UPDATE ON TITAN’S
NIGHT-SIDE AIRGLOW
P. LAVVAS, R.A. WEST, G. GRONOFF, P. RANNOU
UVIS Team meeting - Jun 2014
1
Combined study of
UVIS and ISS observations
during Titan’s eclipse
L18204
WEST ET AL.: TITAN AIRGLOW
West et al. 2012
2
Figure 1. (top) UVIS Titan FUV airglow data on 7 May
indica
dayglo
(bottom
is dom
and w
than th
[8]
so we
1000–
In Fig
synthe
then s
lution
or sys
multip
same
FUV
(0.5 #
with t
multip
the da
2.2.
[9]
sure ti
UV (3
coveri
et al.,
era as
Observations
Table 1: List of ISS images analyzed along with the derived DN values.
Image
W1620379032
W1620381930
W1620383876
W1620385882
Image mid time
(UTC)
2009-127T08:31:22.675
2009-127T09:19:40.705
2009-127T09:52:06.718
2009-127T10:25:32.731
Filters
CL1, BL1
CL1, VIO
CL1, CL2
IR2, CL2
c
(nm)
460
420
635
853
Distance
(km)
644030.77
658002.55
667326.14
676887.08
photons
befield
of have
view.wavelengths
This correction
was
performed by subTable 2: Observed emissions (in Rayleighs) during Titan eclipse by
is spectral
behavior
is a result
racting
a scaled
imaged
of Titan UVIS
captured during direct
en
the spectral
West et al. (2012)
Ajello et al. (2012)
unlight
at the distribution
same phaseofangle andFeature
filter combination,
on,
spectral
dependence
of size. The absolute scaling
Limb
Limb
Disk
ndthe
scaled
at the
same body
a
n, and finally the transmission
LBH
7.2±5.1
⇠10
f the dayside image was performed in such a manner
s we show below.
VK
3.9±2.6
-b
hat the residual intensity is continuous across the terCY
0.3c
minator as expected for a nightglow process. The reNI 1200
>
3.1
2.0
ion of interest of the final image isNI
shown
in Fig. 4.4±1.4
1 and
1243
0.94
0.52
observations
captured
Titan’s
emonstrates the bright emission atNIthe1493
main haze layer
?
1.93
0.55
h a pixel scale of 600 km. Due
c
op (300 km) and the faint emission
between
3000.4±0.1
and
NII
1085
0.2
he coarseness of the grid the
a
000
km.
LBH was not discernible on disk observations.
tra (57 in total) had to be cob
Observed but emission rate not reported.
anTo
acceptable
signal
to noiseof the very
get a robust
estimate
weak signal at the
c
These values are averages over both disk and limb spectra.
Ajello
et al.,
Although
imb we
can2012).
compare
the pixel values of the limb with
ctravalues
demonstrate
strong
noisefrom the limb. For this purhe
of pixels
away
oose
discriminate
the signature
we examined
histograms
ofThis
signal
values for
two
264
calculation
will
provide us an idea about the minmolecular features. In the
3 to generate the airglow
nnuli, one for the limb between
300 and
1000budget
km, and
265
imum
energy
required
m the Lyman-Birge-Hopfield
<Limb DN>
<Disk DN>
1.1
0.2
11
0.5
4.5
0.7
71.7
6.5
Energy Budget
ISS - Visible
Limb : 3x10-6 erg cm-2 s-1
Disk : 2x10-5 erg cm-2 s-1
UVIS - UV
VK : 8.6x10-4 erg cm-2 s-1
LBH : 1.4x10-4 erg cm-2 s-1
CY : 4.0x10-6 erg cm-2 s-1
N,N+ : 9.7x10-5 erg cm-2 s-1
Total Limb :
~10-3 erg cm-2 s-1
4
INCLUDING UNOBSERVED
COMPONENT
POTENTIAL ENERGY SOURCES
1200
1000
Altitude (km)
800
600
Type
Magnetospheric electrons
Magnetospheric protons
~800
~500 (silicates)
~700-800 (H2O)
Chemi-luminescence
(erg cm-2 s-1)
(depending on B)
~800
Meteoroids
Energy Flux
~1000
Magnetospheric ions (O+)
400
200
Deposition
Altitude (km)
~4-8x10-3
small
~200 (C2H2)
Cosmic Rays
~65
5
~2x10-3
Production rates for N2 states
High
~10
Low
Solar
GCR
LOCAL EMISSION RATES
7
COMPARISON WITH OBSERVATIONS
Night
Day
10 of 17
Complete Band Emissions
Night profiles = Day profiles /10
(roughly)
8
Stevens et al. 2011
COMPARISON WITH OBSERVATIONS (UVIS)
Table 3: Comparison between simulated emissions and UVIS observations from West et al. (2012). All values are in Rayleighs.
Band
LBH
VK
NI
NII
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
9
634
635
Model
Total UVIS
10.3
8.9
39.4
1.1
1.3
0.9
0.2
0.2
Observation
Ratio
7.2±5.1
3.9±2.6
4.4±1.4
0.4±0.1
0.8
3.5
4.7
2
by aerosols) below 400 km tangent altitude, while emissions from the Meinel band dominate at all tangent altitudes, followed by emissions from the Vegard-Kaplan
and 1 st positive bands.
The UVIS spatial pixel size for these observations
is large (⇠ 650 km) and we can not derive spatial information of the emitted radiation. Instead, the signal of pixels that correspond approximately to the altitude range between 0 and 2000 km above Titan’s surface was added in order to derive the spectra reported
in West et al. (2012). These correspond to about 3
pixels from each side of Titan along the UVIS track.
