Water Production Rates of Comets Derived from Hydrogen Images Obtained by the All-Sky Lyman-alpha Imager Solar Wind ANisotropies (SWAN) Instrument on the SOlar and Heliospheric Observatory (SOHO) Satellite Data Set Overview ================= This data set presents water production rates of comets taken from 1996 to 2012 derived from observations of the hydrogen Lyman-alpha coma made with the Solar Wind ANisotropies (SWAN) Instrument on the SOlar and Heliospheric Observatory (SOHO) Satellite. The raw images are archived at Goddard Space Flight Center and by the European Space Agency. The SOHO archive maintained at NASA/GSFC and at ESAC is available at http://sohowww.nascom.nasa.gov/data/archive/ and at https://seal.nascom.nasa.gov/cgi-bin/gui_seal. The instrument, its operation and application to comet observations are described by Bertaux et al. (1998). Fully calibrated images are not available in a public archive yet. For now the SWAN PI can provide fully calibrated images on a case-by-case basis. The SWAN instrument has an instantaneous instrument field of view consisting of a square of five by five 1 degree pixels that are scanned across the sky to construct an image. In normal operations this is done once per day over the entire sky as seen from SOHO to make an all-sky image of the interplanetary hydrogen streaming through the solar system. Because the neutral hydrogen is ionized by impact from the solar wind and solar ultraviolet radiation, its distribution provides a mapping of the spatial distribution of the solar wind itself. The main gaseous constituent of comets is water, which is primarily photodissociated into H and OH and the OH is further photodissociated into H and O. Therefore, observations of the distribution of atomic hydrogen in the comae of comets can be used to calculate the water production rate of the comet through the use of models appropriately parameterized after many independent studies. The brightness of the Lyman-alpha emission is converted into column densities using the daily solar Lyman-alpha fluxes compiled by the Laboratory for Atmospheric and Space Physics (LASP) of the University of Colorado and available at http://lasp.colorado.edu/lisird/lya/. The emission rate factor, or g-factor, at any comet's particular heliocentric velocity is determined from this solar flux and the shape of the solar Lyman-alpha line profile by Lemaire et al. (1998; 2005). Because the comet is at different heliographic longitude than near-Earth-based measurements that contribute to the LASP data base, the solar Lyman-alpha flux used is advanced or delayed by the number of days corresponding to the nearest angular difference between the comet's longitude and that of the Earth. Typically the solar Lyman-alpha flux depends on the number and intensity of active regions on the photosphere of the Sun and the pattern is reasonably reproducible from one solar rotation to the next. The comet's heliocentric velocity is also accounted for in the position along the solar line used to calculate the g-factor. The results of most of the observations contained in this data set were derived from full-sky images. In the early years of SWAN operation there were quite a few specialized comet observation campaigns, where the region of the sky centered on the comet was oversampled spatially by a factor of 2 (half degree spacing) and integration times were longer. In theory this should give somewhat higher quality data with lower uncertainties. In this data set, all the comet-specific and full-sky observations are folded together and the given 1-sigma uncertainties reflect the quality of the resulting production rate and its mode of acquisition. Parameters ========== The data are presented as ASCII tables with seven columns. The first column is the time in UTC when the comet nucleus was sampled. The second column is that same time given in fractional days from the comet's perihelion day and time. The third column gives the heliocentric distance of the comet at the time of the observation in astronomical units. The fourth column gives the spacecraft to comet distance in astronomical units. The fifth column gives the photon emission rate, or g-factor, in photons per atom per second at a distance of one astronomical unit. The sixth column gives the water production rate in 1.0e27 molecules per second. The seventh column gives a one-sigma uncertainty calculated from the scatter in the image data and the quality of the model fit to the interplanetary background signal and the comet coma distributions. Owing to calibration uncertainty, model and model parameter uncertainties, as well as the influence of faint background stars, a systematic uncertainty on the order of 30% has been estimated. The heliocentric distance and distance from the comet to SOHO, and not the geocentric distance of the Earth, at the time of the observation were calculated from the orbital elements taken from the JPL Horizons web site at the time the observations were analyzed. Orbital elements are typically updated by JPL from time to time and so some small differences might be found between heliocentric and