UPDATE ON TITAN S NIGHT-SIDE AIRGLOW P. LAVVAS, R.A. WEST, G. GRONOFF, P. RANNOU

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1 UPDATE ON TITAN S NIGHT-SIDE AIRGLOW P. LAVVAS, R.A. WEST, G. GRONOFF, P. RANNOU UVIS Team meeting - Jun

2 Combined study of UVIS and ISS observations during Titan s eclipse L18204 WEST ET AL.: TITAN AIRGLOW West et al

3 Observations Table 1: List of ISS images analyzed along with the derived DN values. Image Image mid time Filters c Distance <Limb DN> <Disk DN> (UTC) (nm) (km) W T08:31: CL1, BL W T09:19: CL1, VIO W T09:52: CL1, CL W T10:25: IR2, CL Table 2: Observed emissions (in Rayleighs) during Titan eclipse by UVIS Feature West et al. (2012) Ajello et al. (2012) Limb Limb Disk LBH 7.2± a VK 3.9±2.6 - b CY 0.3 c NI 1200 > NI ± NI 1493? NII ± c a LBH was not discernible on disk observations. b Observed but emission rate not reported. c These values are averages over both disk and limb spectra. 3

4 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 INCLUDING UNOBSERVED COMPONENT Total Limb : ~10-3 erg cm -2 s -1 4

5 Altitude (km) POTENTIAL ENERGY SOURCES 1200 Type Deposition Altitude (km) Energy Flux (erg cm -2 s -1 ) 1000 Magnetospheric electrons ~1000 (depending on B) ~4-8x Magnetospheric ions (O+) ~800 Magnetospheric protons ~ Meteoroids ~500 (silicates) small 400 ~ (H2O) 200 Chemi-luminescence ~200 (C2H2) Cosmic Rays ~65 ~2x10-3 5

6 Production rates for N2 states High Low ~10 Solar GCR

7 LOCAL EMISSION RATES 7

8 COMPARISON WITH OBSERVATIONS Night Day 10 of 17 Complete Band Emissions Night profiles = Day profiles /10 (roughly) 8 Stevens et al. 2011

9 COMPARISON WITH OBSERVATIONS (UVIS) Table 3: Comparison between simulated emissions and UVIS observations from West et al. (2012). All values are in Rayleighs. Band Model Observation Ratio Total UVIS LBH ± VK ± NI ± NII ±

10 COMPARISON WITH OBSERVATIONS (ISS) CL1,BL1 CL1,CL2 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). Limb Average ( km) Disk Average CL1,CL2 CL1,VIO CL1,BL1 IR2,CL2 CL1,CL2 CL1,VIO CL1,BL1 IR2,CL2 Observed Modeled Ratio

11 I. Chemi-luminescence OTHER POTENTIAL CONTRIBUTIONS C2H2 ~300 ns C4H2 ~100 ms II. CH4 emission Only from dissociation fragments CH ( nm) but too weak Hodyss et al 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.

12 OTHER POTENTIAL CONTRIBUTIONS IV. Star Light Direct Star light source: ~10 5 photons cm -2 s -1 nm -1 sr -1 Zodiacal light source: ~10 4 photons cm -2 s -1 nm -1 sr -1 CL1/CL2 DN~10 (observed = 71.7) 12

13 OTHER POTENTIAL CONTRIBUTIONS IV. Star Light Stellar background illuminating Titan s disk * ~ 4 Stellar background on ISS field of view 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 10 o x10 o FOV around the ISS bore sight. (Girardi et al. 2005, 13

14 CONCLUSIONS I. Strongest emissions (ISS) in the upper atmosphere come from the Vegard-Kaplan, 1 st positive and Meinel bands, while for the lower atmosphere 1 st negative and 2 nd 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 14