Cassini Team Meeting June 2008
INTRODUCTION:
Slide 1-INTRODUCTION
-Present Limb Profiles of 13Dec04 since controversial
VI work 25 years ago (SS=1100 km, Hunten 1300km,
on c’ bands), which may have been mostly NI lines
indicated ME (exobase) originally and later was leaning
to solar PEs
-Establish limb profiles of LBH NI and other lines and
Rayleigh Scattering
- Model approach includes laboratory data.
Slide 2-GEOMETRY
Trajectory 160,000 to 85,000 km limb to limb over
4 hours
-overhead sun and 90o emission angle
Inbound orbit is one of a dozen or so observations to
date Titan B orbit on 13dec2004, second initial orbit .
orbit grazes the magnetopause Titan Tb observations
take place inside the magnetosphere. We see fully
illuminated disk and limb
Slide 3-LIMB SPECTRA AT 9 ALTITUDES
Limb spectra 90o emission and solar incidence
angles
Note regions of strong solar reflectance near
surface and strong LBH near 1100km
Slide 4-REVIEW OF DISK REGRESSION
Review of FUV results explains modeling technique
solar angles are average over disk
the UVIS disk airglow spectrum and regression
model fit over the spectral range of 1150-1750 Å is
shown in Figure. It is observed that there are about
20 well-defined emission features from the LBH
bands of N2 and the atomic multiplets of H, C and
N from 1150-1570 Å along with about a half-dozen
solar chromospheric lines from 1570-1750 Å
excellent fit verifying spectrally the
appearance of tholins
FUV spectra
- Day or night
- Bright on dayside and dark on nightside-hint of
NI 1200.
- Day intensities w/o Rayleigh scattering using my
lab spectrum to normalize out the LBH band
system-deep in atmosphere=300m
- LBh =44 R PE (optically forbidden) =hi in
atmosphere
- NI =30R\
- HI =170 R PDI
In order
-Rayleigh scattering
-LBH PEs
-NI Lines PDI
-H Ly 
-two strong vectors LBH and Rayleigh scattering
Solar spectrum is TIMED SEE from 13dec04
Reflectance spectra is now SORCE SOLSTICE TIEM
SHIFTED TO -2.3 DAYS=-31 DEG so use dec 11
SORCE spectrum
Slide 5-MIXING RATIOS ARE PARAMAERS IN
MODEL COMPARE TO WILSON AND ATREYA
AND LIANG
Compare to other results at 300 km Liang et al and
Wilson and Atreya
- 1723 pixels with average u and u0 of ~45 deg is
modeled separately and is average disk spectrum
- Main HCs in stratosphere are c2h2,
c2h4(ethylene), c4h2(diacetylene) that absorb
1500-1800
- Need tholin to fit
-first vector
Interaction between solar XUV radiation
field shortward of 1500A produces a) ionosphere
at 900 km and shortward of 1000A
photodissociates N2 and shortward of 1350
photdissociated CH4 to produce radicals like HC
polymers while longward of 1400 a not involved in
chemistry and penetrates deeper and is Rayleigh
scattered at unit optical depth ~1 x 10(24) cm-2 and
absorbed by c2h2 and others at ~300 km
-we are probing to 300 km
-reflected lite depends on mixing ratio at 300
km thru the albedo for single scattering
-tried to fit with and without tholins
-Lommel-Seeliger law)
spectral vector is created from best fit of HCs
and tholins by modeling long wavelength is second
verifications of tholins from a spectrum using
Liang et al cross sections fro 125 A particles
- 1723 pixels with average u and u0 of ~45 deg is
modeled separately and is average disk spectrum
- Main HCs in stratosphere are c2h2
ACETYLENE, c2h4(ethylene), c4h2 (DIACETYLENE) that absorb 1500-1800
- Need tholin to fit
Slide 6 TITAN REFLECTIVITY INTO FUV
-IUE and UVIS decrease due to HC and tholin
extinction 3% at 3000A to 1.5% at 1500A
-Function of phase angle –not corrected to
zero phase angle?
-C2h2 bands are strongest absorption feature
mcGrath et al. mention bi-modeal distribution
geometric albedo is the most useful way for
studying the nature of the haze (McKay et al.,
2001). We can show for the first time the FUV
geometric albedo of Titan from 1500-1900 Å in Fig.