In order to compare these observations with our model
we calculated the average emission for each simulated
band over the model altitude range (0-1500 km) and
compare these average emissions with the UVIS observations (Fig. 12). The simulated FUV spectrum is
dominated by emissions from the Vegard-Kaplan and
F
l
H
f
c
COMPARISON WITH OBSERVATIONS (ISS)
CL1,CL2
CL1,BL1
IR2,CL2
CL1,VIO
Table 4: Comparison between observed DN values for disk and limb observations from di↵erent ISS filters, and the corresponding DN values from
our model. The uncertainty on the observed DN values are of 0.2 DN (see text).
Observed
Modeled
Ratio
Limb Average (300-1000 km)
CL1,CL2 CL1,VIO CL1,BL1 IR2,CL2
11
0.2
1.1
0.5
4.35
0.17
0.42
0.39
2.53
1.18
2.62
1.28
10
CL1,CL2
71.7
0.1
717
Disk Average
CL1,VIO CL1,BL1
0.7
4.5
0.005
0.014
140
321
IR2,CL2
6.5
0.004
1625
OTHER POTENTIAL CONTRIBUTIONS
I. Chemi-luminescence
C2H2 ~300 ns
C4H2 ~100 ms
II. CH4 emission
Only from dissociation fragments
CH (420-440 nm) but too weak
Fluorescence of T
Hodyss et al. 2004
III. Aerosol fluorescence
~10 photons cm-2 s-1
too small
11
Fig. 5. Fluorescence spectra of tholin. (a) Cut through the plot in Fig. 2b
at an excitation wavelength of 360 nm. (b) Fluorescence of tholin in ice at
77 K. (c) Fluorescence of tholin in ice at 77 K, after boiling for 5 minutes.
(d) Fluorescence of solid tholin at 77 K.
OTHER POTENTIAL CONTRIBUTIONS
IV. Star Light
OPTICAL EBL. I. RESULTS
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band filters and photometers, as were used by Dube et al.
(1977, 1979). Finally, IRAS has provided maps of the thermal emission from dust at high Galactic latitudes. We have
used the IRAS maps to select a line of sight for these observations that has a low column density of Galactic dust in
order to minimize the DGL contribution caused by dustscattered starlight and also to estimate the low-level DGL
that cannot be avoided.
Our measurement of the EBL utilizes three independent
data sets. Two of these are from HST: (1) images from the
Wide Field Planetary Camera 2 (WFPC2) using the F300W,
F555W, and F814W filters, each roughly 1000 Å wide with
central wavelengths of 3000, 5500, and 8000 Å, respectively,
and (2) low-resolution spectra (300 Å per resolution element) from the Faint Object Spectrograph (FOS) covering
3900–7000 Å. The FOS data were taken in parallel observing mode with the WFPC2 observations. While flux calibration of WFPC2 images and FOS spectra achieve roughly
the same accuracy for point-source observations, the
increase in spatial resolution, a 104 times larger field of view,
lower instrumental background, and absolute surface
brightness calibration achievable with WFPC2 make it betFig. 1.—Relative surface brightnesses of foreground sources, upper
ter suited than FOS-2to an
-1absolute
-1 surface
-1 brightness meaStar
light
~105 photons
cm
s
nm
sr
limits on the EBL23 Direct
(see x 1), and lower
limits
based onsource:
the integrated flux
surement of the EBL. Nonetheless, the FOS observations
from resolved galaxies (V555 > 23 AB mag) in the HDF (Williams et al.
4
do provide a second,
of the total
lightlight
source:
photons
cm-2independent
s-1 nm-1measurement
sr-1
1996). The spectral shape Zodiacal
and mean flux of zodiacal
and of DGL ~10
are
background flux of the night sky, also free of terrestrial airshown at the levels we detect in this work. The airglow spectrum is taken
CL1/CL2
DN~10
(observed
= 71.7)
from Broadfoot & Kendall (1968) and is scaled to the flux level we observe
glow and extinction,
but with greater spectral resolution
at 3800–5100 Å (see x 9). The effective bandpasses for our HST observathan the WFPC2 images. The third data set consists of longtions are indicated at the bottom of the plot.
slit spectrophotometry of a region of ‘‘ blank ’’ sky within
12 the WFPC2 field of view. These data were obtained at the
‘‘ blank screens,’’ spatially isolating all foreground contribu2.5 m du Pont telescope at the Las Campanas Observatory
OTHER POTENTIAL CONTRIBUTIONS
IV. Star Light
Stellar background illuminating Titan’s
*
disk
Stellar background on ISS field of view
~4
This ratio would have to be 2-3 times larger to match the
observations, which could be the case if all possible light
sources are taken into account.
*Using TRILEGAL model we compared the stellar fluxes from 37 bright sky locations visible from Titan’s
disk, with the corresponding fluxes from a 10ox10o FOV around the ISS bore sight.
(Girardi et al. 2005, http://stev.oapd.inaf.it/cgi-bin/trilegal)
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CONCLUSIONS
I. Strongest emissions (ISS) in the upper atmosphere come from the
Vegard-Kaplan, 1st positive and Meinel bands, while for the lower
atmosphere 1st negative and 2nd positive dominate. For UVIS/FUV
Vegard-Kaplan, LBH and atomic N emissions dominate.
II. Simulated limb emissions are consistent with observations and are
dominated by nominal magnetospheric energy input.
III. Disk observations are much higher than the simulated emissions.
Reflection of stellar light from Titan’s disk appears to be the most likely
explanation.
Manuscript under review in Icarus
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