Comets in full sky L-alpha maps of the SWAN instrument - I. Survey from 1996 to 1998

The SWAN instrument onboard the SOHO spacecraft is a Lyman cu scanning phot ometer cabable of mapping the whole sky with 1 degrees resolution. Since Ja nuary 1995 the instrument has produced on average three full sky maps a wee k with the principal scientific objective of observing the distribution of heliospheric neutral hydrogen. In addition, these systematic observations a re a valuable source for studying comets brighter than a visual magnitude o f 7-11 the observing limit depending on the abundance ratios of produced ra dicals and the location of the comet relative to the galactic plane. When t he data before the temporary loss of control of SOHO st the end of June 199 8 were processed, altogether 18 comets were positively identified, of which one is a new discovery and another 5 can be detected on SWAN images before their actual discovery date. This demonstrates the feasibility of SWAN as an instrument for cometary surveys. The observations are used to estimate t he water production rates of the detected comets near their perihelion pass ages.