comet-SOHO distances from the most current orbital elements. Also, SOHO is not located at the L1 Lagrange point but is in a halo orbit around the L1 Lagrange point, and the comet-SOHO distance uses the update SOHO orbit information from the SOHO project. Processing ========== SWAN was recalibrated against Hubble Space Telescope observations of the hydrogen Lyman-alpha as described in various references cited in the paper by Combi et al. (2011). Recalibration factors for comet results published before 2011 are given in that same paper. Results contained in the PDS archive include the new 2011 calibration and so will be different from results published before the 2011 paper. The calibration has remained the same since that paper. Water production rates were calculated by normalization of modeled distributions of the hydrogen coma as described in the paper by Makinen and Combi (2005). The so-called time-resolved model (TRM) is a model calculation for hydrogen atoms produced by the photodissociation chain of H2O and OH. The production of hydrogen atoms as well as their speeds after they exit the collisionally thick part of the coma are taken from a parameterized version of the Monte Carlo models from Combi and Smyth (1988a; 1988b) that were subsequently tested against a range of spatial observations of comet hydrogen observations (Combi et al. 2000) as well as doppler-resolve hydrogen Lyman-alpha line profiles obtained the Hubble Space Telescope Goddard High Resolution Spectrometer (Combi et al. 1998; and Richter et al. 2000). The models have been since tested against comet observations covering a wide range of heliocentric distances and water production rates. In the TRM the trajectories of hydrogen atoms in the coma under the influence of solar gravity and solar radiation pressure are calculated using a combination of the original syndyname approach described for hydrogen by Keller and Meier (1976) and the vectorial model of Festou (1981). To compare the model distribution with the observed images, apertures of radius 4 degrees to 8 degrees were used in the analyses and varied from image-to-image and comet-to-comet depending on the density of local field stars. Field stars within the analysis apertures were either subtracted using two images or excluded. Faint field stars not possible to identify contribute to the uncertainty in both the fit to the background interplanetary medium signal that is subtracted from the comet as well as to the comet signal itself. In the large majority of observations, field stars within the 4 to 8 degree field of view were excluded with an interactive process so generally the total brightness used for each observation was not exactly that which would be expected to fill the entire circular aperture of the nominal radius. References Related to the Analysis, Calibration and Processing of these Data ============================================================================ Bertaux, J.L., Costa, J., Quemerais, E., Lallement, R., Berthe, M., Kyrola, E., Schmidt, W., Summanen, T., Makinen, T., Goukenleuque, C. 1998. Lyman-alpha observations of Comet Hyakutake with SWAN on SOHO. Planet. Space Sci. 46, 555-568. Combi, M.R., and Smyth, W.H. 1988a. Monte Carlo Particle Trajectory Models for Neutral Cometary Gases. I. Models and Equations. Astrophys. J. 327, 1026-1043. Combi, M.R., and Smyth, W.H. 1988b. Monte Carlo Particle Trajectory Models for Neutral Cometary Gases. II. The Spatial Morphology of the Lyman-alpha Coma. Astrophys. J. 327, 1044-1059. Combi, M.R., Brown, M.E., Feldman, P.D., Keller, H.U., Meier, R.R., and Smyth, W.H. 1998. Hubble Space Telescope Ultraviolet Imaging and High Resolution Spectroscopy of Water Photodissociation Products in Comet Hyakutake (C/1996 B2). Astrophys. J. 494, 816-821. Combi, M.R., Reinard, A.A., Bertaux, J.-L., Quemerais, E., and Makinen, T. 2000. SOHO/SWAN Observations of the Structure and Evolution of the Hydrogen Lyman-alpha Coma of Comet Hale-Bopp (1995 O1). Icarus 144, 191-202. Combi, M.R., Lee, Y., Patel, T.S., Makinen, J.T.T., Bertaux, J.-L., and Quemerais, E. 2011. SOHO/SWAN Observations of Short-Period Spacecraft Target Comets. Astron. J. 141, 128 (13pp). Festou, M.C. 1981. The Density Distribution of Neutral Compounds in Cometary Atmospheres I. Models and Equations. Astron. Astrophys. 95, 69-79. Keller, H.U. and Meier R.R. 1976. A Cometary Hydrogen Model for Arbitrary Observational Geometry. Astron. Astrophys. 52, 273-281. Lemaire, P., Emerich, C., Curdt, W., Schuehle, U., and Wilhelm, K. 1998. Solar H I Lyman alpha full disk profile obtained with the SUMER/SOHO spectrometer. Astron. Astrophys. 334, 1095-1098. Lemaire, P., Emerich, C., Vial, J.-C., Curdt, W., Schuhle, U., and Wilhelm, K. 2005. Variation of the full Sun hydrogen Lyman profiles through solar cycle 23. Adv. Sp. Res. 35, 384-387. Makinen, J.T.T. and Combi, M.R. 2005. Temporal Deconvolution of the Hydrogen Coma. I. A Hybrid Model. Icarus 177, 217-227. Richter, K., Combi, M.R., Keller, H.U., and Meier, R.R. 2000. Multiple Scattering of Hydrogen Lyman-alpha Radiation in the Coma of Comet Hyakutake (C/1996 B2). Astrophys. J. 531, 599-611.