Proposal_Information Title: The Ultraviolet Emissions of Titan Proposal_Category: GO Scientific_Category: Solar System Cycle: 5 Investigators PI_name: Melissa McGrath PI_Institution: Space Telescope Science Institute CoI_Name: Paul Feldman CoI_Institution: Johns Hopkins University CoI_Name: Regis Courtin CoI_Institution: Observatoire de Paris-Meudon CoI_Name: Darrell Strobel CoI_Institution: Johns Hopkins University CoI_Name: Doyle Hall CoI_Institution: Johns Hopkins University Abstract: ! Free format text (please update) Observations are proposed using the High Resolution Spectrograph to accurately measure the far-ultraviolet (~1175-1525Angstrom) spectrum of Titan at ~2Angstrom resolution, which has been observed previously only by the Voyager Ultraviolet Spectrometer at very low spectral resolution (~30Angstrom). Models of the bright emissions from atomic and molecular nitrogen and N+ in the Voyager data provide poor fits longward of ~ 1000Angstrom. In addition, several unidentified emissions remain which cannot be explained by N+ or its dissociation products, including a strong feature at ~1336Angstrom. Positive identifications of these emissions will help determine the relative contributions by magnetospheric particle precipitation, photoelectrons, and direct solar excitation to the observed UV dayglow, and will allow comparison with UV observations of the Earth's airglow and aurora. The proposed observations will therefore address several principal scientific objectives for Titan: the investigation of the upper atmosphere, its interaction with Saturn's magnetosphere and the solar wind, and its role as a source of neutral and ionized material to the magnetosphere. Questions ! Free format text (please update) Observing_Description: We request a very straightforward set of observations with only two integrations using the same instrument on a single target, Titan. Two back to back integrations using the GHRS in ACCUM mode with Titan centered in the large science aperture are requested. One integration will be performed using G140L centered at each of two wavelengths, 1320Angstrom(covering 1175-1460Angstrom) and 1385Angstrom (covering 1240-1525Angstrom), to cover the far UV spectral region from 1175-1525Angstromfor the purpose of accurate measurement of the N Imultiplets at 1200, 1243, and 1493Angstrom, and the molecular nitrogen (N2) LBH bands (~ 1260-2000Angstrom; see Fig. 2) which are brightest in the far UV; detection of the N2 BH bands (fainter than the LBH bands) may also be possible. In addition, the unidentified features present in the Voyager UVS spectra will be detected and identified. We have used existing Voyager UVS data to crudely estimate a disk averaged LBH band brightness of ~ 75R between 1250-1550Angstrom, which implies 5-10R per line. Titan's disk size of ~ 0.8'', covering ~ 3 science diodes, implies spectral resolution of ~ 2Angstrom. We know by experience using the GHRS with grating G140L to detect faint emissions in the far-UV Jupiter observations in cycle 4, program 5373, PI P. Feldman; Europa observations in cycle 4, program 5376, PI D. Hall; and Io torus observations in cycle 4, program 5642, PI M. McGrath that we can detect ~ 5R features at a S/N of ~ 5 in ~ 300 minutes at peak response near 1300 Angstrom where the strongest LBH bands occur. We therefore request ~ 300 min of total integration time. We account for the fact in these calculations that Titan and its extended atmosphere (radius ~ 3000 km) do not fill the large aperture. We also know by experience that we can typically get ~ 25- 30 min of integration time per orbit, except in the acquisition orbits. Allowing for a one orbit target acquisition sequence (onboard GHRS acquisition using BRIGHT=RETURN, followed