Analysis of Cassini UVIS observations of Titan VUV

emissionX. Liu

D. Shemansky10/11/07

UVIS Imaging Spectrographs•Telescopic spectrographs imaging 64 contiguous spatial pixels 1 mrad/px projected on sky. (Full width 3.2o)

•1024 spectral pixels 0.25 mrad/px projected on sky.

•EUV: 55. nm – 115. nm with solar occ port

•FUV: 110. nm – 190. nm

•Spectral resolution 0.23 nm – 0.48 nm

Atomic and molecular emission

• N2, NI, NII, CI identified and modeled• Results for nitrogen are comparable or

lower than Strobel & Shemansky(1982), and Hall et al (1992) from Voyager UVS.

• Scattered solar photons (λ > 154 nm) from aerosols are measurable at high and low phase from 1400 km downward.

1 1 0 0 1 2 0 0 1 3 0 0 1 4 0 0 1 5 0 0 1 6 0 0 1 7 0 0 1 8 0 0 1 9 0 0

λ ( A )

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

Cou

nts

1872

0 s-1

pxs

-1 p

xw-1

f u v2 0 0 4 _ 1 8 4 _ 1 0 _ 4 0 _ 0 6 _ u _ c m 2 s _ t

N 2 L B H

N I

N II

1 8 4 _ r 2 3 _ r 3 9 _ a v gm o d e l_ to ta ls o lr e f l_ t it_ 0 2 _ 0 5C I_ m o d e l

1100 1200 1300 1400 1500 1600 1700 1800 1900λ (A)

0

10

20

30

40

50

60

70

80

90

Cou

nts

1872

0 s-1

pxs

-1 p

xw-1

UVIS FUV Titan 2004 DOY 184 disk average

NI Solarreflection

UVIS Titan datamodel_total

CI

1200 1300 1400 1500 1600 1700 1800 1900

λ (A)

0

10

20

30

40

50

Cou

nts

px-1

cuv_fu1_n2lbh_tit_00Transmission of N2 LBH throughTitan atmosphere

00_00: no extinction

00_03: CH4= 2.06 X 1018; C2H2=2.05 X 1016

C2H4=1.1 X 1016; C2H6=5.0 X 1016

00_04: CH4= 1.12 X 1017; C2H2=1.11 X 1015

C2H4=5.97 X 1014; C2H6=2.71 X 1015

N2LBH_00_00N2LBH_00_03N2LBH_00_04

Distribution of N2 states a’ 1Σ-

u (9) τ ~ 3 ms a’ 1Σ-

u (5) τ ~ 9 ms w 1Δu (0) τ ~ 67 μs higher v levels 100% predissociative

a’ 1Σ-u (1) τ ~ 7 s

a 1Πg (0) τ = 140 μs a’ 1Σ -

u (0) τ unknown W 3Δu (0) B’ 3Σ -

u (0) B 3Πg (0) A 3Σ +

u (0) τ ~ 2 s X 1Σ +

g

1200 1300 1400 1500

λ (A)

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.0010

0.0011

n2_x

_z_0

6_03

_sc

n2_x_z_06_03_scn2_lbh_04_cm2_scn2_06_06_cm2_sc

cuv_fu1_n2_x_z_06_xx_00aN2 multistate model

1400 1500 1600 1700 1800 1900

λ (A)

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.0010

0.0011

n2_x

_z_0

6_03

_sc

n2_x_z_06_03_scn2_lbh_04_cm2_scn2_06_06_cm2_sc

cuv_fu1_n2_x_z_06_xx_00

A - X (6,0)

a - X (6,7)A - X (10,0)w - X (0,4)

N2 multistate model

0 2 4 6 8 10 12 14 16 18 20

state

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

24

24

24

24

24

24

24

24

24

[N(n

)]

[e]=1010 cm-3

[e]=108 cm-3

[e]= 107 cm-3

[e]=106 cm-3

n2_x_z_06_xx_dmN2nonLTE state densities vs ambient electron density

[NX] > 1 - 11

[NA] > 12 - 19,22,25,28,30,32,36,

38,43,48.50,56,60,64,69

Te= 200 K

Titan TA

-2 -1 0 1 2

RT

0.0

0.5

1.0

1.5

2.0

2.5

3.0C

ount

s s-1

cm

-2FUSC4_titan_revA_gxz100_vims

H LyαSolar reflection X 1/3NI_1200 x 10N2_LBH_1380 X 40

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

r (km)

0

10

20

30

40

50

60

70

80

90

100

110

120

I (R

)

TA_em_dist_00

N2_LBHTA_em_dist_00TA_em_dist_00NI_1200NI_1243X10NI_1493X10NII_1085X20HLyα X 10-1

