PS12-D4-PM2-P-020
National Aeronautics and Space Administration
Experimental and Coupled-channels Investigation of N2 c4′ 1+u(0) and b′ 1+u(1) - X 1+g(v′) transitions and
Application to the Analysis of Terrestrial-thermospheric Dayglow Emissions Observed by FUSE
Xianming Liu1,3, Alan N. Heays2, Brenton R. Lewis2, Charles P. Malone1, Paul V. Johnson1, Donald E. Shemansky3,
Stephen T. Gibbson2, Glen Stark4 Paul D. Feldman5 and Isik Kanik1
1
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA,
Research School of Physical Sciences and Engineering, The Australian National University, Canberra, Australian Capital Territory, Australia
Planetary and Space Science Division, Space Environment Technologies, Pasadena, CA, USA
4
Department of Physics, Wellesley College, Wellesley, MA, USA
5
Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, USA
2
3
Introduction
The radiative properties of molecular nitrogen are critical for the
determination of the state of the gas in the thermospheres and
ionospheres of the nitrogen dominated atmospheres of Earth, Titan and
Triton. N2 is transparent to solar radiation from the IR to FUV where
the forbidden Lyman-Birge-Hopfield and Vegard-Kaplan band systems
show weak, discrete absorption transitions. Strong absorption of solar
radiation occurs only at wavelengths below ~100 nm where the singlet
ungerade electronic states show strong discrete blended structure. The
singlet ungerade states are closely spaced in energy and strongly
coupled. Many of the states are strongly predissociative and a major
source of chemical radicalization in the atmosphere.
The emission properties of the N2 c4′ 1+u–X 1+g band system have
been investigated in a joint experimental and coupled-channels
theoretical study. Relative intensities of the c4′ 1+u(0)–X 1+g transitions,
measured via electron-impact-induced emission spectroscopy, are
combined with a coupled-channel Schrödinger equation (CSE) model,
enabling determination of the diabatic electronic transition moment for
the c4′ 1+u–X 1+g system as a function of internuclear distance. The CSE
rotational transition probabilities are further verified by comparison
with a high-resolution experimental spectrum.
The Far Ultraviolet Spectroscopic Explorer (FUSE) observation of
terrestrial-thermospheric dayglow shows strong emission of the
coupled c4′ 1+u(0) and b′ 1Σ+u(1)–X 1+g transitions. The CSE calculated
radiative properties are utilized to analyze the observed dayglow
emission spectra. Model emission spectra for the c4′ 1+u(0) and b′ 1Σ+u
(1)–X 1+g(v=2, 6-9) transitions, calculated for the case of excitation
by photoelectron impact, are in excellent agreement with the
observations. While the principal excitation mechanism for N2 in the
thermosphere is photoelectron impact, evidence is also found in other
transitions of resonant fluorescence, induced by lines in the solar
atomic-hydrogen Lyman-series, atomic-oxygen transitions, and other
N2 bands. The observed emission rate of the c4′ 1+u(0) and b′ 1Σ+u(1)–X
1 +
g(0) band is ~1% of that inferred from the emission rates to X 1+g
(v>2) levels. Estimates of the total electron-excitation rates for the
nominal c4′ 1+u(0) and b′ 1Σ+u(1) levels are determined from the spectrum
by extrapolating the model through regions containing unmeasured
and/or resonantly-absorbed bands.
Results from measurement and CSE analysis
Application to the analysis of terrestrial-thermospheric
N2 dayglow emission observed by FUSE
Experiment
The experimental system consists of an Acton VM-523-SG 3-m
spectrometer and an electron collision chamber. Electrons generated by
heating a thoriated tungsten filament are magnetically collimated by an
axially symmetric magnetic field of ~100 G and accelerated to a kinetic
energy of 20 or 100 eV. Horizontally accelerated electrons collide with
either a vertical beam of N2 gas formed by a capillary array (crossed
beam mode) or with a uniform swarm of N2 gas (‘swarm’ mode). In the
crossed beam mode, the cylindrical interaction region is ~3 mm in
length and ~2 mm in diameter. Optical emission from electron-impactexcited N2 is dispersed by the spectrometer that is equipped with a
1200/mm grooves grating. The spectrometer has an aperture ratio of
f/28.8 and a field of view of 3.8 mm (horizontal) by 2.4 mm (vertical).
The dispersed radiation is detected with a channel electron multiplier
coated with CsI. A Faraday cup is utilized to monitor the beam current
and to minimize the backscattered electrons. A calibrated ionization
gauge is used to monitor the N2 pressure in the collision chamber.
Measurements were carried out in swarm and crossed beam modes at
various pressures to ensure that self-absorption at the c4′ 1+u(0)–X 1+g
(0) band is negligible.
Instrumental range and
resolution
1st order – 64 mÅ, 300-3700Å
2nd order – 32 mÅ, 300-1850Å
3rd order – 21 mÅ, 300-1230 Å
(FWHM measured with e+H2 point
source, 10mm slits, and the 1200
G/mm grating)
Relative instrumental sensitivity
was calibrated by e+H2 model
with error <8% in 900-1630Å
and <11% in 790-900Å
N2 CSE Model
Considers interactions among b′, c4′ and c5′ 1+u, b, c3 and o3 1u, and C,
C′, F and G 3u states.
Homogeneous electrostatic interactions among the states within the 1+u,1 u and
3
 u Rydberg-valence manifolds.
Homogeneous spin-orbit interactions between the 1+u, 1 u and 3 u manifolds.
Heterogeneous L-uncoupling interactions between the 1u+ and 1 u manifolds
All interactions depend on internuclear distance, R.
Utilizes ab initio potential curves refined by experimental term values
and transition moments refined by photoabsorption and electron impact
induced emission measurement.
Fits CSE cross section to Fano profile to obtain resonance energy,
width and oscillator strength.
Summary
Terrestrial thermosphere temperature of 500±50 K is inferred.
Combined column integrated excitation rate to c4′ 1+u(0) and b′ 1Σ+u(1)
states is determined to be 3.4×109 cm−2 s−1, significantly larger than
1.4×109 cm−2 s−1 estimated by Bishop et al. (2007).
Multiple scattering and predissociation results in 99% and 75%
radiation loss for the c4′ 1+u(0)–X 1 +g(0) & X 1+g(1) bands, respectively.
In addition to the excitation by photoelectrons, some N2 states such as
the v′=4, 6, and 7 levels of b′ 1Σ+u and the v′=3 and 6 levels of b 1u are
also excited by solar H Lyman lines.
References
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Point of Contact:
E-mail:
Phone:
National Aeronautics and Space Administration
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California
www.nasa.gov
Dr. Xianming Liu
Xianming.Liu@jpl.nasa.gov
(818) 390-1721
Acknowledgment:
This work was performed at the Space Environment Technologies, the Australian National University, Wellesley
College, Johns Hopkins University, and at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under
a contract with the National Aeronautics and Space Administration (NASA). We gratefully acknowledge financial
support through NSF-ATM and NASA PATM and Cassini UVIS programs.