4. We plot in Fig. 4 the modeled geometric albedo
and the UVIS FUV integrated intensity divided by
the TIMED SEE solar flux (4I/F) and compare
the results to complementary results for the middle
ultraviolet (MUV) from International Ultraviolet
Explorer (IUE) (McGrath et al., 1998) and for
visible near IR (based on spacecraft and groundbased results compiled by McKay et al., 2001). The
modeled geometric FUV albedo (inset: Fig. 4)
indicates a value of 0.017 at about 1850 Å with a
general trend to decline with decreasing
wavelength.
Slide 7
-
REPEAT OF LIMB SPECTRA
SLIDE 8-ALTITUDE DISTRIBUTION OF THE 4MAIN VECTORS/SPECTRAL COMPONENTS
Slide 9 MODEL OF LONG WAVELENGTH FUV
REFLECTANCE BY ALTITUDE
"bestfit_titan_limb_200_250.idl" is the result for emission
>from the pixels at rayheight
>from 200 km to 250 km. Each file has:
>- TITAN_FUV_LAMBDA: wavelength
>- TITAN_FUV_KR_A: limb observation
>- SYNTH: best fit
>- RESULT: contains the best values: result(0): N2 mixing ratio, result(1):
>C2H2 mixing ratio,
>result(2): tholin mixing ratio, result(3): C2H4 mixing ratio, result(4):
>C4H2 mixing ratio.
>The S/N ratio is still good for 50km bins, so I guess that's the spatial
>resolution we should use.
-reflected lite depends on mixing ratio at
MINIMUM RAY HEIGHT altitude km thru the
albedo for single scattering
fit with tholins
-Lommel-Seeliger law)
Second spectral vector is created from best fit
of HCs and tholins by modeling long wavelength is
second verifications of tholins from a spectrum
using Liang et al cross sections fro 125 A particles
16 strongest blended features ID based on lab
spectra and FUSE Ajello et al.,
- Feature 6 in detail with red cassini and black 20 ev lab
low res and blue 100 eV high res 0.2 A FWHM
Slide 10-LOWER ATMOSPERE 100 KM
STRONGEST IS BUE=RAYLEIGH scattering
Lyman alpha is green
LBH is red
Atomic N is purple
Regression is orange
Deficits are 1561, 1597, 1657, 1722A
Slide 11-MIDDLE ATMOSPERE
STILL STRONG Rayleigh scattering
SLIDE 12-UPPER ATMOSPHERE AT 1100 KM
Long wavelength deficit in LBH bands
MODELS AT 0 and 40% enhancement beyond
1400 A CH4 methane edge
SLIDE 13 LBH BANDS LAB SPECTRUM
HIGH VIBRATIONAL NUMBER OR LONG
WAVELENGTHS ARE ENHANCED
LBH first vector
-LBH band system extends from 1250-2600 A
-complicated system (mag dipole+ electric
quadrupole uused lab spectra at high resolution to
convolve with UVIS 5A FWHM
-strongest 1300-1500A ~ as PE flux ie solar XUV
-source of Space Weather on Nitrogen bearing
planets TIMED=10% accuracy based on Q
Slide 14 MODEL INCLUDING SCATTERED LIGHT
FOV TO 0.6 O
FUV last line-of-sight that sees full column is at
1200 km then we see near field near S2 only for
short wavelengths and full column till 1024 for long
wavelengths whch reaches iptical depth unity at
minimum ray height of 200-300km
Fuv allows lite shortward of 1400A come from
slant columns of 4.* 1016 while long wavelengths
come from 1024
FOV allow lower atmosphere signal to contribute
Rayleigh scattering and LBH from Lower
atmosphere
Slide 15 MODEL WITH FULL SLANT COLUMN
DENSITIES OF STELLAR OCCULTATION
FROM TABLE 1 OF LIANG.
UVBL does not see far field S1 only near field S2
to 10-3 level of principal ray
We do not see to 1019 column density of CH4 since
optical depth unity occurs at 1016-17 or 1/100 of full
slant column
SLIDE 16 SUMMARY OF LIMB PROFILES
Mystery line seems a lower atmosphere phenomena
SLIDE 17 – THE STRENGTH OF Rayleigh
scattering at the long FUV wavelengths
SLIDE 18 –LIMB MIXING RATIOS
What do they mean. We are seeing high in the
atmosphere on the average above the minimum ray
height altitudes
Chapman factor is 20 at 1000 km
SLIDE 19 -summary
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