SOLAR AND HELIOSPHERIC OBSERVATORY/SOLAR WIND ANISOTROPIES OBSERVATIONS OF FIVE MODERATELY BRIGHT COMETS: 1999-2002

Abstract

Solar Wind Anisotropies (SWAN), the all-sky hydrogen Lyman-alpha camera, on the SOHO spacecraft makes routine all-sky images of the interplanetary neutral hydrogen around the Sun and thus monitors the effect of the variable solar wind on its distribution. SWAN has an ongoing campaign to make special observations of comets, both short- and long-period ones, in addition to making serendipitous observations of comets as part of the all-sky monitoring program. We report here on a study of the moderately active Oort cloud comets observed by SWAN during the period of 1999-2002: 1999 H1 Lee, 1999 T1 McNaught Hartley, 2000 WM1 LINEAR, 2001 A2 LINEAR, and 2002 C1 Ikeya Zhang (P153). SWAN is able to observe comets almost continuously over most of their visible apparitions and provide excellent temporal coverage of water production. In addition to calculating production rates from each single image, we also present results using our time-resolved model (TRM) that analyzes an entire sequence of images over many days to several weeks/months, and from which daily-averaged or two-day-averaged water production rates are extracted over continuous periods of several days to months. The short-term (outburst) behavior is correlated with other observations and is examined and associated with fragment release. The long-term heliocentric distance-dependent variations of water production rate are examined and compared and contrasted with the measured volatile compositions of the comets as well as their absolute production rate levels. The overall long-term variation is also distinguished from seasonal effects seen in the pre- to post-perihelion differences.