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Disk-‐Integrated Measurements of the Moon in the Ultraviolet

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Disk-‐Integrated Measurements of the Moon in the Ultraviolet Disk-integrated measurements of the Moon in the ultraviolet Greg Holsclaw [email protected], Mar=n Snow, Bill McClintock, Tom Woods Laboratory for Atmospheric and Space Physics University of Colorado CALCON 2012 Outline • The SOLSTICE instrument • Observaons • Data Processing • Results – Reflectance – Photometric fits – Polarizaon • Conclusions 2 SOLar STellar Irradiance Comparison Experiment (SOLSTICE) • Measures solar irradiance in ultraviolet – 115-180 nm Far Ultraviolet (FUV) – 180-300 nm Middle Ultraviolet (MUV) • Measures stellar irradiance for tracking long term degradaon • On board SORCE: – Low Earth Orbit 3 Solar-Stellar Rao Need a large dynamic range (~109) to measure both Sun and stars with same op=cs and detectors 4 SOLSTICE Layout • Scanning grang monochromator • Operates as an objec=ve grang spectrometer in stellar mode • Change entrance aperture 2x104 • Widen exit slit 10 or 20 • Lengthen integraon 103 5 UV presents unique challenges • Solar variability – Visible wavelengths (>400nm): ~0.1% – Middle-ultraviolet (200-300nm): ~2-3% – Far-ultraviolet (115-200nm): 10-40% • Poorly characterized calibraon targets – Ground calibraon of UV instruments has a high uncertainty – Hot, bright stars are frequently used in-flight but previous measurements also have a high uncertainty 6 Lunar Observing Campaign • June 2006 to December 2010 • Added the Moon to the schedule of eclipse calibraon ac=vi=es • On average, we observed one disk-integrated spectrum per day in each channel – Over 1000 complete spectra in each channel • NASA Grant: LASP Lunar Albedo Measurement and Analysis from SOLSTICE (LLAMAS) 7 Phase angle coverage 30 bin size = 5 deg FUV MUV 25 20 Min Abs( phase ) 0.1° Min phase -171.7° 15 Max phase 168.6° 10 number of observations 5 0 -180 -135 -90 -45 0 45 90 135 180 phase angle (degrees) 8 Wavelength and phase coverage in context SSBUV (MESSENGER) ROLO GOME SOLSTICE (Galileo) (SNOE) (Cassini) (HUT) (Mariner 10) (Apollo 17) NOZOMI 9 Derivaon of reflectance Note that by making both Solar and Lunar EM CM AS ΔλS = Γ irradiance measurements ES CS AM ΔλM with SOLSTICE, all calibraon terms cancel out except for E / Ω knowledge of the r = M M aperture areas and a single correc=on factor ES /π (Gamma). 10 Example reflectance and phase curves 0.08 100 20deg phase 10-1 0.06 MUV, 280nm 10-2 0.04 FUV, 170nm -3 reflectance reflectance 10 0.02 10-4 0.00 10-5 100 150 200 250 300 -180 -135 -90 -45 0 45 90 135 180 wavelength (nm) phase angle (degrees) 11 242 7. Absolute Irradiance of the Moon Systemac correc=ons • Vigneng (1-3 % for MUV, 5-10% for FUV) • Wavelength shiks (~1-2 nm) • Changes in resolu=on with phase and roll angle • Polarizaon (not yet applied, discussed later) Figure 7.5: Position of the Moon in the field-of-view of the instrument for a few typical observations. The axes are the pitch and yaw directions for the instrument. 12 The crosses show the position of the center of the lunar disk as it drifts away from the center of the FOV during the observation. The spacecraft is periodically re-pointed to the new position of the Moon to re-center it, but during each such maneuver, the roll angle of the spacecraft is allowed to change, so the new direction of the Moon’s drift can be different after each re-pointing. Vigneng dispersion plane cross-dispersion plane 1.00 1.00 0.95 0.95 0.90 0.90 100nm 100nm 0.85 150nm 0.85 150nm fraction 200nm fraction 200nm 250nm 250nm 0.80 300nm 0.80 300nm 350nm 350nm 0.75 0.75 0.70 0.70 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 angle (arcmin) angle (arcmin) Frac=on of light that reaches the detector as a func=on of target posi=on (from Zemax raytrace) 13 Variaon in resolu=on FUV MUV 1.0 1.0 0deg 0deg 150nm 60deg 250nm 60deg 90deg 90deg 0.8 120deg 0.8 120deg 0.6 0.6 0.4 0.4 0.2 0.2 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 wavelength (nm) wavelength (nm) Point-spread func=on in each channel at four different phase angles (0deg s/c rotaon) 14 Photometric modeling • Polynomial – 7th order polynomial used to fit wide phase range Mercury observaons from SOHO [Mallama et al, 2002, Icarus 155] – Simple, but nonphysical • ROLO – Empirical model, but tailored to the shape of the lunar phase curve – Achieves good fits (~1% rms) of visible, IR measurements [Kieffer and Stone, 