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                                                             
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This data set presents water production rates of comets taken from 1996 to    
2021 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://www.cosmos.esa.int/web/soho/soho-science-archive. The instrument, its
operation and application to comet observations are described by Bertaux et
al. (1998). Fully calibrated images are an intermediary step in the modeling
process as described in Combi et al. (2011) and are not an output of the
processing and thus not currently available in a public archive. 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                                                                    
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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                                                                    
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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. Since the 
resulting production rates are dependent on normalized distributions 
from the best fit models and are not single valued, heliocentric velocities 
and column densities are not reported in the data tables, as reporting 
single values would be somewhat arbitrary.
                                                                              
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 and are irregular 
in size and magnitude from measurement to measurement (Combi et al. 2019). 
Therefore, 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  
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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).                                  
                                                                              
Combi, M.R., Makinen, T.T., Bertaux, J.-L., Quemerais, E., Ferron, S., 
  Avery, M., and Wright, C. 2018. Water production activity of nine long-
  period comets from SOHO/SWAN observations of hydrogen Lyman-alpha: 2013-
  2016. Icarus 300, 33-46.

Combi, M.R., Makinen, T.T., Bertaux, J.-L., Quemerais, E., and Ferron, S.
  2019. A survey of water production in 61 comets from SOHO/SWAN 
  observations of hydrogen Lyman-alpha: Twenty-one years 1996-2016. Icarus 
  317, 610-620.

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.                                    
                                                                              
