely wavelengths, spontaneous emis-
sion coefficients, and electron recombination cross-sections. Given that many of the emitting
levels are metastable and therefore difficult to observe in the laboratory, a great reliance has
been placed on theoretical calculations of these parameters. However, due to the rarefied na-
ture of the atmosphere and the abundance of emitters in the nightglow layers, the nighttime
sky acts as a laboratory where these parameters can either be directly measured or inferred,
and compared to theoretical values.
https://ntrs.nasa.gov/search.jsp?R=20060052477 2019-08-29T18:55:02+00:00Z
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Table 1: HIRES Measurements of Atomic Nightglow Line Wavelengths versus NIST
Atomic Line HIRES λ (A˚) NIST λ (A˚) HIRES-NIST λ (A˚)
Slanger et al., J. Chem. Phys., 2000
[O I] 2p4 1D2–2p
4 1S0 5577.335 5577.339 -0.004
[O I] 2p4 3P2–2p
4 1D2 6300.302 6300.304 -0.002
[O I] 2p4 3P1–2p
4 1D2 6363.780 6363.776 0.004
[N I] 2p3 4So
3/2–2p
3 2Do
3/2 5197.922 5197.902 0.020
[N I] 2p3 4So
3/2–2p
3 2Do
5/2 5200.281 5200.257 0.024
Sharpee et al., Astrophys J., 2004
[O II] 2p3 2Do
5/2–2p
3 2Po
1/2 7319.044±0.004 7318.92 0.124
[O II] 2p3 2Do
3/2–2p
3 2Po
1/2 7320.121 7319.99 0.131
[O II] 2p3 2Do
5/2–2p
3 2Po
3/2 7329.675 7329.67 0.005
[O II] 2p3 2Do
3/2–2p
3 2Po
3/2 7330.755 7330.73 0.020
2. Forbidden Line Wavelengths
The High Resolution Echelle Spectrometer (HIRES) on the Keck I telescope, W.M.
Keck Observatory, has provided a wealth of individual nightglow spectra, which have been
co-added together (∼100 hours) to provide a very high signal-to-noise spectrum that has
been proven useful for transition wavelength measurements (Slanger et al. 2000). Wavelength
calibration by ThAr lamp and comparison with numerous OH Meinel band system nightglow
line wavelengths provide a level of accuracy of about 0.004A˚. Comparisons with National
Institute of Standards and Technology (NIST) wavelengths for various atomic lines, listed
in Table 1, show good agreement for some lines, but poorer agreement for others.
Other HIRES nightglow spectra, supplemented by selected planetary nebula and H II
region spectra, have been used to establish new energy levels for the 2p3 ground electron
configuration of O II, and for inferring new wavelengths for the [O II] λλ7320,7330 doublet
lines (Sharpee et al. 2004). As can be seen in Table 1, the difference between NIST and
observed values is quite substantial (upwards of 5.4 km s−1) adversely affecting, for example,
the use of these lines as a Orion Nebula outflow velocity indicator (Baldwin et al. 2000).
3. Diagnostic Line Intensity Ratios
Nightglow spectra from HIRES and the Echellette Spectrograph and Imager (ESI)
on Keck II, have been used to measure the optically forbidden line intensity ratios [O I]
λ6300/λ6364 and [N I] λ5198/5200. While not providing a direct determination of these
lines’ spontaneous emission coefficients, these measurements allow discrimination between
competing and sometimes widely varying theoretical values.
For [N I] λ5198/λ5200, the observed value, 1.759±0.014, departs significantly from that
– 254 –
Fig. 1.— LEFT: [N I] λ5198/λ5200 intensity ratio from 190 HIRES and ESI nightglow
spectra, best fit (1.759), and ratio determined from NIST spontaneous emission coefficients
(2.61). RIGHT: Observed [O I] λ6300/λ6364, best fit (2.997), and NIST (3.10).
calculated with the spontaneous emission coefficients recommended by NIST (2.61), as is seen
in Figure 1 (Sharpee et al. 2005). The ratio is used as an electronic density diagnostic, and
the difference in densities determined using the NIST coefficients and those yielding the best
agreement with observation can reach an order of magnitude for an observed ratio at typical
nebular temperatures. By contrast, the observed [O I] λ6300/λ6364 ratio of 2.997±0.016
shows better agreement with the consensus theory value of ∼3.1 including NIST, as is seen
in Figure 1 (Sharpee & Slanger 2006), but is slight lower reflecting the subtle effects of the
inclusion of relativistic corrections to the magnetic dipole operator made during the recent
coefficient calculation of Storey & Zeippen (2000).
4. Weak Permitted Lines From Electron Radiative Recombination of O+
HIRES and ESI spectra have revealed numerous weak permitted lines of neutral oxygen
that are produced by electron radiative recombination of O+ to excited levels, and subsequent
radiative cascade (Slanger et al. 2004). Recombination lines, while generally much weaker
than their forbidden counterparts in planetary nebulae and H II regions, are the lines of choice
for abundance determinations due to their insensitivity to variations in electron temperature
and density along a line of sight through a nebula.
The effective recombination coefficients that directly link the intensities of these lines
with parental abundances combine several fundamental parameters, including electron re-
capture cross-sections and cascade branching ratios. Line intensities from both the triplet
and quintet sequences measured in HIRES and ESI spectra are found to be proportional to
their effective recombination coefficients (Escalante & Victor 1992) by the same constant, the
– 255 –
product of electron and O+ densities, which in turn yields a sensible O+ abundance for con-
ditions likely to prevail in the ionosphere at the time of the observation. This validates their
coefficient calculation methods, providing greater confidence in similar calculations made for
other astrophysically important ions.
This work was supported by NSF grant ATM-0221700, NSF CEDAR program grant
ATM-0123136 and the NASA Office of Space Science.
REFERENCES
Baldwin, J.A. et al. 2000, ApJS, 129, 229
Escalante, V. & Victor G. A. 1992, Planet. Space Sci., 40, 1705
Sharpee, B. D., Slanger, T. G., Huestis, D. L., & Cosby, P. C. 2004, ApJ, 606, 605
Sharpee, B. D., Slanger, T. G., Cosby, P. C.,& Huestis, D. L. 2005, Geophys. Res. Lett.,
doi:10.1029/2005GLO23044, L12106
Sharpee, B. D. & Slanger T. G. 2006, J. Phys. Chem. A, in press
Slanger, T. G., Huestis, D. L., Cosby, P. C., & Osterbrock, D. E. 2000, J. Chem. Phys., 113,
8514
Slanger, T. G., Cosby, P. C., & Huestis, D. L. 2004, J. Geophys. Res.,
doi:10.1029/2004JA010556, A10309
Storey, P. J. & Zeippen, C. J. 2000, MNRAS, 312, 813


Studying Atomic Physics Using the Nighttime Atmosphere as a Laboratory

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