UVIS solar occultation results at
T10
D. Shemansky
12/13/2006
Summary
• Accurate N2 and CH4 densities have been obtained
from the UVIS solar occultation at T10.
• Based on a statistic of 3 UVIS occultations,
variability in observed thermospheric mass density
is consistent with variable magnetospheric heat
deposition at the top of the thermosphere.
• The mass densities derived from the UVIS
occultations at the top of the thermosphere are
consistent with the INMS results, and out of bed
with AACS derived values by an order of magnitude.
• A model of atmospheric structure determined from
UVIS results has been developed from the
stratosphere through the top of the thermosphere.
•This report presents the results of the analysis of the UVIS
T10 solar occultation. Data reduction establishing N2 and CH4
vertical abundance profiles has been completed.
Severe pointing instability, producing motion of the solar
spectral image on the spectrograph detector throughout the
occultation period, caused a substantial delay in data processing.
A methodology was developed to correct the solar spectral data
to a common phase without resorting to the use of a solar
simulation model.
This process has provided accurate vertical profiles.
•A hydrostatic model with a temperature profile common
to both N2 and CH4 abundance profiles has been calculated to fit
the data, showing internal consistency, and agreement with
CIRS/RSS densities at altitude h=100 km. This model is
considered accurate enough to be used as the basis for a Titan
atmospheric model.
Data properties
• The pointing instability at T10 has limited useable data to
altitudes above h = 850 km in egress only.
• Accuracy in optical depth measurement: Signal count
statistics in the integrated spectra are not a limiting factor
over the range of measured extinction levels. The primary
limiting factor in data accuracy is systematic error
introduced through imperfect phase correction. This limits
reliable extinction measurements to a 1% - 2% level at the
top of the atmosphere.
Reduction methodology
• N2 abundances are obtained in measured extinction of the
H Lyγ solar line at 972 A by the N2 b(3,0) discrete band in
combination with extinction of the solar He 584 A line in
the ionization continuum. In the altitude range of overlap
of these measurements the same abundance values are
obtained to within 5%, confirming accuracy in the cross
sections. Cross sections are known to within ±10%. Cross
section temperature dependence shows negligible effect on
derived optical depth.
• CH4 abundance is obtained from extinction of the N+ 1085
A solar line in the CH4 ionization continuum. N2 is
transparent at 1085 A.
Data and reduction quality
• Figure 1 shows a limited region of the transmitted
spectrum around the H Lyγ line to demonstrate data
quality. The spectra are shown after phase correction at
three altitudes, the first of which is at high altitude with no
measurable extinction.
• Data are reduced to abundances through simulation of
instrument response to a solar spectral model, accounting
for the internal effect of the point spread function.
Figure 1
rev20_ti_occ_12s_r0_I_shft_01
200
180
H Lyγ
I0 UVIS solar spectrum
I(1721km) phase shifted
I(1556 km) phase shifted
Normalized signal counts
160
140
120
100
80
60
40
20
0
950
960
970
980
λ (A)
990
1000
Results
• Figure 2 shows the measured N2 and CH4 abundances
plotted against a hydrostatic model using a temperature
profile common to both species.
• Model properties: 1) The N2 model passes through the
measured CIRS/RSS density at h= 100 km and the UVIS
measured abundances h> 850 km. 2) The CH4 model
projects downward to the 100 km level to predict a CH4/N2
number density mixing ratio of 0.013 (Flasar etal obtain
0.016 ± 0.005) . 3) The temperature profile from 100 km to
850 km is derived from CIRS measurements to 450 km and
the UVIS TB λsco occultation at -36o latitude from 450 to
850 km. The profile above 850 km is determined by the
T10 CH4 and N2 abundance distributions. Temperature at
the top of the thermosphere is T∞ = 199 ± 10 K (Lat. -62o -51o).
Figure 2
sol_T10_rv1_r0_03ev_012_abnd_syn
2500
N2 model 12_8
CH4 model 11_1
T10 N2 rv1_2 data
T10 CH4 rv1_1 data
h (km)
2000
1500
1000
500
0
11
13
15
17
19
Log([N] (cm-2))
21
23
25
27
Comparison of physical properties
internal to UVIS occultations
• The common species directly measured in the
UVIS occultations is CH4 . The indication is that
differences are primarily caused by the hydrostatic
effect of different temperatures at the top of the
thermosphere.
• It is evident that the two TB occultations differ
from each other and both infer substantially
colder states of the thermosphere than at T10.
UVIS T10 and TB results for
CH4
• Figure 3 shows CH4 derived abundances
against models. The difference in values of
T∞ is 50 K. A detailed analysis on the αVir
occultation has not been made but the data
plotted in this figure suggests a temperature
intermediate between the profiles for T10
and λSco.
LSCO_AVIR_T10rv1_1_CH4_abnd
Figure 3
2200
λSco
αVir
Sun
T10 solar model
TB λSco model
2000
1800
h (km)
T10:
T∞ = 199 K
TB λSco: T∞= 149 K
1600
1400
1200
1000
800
13
14
15
16
Log([CH4] (cm-2)
17
18
Comparison of measured N2 abundance
at T10 with inferred abundance at TB
• Figure 4 shows N2 abundance derived from
T10 against the models for T10 and Tb
(λsco). The λsco predicted abundance
profile is based on the density at 100 km
and the derived temperature profile. The
T10 model is confined by the measured
abundance above 850 km and the
CIRS/RSS density at 100 km.