by an ACQ/PEAKUP in the large science aperture) in which we will get very little integration time, and accounting for the fact that we wish to limit ourselves primarily to dark time to avoid contamination of the 1300Angstromspectral region by Earth emission, we calculate we will need 12 orbits total to get a total integration time of ~ 300 min. We have chosen to expand the nominal 285Angstromspectral coverage of the G140L grating by using two grating settings separated by 65 Angstrom. This will allow detection of the features near 1200 Angstrom and 1500Angstrom, while maintaining a total integration time of 300 min for a large portion of the spectral region covered at both grating settings, where the weakest expected features, the LBH bands, are located. HST is the only instrument capable of performing this program at the present time. We require its UV sensitivity, and superior spectral resolution relative to that of the Voyager UVS, and the capabilities of the IUE satellite. Real_Time_Justification: We require that the majority of the integration time be performed in dark time to avoid severe contamination by Earth O I 1304 emission, which would seriously compromise the scientific objectives of the program. However, we will perform frequent readouts, so that we can allow portions of the orbits in bright time to be used, maintaining a high observing efficiency, and increasing the scheduling flexibility of the program. Additional_Comments: Solar_System_Targets Target_Number: 1 Target_Name: Titan Description: satellite titan Level_1: std=Saturn Level_2: std=Titan Window: OLG OF SATURN BETWEEN 345 15 Flux: SURF(V)=9.5 Visits Visit_Number: 1 Visit_Comments: *PLEASE MAXIMIZE DARK TIME FOR THIS PROGRAM* by scheduling as close to opposition as possible. Since 12 orbits in one visit is not possible, program has been redesigned (broken into 2 visits grouped within a short time, additional acq added, exposure times reduced). Exposure_Number: 1 Target_Name: Titan Config: HRS Opmode: ACQ Aperture: 2.0 Sp_Element: MIRROR-N2 Optional_Parameters: BRIGHT=RETURN LOCATE=EXTENDED Number_of_Iterations: 1 Time_Per_Exposure: 13.5S Special_Requirements: ONBOARD ACQ FOR 2-4 Comments: STEP-TIME=1.5S; EXPECT ~1000 CTS PEAK DWELL Exposure_Number: 2 Target_Name: Titan Config: HRS Opmode: ACCUM Aperture: 2.0 Sp_Element: G270M Wavelength: 1975 Optional_Parameters: FP-SPLIT=TWO,STEP-PATT=5,FLYLIM=3 Number_of_Iterations: 1 Time_Per_Exposure: 1468.9S Exposure_Number: 3 Target_Name: Titan Config: HRS Opmode: ACCUM Aperture: 2.0 Sp_Element: G140L Wavelength: 1385 Optional_Parameters: STEP-PATT=5 Number_of_Iterations: 17 Time_Per_Exposure: 680.1S Exposure_Number: 4 Target_Name: Titan Config: HRS Opmode: ACCUM Aperture: 2.0 Sp_Element: G140L Wavelength: 1320 Optional_Parameters: STEP-PATT=5 Number_of_Iterations: 4 Time_Per_Exposure: 680.1S Visit_Number: 2 Exposure_Number: 1 Target_Name: Titan Config: HRS Opmode: ACQ Aperture: 2.0 Sp_Element: MIRROR-N2 Optional_Parameters: BRIGHT=RETURN LOCATE=EXTENDED Number_of_Iterations: 1 Time_Per_Exposure: 13.5S Special_Requirements: ONBOARD ACQ FOR 2-3 Comments: STEP-TIME=1.5S; EXPECT ~1000 CTS PEAK DWELL Exposure_Number: 2 Target_Name: Titan Config: HRS Opmode: ACCUM Aperture: 2.0 Sp_Element: G270M Wavelength: 1975 Optional_Parameters: FP-SPLIT=TWO,STEP-PATT=5,FLYLIM=3 Number_of_Iterations: 1 Time_Per_Exposure: 1468.9S Exposure_Number: 3 Target_Name: Titan Config: HRS Opmode: ACCUM Aperture: 2.0 Sp_Element: G140L Wavelength: 1320 Optional_Parameters: STEP-PATT=5 Number_of_Iterations: 9 Time_Per_Exposure: 680.1S Data_Distribution Medium: 8MM ! 8MM or 6250BPI or 1600BPI Blocking_Factor: 10 ! 10 or 1 Ship_To: PI_Address ! STSCI or PI_Address or Ship_Via: UPS ! UPS (2-day) or OVERNIGHT