Emission Brightness across sunlit disk at TA

T0

1100 1200 1300 1400 1500 1600 1700 1800 1900

Wavelength (A)

0

5

10

15

Sign

al (c

/480

s)Titan T0 Scan 0A

Area: X200 km x Z1200 km

BIN_SCAN0ANI_it_06_sctot_tit_modelsolrefl_02_05_scci_1b_tit_01_sc

I(NI(1200))= 35 RI(NI(1493))= 13 R; VOY(S&S(1982))= 49 RI(N2 LBH)= 163 R; VOY(S&S(1982))= 290 R

[CH4]l= 3. X 1016 cm2

[NI]/[N2] = 0.07

h = 1030 -- 1416 km

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

r (km)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Alb

edo

X 1

00TA_T0_albedo_1827

Albedo at 1827 A X 100

albedoX100 TAalbedoX100 T0

T0 sub solar 90 deg phaseTA zero phase pole to pole

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

r (km)

10

30

50

70

90

110

130

150

170

190

I (R

)

T0_TA_LBH

N2 LBH T0 v s TA

TAT0

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

r (km)

10

20

30

40

50I

(R)

T0_TA_NI_1200

TAT0

Consistency of TB (occultation) and T0 (dayglow) analyses•Extinction of NI emission at T0 at 1030 km sub-solar limb indicates CH4 abundance of 3X1016 cm-2 compared to TB value of 3.75X1016 cm-2

•Solar reflection spectrum at h= 0 km shows extinction by C2H2and C2H4 abundances of 4.X1016 cm-2 and 2.2X1016 cm-2 . From TB this compares to the abundance of C2H2 at 630 km (peak of 2.5 X1017 cm-2 at 546 km), and to the value of peak abundance of C2H4 at 600 km.

1 1 0 0 1 2 0 0 1 3 0 0 1 4 0 0 1 5 0 0 1 6 0 0 1 7 0 0 1 8 0 0 1 9 0 0

λ ( A )

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

Cou

nts

1872

0 s-1

pxs

-1 p

xw-1

f u v2 0 0 4 _ 1 8 4 _ 1 0 _ 4 0 _ 0 6 _ u _ c m 2 s _ t

N 2 L B H

N I

N II

1 8 4 _ r 2 3 _ r 3 9 _ a v gm o d e l_ to ta ls o lr e f l_ t it_ 0 2 _ 0 5C I_ m o d e l

F

900 1000 1100 1200λ (A)

0E+000

1E-003

2E-003

3E-003

4E-003

5E-003

6E-003

7E-003

8E-003

9E-003

1E-002

B (1

0-6 c

s-1

cm

-2)

H: - 3650 km, Mesa SubtractedN2 EUV thick Model

E4TI_RA_GXZ100_VIMS_BI1_HM3650

N2 c4' (0,0)

Titan TA

NII 1085. A

NII mdel

1100 1200 1300 1400 1500 1600 1700 1800 1900λ (A)

0.000

0.001

0.002

0.003

0.004

0.005

0.006ZM

345E

1a

h = 875 kmh = 1775 kmh = -125 km

fusc_titan_reva_gxz100_vims_spectra_1_a

South pole region

1100 1200 1300 1400 1500 1600 1700 1800 1900λ (A)

0.000

0.001

0.002

0.003

0.004

0.005

0.006Z3

45E

1a

h = 875 kmh = 1775 kmh = -125 km

fusc_titan_reva_gxz100_vims_spectra_2_a

North pole region

Observation vs electron impact model: TA at h =1050 km

[ea]= 2000 cm-3 [eh]= 0.045 cm-3

Tea= 200 K Teh= 190000 K

e + N2 › products

Transition Observed(R)

Rate (10-10 cm3 s-1)

Model(R or cm-3 s-1)

N2 LBH NI 120 nmNI 149.2 nmNII 108.5 nmN(4S) + N(2D)N2

+

115.45.8.3.6

52.44.72.41.7400200

11510.45.33.616.8.

e + NI › NI + hν

NI 120 nmNI 149.2 nm

45.8.

1107.0

241.5

Conclusions•The emission peak in the dayglow in the range 1050 to 1400 km is explained as electron impact excitation of N2 and NI. The NII emission is primarily from dissociative ionization-excitation of N2.

•In the ionosphere atomic nitrogen can be as much as 10% of the N2 population.

•The subsolar ionosphere peak shows 70% of the NI is in the 2D state indicating strong cold electron pumping through direct e + NI excitation.

Conclusions

• The vertically extended aerosols reaching 1400 km observed in scattered solar photons, λ > 154. nm are undetectable in the polar regions. The location and structure of the transition region has not yet been determined.

• The λ > 154. nm albedo peaks sharply near zero phase in the TA observation.