Titan UVIS Airglow Spectra: Modeling and Laboratory Studies JOSEPH AJELLO JPL MICHAEL STEVENS NRL IAN STEWART KRIS LARSEN LARRY ESPOSITO JOSH COLWELL WILLIAM MCCLINTOCK WAYNE PRYOR GREG HOLSCLAW LASP/CU

UVIS TITAN AIRGLOW PRIMARY GOALS PRESENT FIRST CALIBRATED UV SPECTRA (800-1900 Å) IDENTIFY SPECTRAL CONTENT -c'(0,0) 958Å? IDENTIFY SOURCES BY MODEL SPECTRUM PHOTODISSOCIATIVE IONIZATION EXCITATION (PDI) PHOTOELECTRONS(PE) + N2 MULTIPLE SCATTERING IN c'(0,v'')PROGRESSION MAGNETOSPHERIC ELECTRONS (ME)+ N2 ATOMIC N +e (PE or ME) DEVELOP PE MODEL FOR 200 PERTURBED & STRONG RYDBERG BANDS N2 (b,b',c'-X)

CASSINI UVIS OBSERVATION EUV/FUV AIRGLOW 560-1180 & 1120-1910Å 1024 (spectral)x64 (spatial) 2.0 mrad x 59 mrad FOV Inbound Orbit Tb 13dec04 105 km, 200 km resolution

EUV DAYGLOW OBSERVATION IN N2 RYDBERG BANDS (b 1Pu, b' 1Su, c 1Su  X 1Sg) Dayside (900-1100 Å) 03:43 UT, 13DEC04 phase angle of 16.5o distance 163,000km Nightside 14:13 UT 163o phase angle distance 50,500km Resolution ~100 km

The EUV Airglow of Titan However, (0,0) is optically thick near peak photoelectron exciation. Newer model results showed that (0,0) is weak and many other features are there instead, including NI excited by photodissociative ionization. UVIS data from December, 2004 now show the EUV airglow of Titan in detail. (0,0) is indeed weak or absent and the complexity of the spectrum is revealed. Red curve is the composite of a four component linear regression… Titan V1 UVS airglow data suggested that the N2 c(0,0) and (0,1) bands dominated the EUV due to similarities with electron impact spectra from the lab.

Spectral Fitting of EUV Airglow Electron impact laboratory spectrum without c(0,0-2). Relative intensities of NI and NII PDI [Bishop and Feldman, 2003]. Relative intensities of (0,0-2) based on model results [Stevens, 2001]. HI Lyman- at 1026 Å fit separately. Photon loss and redistribution in c(0)—X shown by comparing blue dotted and green lines.

COMPARISON CASSINI EUV TO LABORATORY SPECTRUM 5.6Å FWHM 5 Feature 6 100eV FWHM=0.2Å

COMPARIONS OF EUV DAGLOW & NIGHTGLOW SPECTRA (800-1140 Å) WITHIN SATURN MAGNETOSPHERE

IDENTIFICATION OF FUV SPECTRUM Dayglow 900km LBH 68R HI 266R NI 30R CH4~1017cm2 Nightglow

GLOBAL MODEL OF N2 DISSOCIATION FOR FAST N (~1 eV) CHEMISTRY FROM EUV INTENSITY

MAJOR RESULTS OF UVIS 13DEC04 DAYGLOW ANALYSIS c’(0,0) IS ABSENT, NI lines are present at 952, 964Å MAIN EUV/FUV PROCESSES INVOLVE N2 PDI(XUV 5-410Å), PE(e~23 eV), PDE PROCESSES(XUV 820-1000Å) ALTITUDE OF EUV DAYGLOW 1100 KM ALTITUDE OF FUV DAYGLOW 900 KM EUV OUTPUT 2.9 X 107 WATTS-measure of Titan atmos. N2 Dissociation and fast N-atoms FUV OUPUT (LBH) 3.6 X 107 WATTS-measure of Solar XUV input FUV OUTPUT (NI,HI,CI) 1 X 108 WATTS N ATOMS ~1 X 1027 GLOBAL ATOMS/S MAGNETOSPHERE EXCITATION WEAKER BY ~20:1 UVS & UVIS IN GOOD CROSS CALIBRATION –SOLAR MAX/MIN~ 3:1 TECHNIQUE & CROSS SECTION FOR N2 PLANETARY ATMOSPHERES

COMPARISON OF VOYAGER UVS AND CASSINI UVIS - 24 YEARS APART