UV Laboratory Studies of N2 and Cassini
UVIS Observations of Dayglow, Twilight
glow and Nightglow at Titan
Joseph Ajello
JPL
Greg Holsclaw
LASP
Todd Bradley
UCF
Bob West
JPL
CIRS 2011 2.5 Hour Set of 36 Titan UVIS Dayglow
Observations Near Zero Degree Phase: Equator to Pole
Titan Dayglow EUV Intensities with
Latitude are a Chapman Layer with SZA
Comparison High Resolution Lab
Spectrum and Titan UVIS

High-resolution
(FWHM = 0.2 Å)
extreme-ultraviolet
(EUV, 800−1350 Å)
laboratory emission
spectra of N2 by
electrons at 20 and
100 eV. A total 491
emission features
observed from N2
electronic-vibrational
transitions and N I,
II multiplets.
The photoelectron-PDI excited dayglow
limb EUVS emission spectrum observed by
the Cassini UVIS from Titan June 22nd 2009
The EUVS limb (8001200km) dayglow of
Titan by Cassini UVIS
(blue curve) compared
to lab spectra at 4–5 Å
FWHM forv20 (red) and
100 eV (green) electron
energies. Strongest
features are indicated
[Stevens et al.,2011].
N2 Diabatic (crossing) Potential-energy Curves for
1S + and 1P excited states for Cassini UVIS Titan
u
u
EUVS Emission
N2 electronic transitions
from X 1Σg+ ground-state
to closely-spaced (12–15
eV) 8 observed states for
Cassini EUVS emission: the
b, c3, c4, and o3 1Πu states;
the b', c'4, and c'51Σu+
states;. Strong
perturbations mix Rydberg
(n>2) and valence states
(n=2) of like and unlike
symmetry with spin-orbit
3Π predissociation, also
u
rotational perturbations. +
Electron-impact-induced fluorescence lab and
model spectra 912 Å - 974 Å for 20 and 100 eV.


Experimental (blue),
model (red) spectra &
residual difference
(grey).
N2 spectra compared
with a quantummechanical model
simulates electronimpact excitation,
fluorescence and
predissociation. Model of
N2 excited states solves
a coupled-Schroedinger
equation (CSE) for radial
(vibrational) motion of
the molecule.
Electron-impact-induced fluorescence lab &
model spectra 974 Å - 1020 Å for 20 & 100 eV.


Factor of 10 Reduction
of c'4(0,0) due to selfabsorption leads to
959Å,… recycling &
enhancement of c'4(0,
v''>0) 980, 1002Å
fluorescence.
CSE model partitions lab
fluorescence between
491mfeaturesbwhich are
partially blended. A clear
example of this utility is
the group of N2 bands
between 1006 and 1012
Å, distinguishable when
band profiles known.
Electron-impact-induced fluorescence
lab & model spectra 1130- 1186 Å for 20
& 100 eV.



Strongest molecular
emission c'41S+u→X 1S+g
Carroll-Yoshino bands.
New members of the c'4 (0,
v'') progression observed in
the lab, unexpected
emission to v''=5–9 as
observed in at Earth by the
FUSE satellite (Bishop et al.,
2007) from homogenous
coupling to b' 1S+u states
like c'4 (0,8) 1156 Å &
c'4(0,9) 1183 Å.
The lab UV emission & model of c'4(0,1)
& c'4(0,9) bands by 20 eV e-impact

Measured 20 eV
spectrum compared
to model strengths
of rotational
transitions. Emission
from broad
envelope of P- and
R-branch rotational
lines for c'4(0,1),
while c'4(0,9)
emission from
excited state levels
with J=10 and 11
The Q(v‘, v‘’’) 20 & 100 eV Emission
cross sections of c'4(v').



Vibrational c'4 cross
sections plotted with
model calculations.
Model emission cross
sections agree with
measured values,
validating CSE &
necessity of modeling
homogeneous coupling
between c'4 1Σ+u and b'
1Σ
+u states, when
calculating c'4 (0,0–3)
emission, a point further
demonstrated for higherv" .
Experiment and model emission cross sections for eight N2
band systems
High Resoltuion e-impact fluorescence
spectrum convolved with 5 Å FWHM IP
Lya Cassini FUV Observation
Titan Dayglow FUV Intensities with
Latitude are a Chapman Layer with SZA
CIRS 2011 48 minute Set of 12 Titan
UVIS Nightglow Observations
o and
(SZA=132 )
FUV Nightglow on Disk
Twilight (SZA=112o) Glow on Limb