Figure 4
N2_t10_rv1_2_tb_abnd_model_vs_data
2500
T10 N2 solar data
T10 N2 _12_8 model
TB λSco N2 _7_2 model
2000
T10:
1500
T∞ = 199 K
h (km)
TB λSco: T∞= 149 K
1000
500
0
11
13
15
17
19
Log([N2] (cm-2))
21
23
25
27
Model densities
• Figure 5 shows the N2 and CH4 number densities
at T10 corresponding to the abundances in Figure
4. The temperature profile is included on the plot.
• The attached Excel file gives the numeric data for
the model densities, abundances, temperature
profile and data derived abundances.
• Exobase location: According to this model, the
exobase is located at 1490 km. The number
densities of N2 and CH4 reach a 1:1 ratio at 1875
km (T10).
Figure 5
130
T (K)
140
2500
150
160
180
190
200
16
18
sol_T10_rv1_r0_03ev_012_dens_syn
N2 _12_8 model
CH4 _12_1 model
T(K) _12_8 model
2000
h (km)
170
1500
1000
500
0
2
4
6
8
10
12
Log([N] (cm-3))
14
Mass densities compared to
INMS and AACS
• Table 1 shows mass densities at selected encounters for the
UVIS results at T10 and TB, compared with INMS and
AACS.
• Significant points of interest: 1) The mass densities
inferred from UVIS TB λsco compared to INMS T16,
indicate similar magnitudes for the three selected altitudes,
950, 1027 and 1174 km. 2) The UVIS T10 density at 950
km is lower by ~1.7 than INMS at T16, but shows a much
hotter vertical distribution, exceeding the T16 value at
1174 km by a factor of ~3.
CH4/N2 partitioning
• Table 1 includes a comparison of number
densities for UVIS T10 and INMS T16. For
N2, at 950 km the T10 value is ~0.6 of the
value at T16. At 1174 km the N2 densities
are similar, inferring higher temperature at
T10. For CH4, the UVIS densities are higher
by factors of 1.15 (950 km) to 1.6 (1174
km), again indicating a higher temperature
at T10.
Table 1
Titan
Mass densities (kg m -3)
Lat
950
1027
1174
T10
UVIS
-62 - -51
4.73E-10
1.50E-10
7.70E-11
TA
INMS
AACS
38
38
1.40E-09 5.50E-09
4.10E-10 1.60E-09
4.60E-11 1.80E-10
TB
INMS
49
1.05E-09
3.00E-10
3.40E-11
UVIS
*Lsco -36
Avir
5.91E-10
1.74E-10
1.97E-11
T5
T16
INMS
74 - 62
AACS
74 - 62
1.40E-10
2.10E-11
6.10E-10
8.00E-11
INMS
7.82E-10
2.10E-10
1.91E-11
* Based on model calculation referenced to [N 2] = 5.6E+17 cm-3 at 100 km
AVIR
963.794
1033.895
1377.518
CH4 results
LSCO_interp T10_interp
16.67881 16.76717104 16.50333
16.32306
16.53629 16.21060
15.2106806
15.18659 15.24817
h
km
T10 UVIS vs T16 INMS number densities (cm -3)
UVIS
INMS
N2.
CH4.
N2.
CH4.
950 1.00E+10 2.54E+08 1.67E+10
2.20E+08
1027 3.20E+09 1.14E+08 4.47E+09
1.00E+08
1174 4.45E+08 3.15E+07 4.00E+08
2.00E+07
Conclusions
• The N2 densities derived from the T10 solar
occultation, given cross section properties and
data quality, in my assessment are limited by
systematic error inside ± 20%. The measured
abundance vertical profile, which provides
constraint from 860 km to 2060 km, confines the
top of thermosphere temperature to ± 10 K, based
on hydrostatic modeling.
• The mass densities from UVIS T10 and TB are
comparable to INMS at T5 and T16 at 1027 km
• No evidence has been found for systematic
calibration differences between UVIS and INMS
mass densities, although there is a tendency for
UVIS CH4/N2 ratios to be higher than the values for
INMS.
Conclusions continued
• Internal to the UVIS results, inferred variation in
atmospheric density in the thermosphere can be
explained by changes in heat deposition at the top
of the thermosphere, attributed to magnetospheric
input. Modeled N2 densities for T10 and TB differ by
factors of 4 (1500 km) and 7 (1750 km) (see Figure
4) because of different values of T∞ , typical of
variability reported in INMS data.
• Inferred heat deposition: For T10 the temperature
profile on the basis of thermal conductivity requires
2.7 X 10-3 erg cm-2 s-1 of heating at the top of the
thermosphere, at TB 1.5 X 10-3 erg cm-2 s-1 . Solar
photoelectron deposition is 1.5 X 10-3 erg cm-2 s-1
but not at the top of the thermosphere.