2005, Astron J 129] – Only valid for phase range 1.6° to 97° • Hapke – Semi-physical coefficients related to surface proper=es – Widely used, long history – Non-unique without resolved data [Domingue and Hapke 1989, Icarus 78] 15 7th order polynomial fit 100 10-1 data fit • 280nm 10-2 10-3 • Residual reflectance 10-4 systemac 10-5 structure -180 -135 -90 -45 0 45 90 135 180 phase angle (deg) evident at small data / fit and possibly 1.4 +/- 5% sdv = 0.068 large phase 1.2 angles 1.0 ratio 0.8 • Posi=ve and 0.6 negave phases -180 -135 -90 -45 0 45 90 135 180 fit phase angle (deg) independently 16 ROLO photometric func=on fit 100 10-1 data fit • Only fit to phase 10-2 range 1.6° to 97° 10-3 • reflectance Missing linear 10-4 term? 10-5 -180 -135 -90 -45 0 45 90 135 180 • No other phase angle (deg) obvious data / fit systemac 1.4 +/- 5% sdv = 0.055 variaon 1.2 • Need to add 1.0 func=onal terms ratio 0.8 to represent 0.6 turn down at -180 -135 -90 -45 0 45 90 135 180 large phase phase angle (deg) 17 Hapke fit 100 10-1 data fit -2 • 10 No separaon 10-3 of posive 10-4 and negave -5 disk-equivalent albedo 10 -180 -135 -90 -45 0 45 90 135 180 phases phase angle (degrees) • data / fit Significant 1.4 systemac +/- 5% sdv = 0.148 1.2 structure 1.0 ratio 0.8 0.6 -180 -135 -90 -45 0 45 90 135 180 phase angle (degrees) 18 Lunar Polarizaon Volcanic “per mille” or Ash “per thousand” symbol Mare Moon Highlands Lyot, 1929 1971, Dollfus & Bowell 19 Instrument polarizaon response 0.025 0.020 H Instrument 0.015 sensi=vity due to 0.010 V sensitivity polarized light at 0.005 0.000 two orientaons 180 200 220 240 260 280 wavelength (nm) 0.8 (H-V)/(H+V) 0.6 Derived linear 0.4 polarizaon polarization 0.2 sensi=vity 0.0 180 200 220 240 260 280 wavelength (nm) 20 Variaon in roll angle enables polarizaon measurement 180 135 90 roll angle (degrees) 45 FUV MUV 0 -180 -135 -90 -45 0 45 90 135 180 Geometry of an observaon. The scaering phase angle (degrees) plane is defined by the Sun, Moon, and observer (spacecra). The spacecra roll Distribu=on of sampled roll angle, θ, is defined between one spacecra axis and the normal to the scaering plane. angles at each phase angle. 21 Calculang Polarizaon Frac=on We can express the measured signal in terms of the spacecra roll angle, θ C(!,") = A + Bcos2" The polarizaon of the lunar irradiance is then given by: B 1 PM = ! A PI 22 Extrac=ng Pm For each wavelength and phase angle, we can find A and B using least-squares fing algorithm. 280nm, 10deg 280nm, 80deg 0.10 0.014 0.012 0.08 0.010 0.06 0.008 0.006 0.04 0.004 disk-equivalent albedo 0.02 data disk-equivalent albedo data model fit 0.002 model fit 0.00 0.000 0 45 90 135 180 0 45 90 135 180 roll angle (deg) roll angle (deg) 23 Polarizaon vs Phase 24 Polarizaon 0.25 WUPPE (not full disk) [8] waning waxing 0.20 SOLSTICE 0.15 0.10 Coyne & Pellicori [7] polarization 0.05 60° phase 0.00 200 300 400 500 wavelength (nm) Fox et al, 1998, WUPPE, MNRAS 298 Coyne & Pellicori, 1970, Astron. J. 75 25 Conclusions • SOLSTICE observes both the Sun and Moon directly, leading to a uniquely accurate measure of reflectance • Wide phase coverage: -170 to +170° • Wide UV wavelength coverage: 121, 130 - 300 nm • Photometric modeling will enable the use of the integrated Moon as a UV calibraon target – FUV requires nearly simultaneous solar spectrum • Need to improve correc=ons for systemac effects (resolu=on, wavelength, vigneng, polarizaon) such that residuals are on the order of random uncertainty • Ul=mately, reduced data and model will be available to the public 26 Acknowledgements & References Coyne GV, Pellicori SF (1970) Wavelength dependence of polarizaon. xx. the integrated disk of the moon. Astron J. 75:54-60. doi:10.1086/110940 Domingue, D. and Hapke, B. (1989) Fing Theore=cal Photometric Func=ons to Asteroid Phase Curves, Icarus 78, 330-336. Fox GK, Code AD, Anderson CM, Babler BL, Bjorkman KS, Johnson JJ, Meade MR, Nordsieck KH, Weitenbeck AJ, Zellner NEB (1998) Solar system observaons by the wisconsin utraviolet photopolarimeter experiment — iii. the first ultraviolet linear spectropolarimetry of the moon. MNRAS 298:303-309. doi:10.1046/j.1365-8711.1998.01633.x Kieffer, H. and Stone, T. (2005) The spectral irradiance of the Moon, Astron. J. 129, 2887-2901 Snow, M., Holsclaw, G., McClintock, W. E., and Woods T. N. (2012) Absolute ultraviolet irradiance of the Moon from the LASP Lunar Albedo Measurement and Analysis from SOLSTICE (LLAMAS) project.
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