SOLAR AND HELIOSPHERIC OBSERVATORY/SOLAR WIND ANISOTROPIES OBSERVATIONS OF MODERATELY BRIGHT COMETS: 1999–2014

The author J.P.Navarro Pina investigated the recient observations of SOHO / SWAN of Observations of hydrogen Lyman-α (Ly-α) at 1215.7 Å in comets and their interpretation are important. Atomic hydrogen is the most abundant species in the atmosphere (or coma) of a comet being produced in a photodissociation chain originating with water molecules and including intermediate OH radicals. Water is the most abundant volatile species in a comet’s nucleus, and water sublimation controls the abundance and activity of the coma when comets are within 3 AU from the Sun. Measurements of the abundance and distribution of hydrogen in the coma, when appropriately modeled, can provide a reliable measure of the water production rate and its variation in time in comets. Virtually all compositional information is compared to water, making water the most important species for obtaining accurate production rates. Variations in production rate with time generally, and with heliocentric distance in particular can provide information about the composition and structure of the nucleus. nucleus. Solar Wind Anisotropies (SWAN), the all-sky hydrogen Ly-α camera, has been operating on the Solar and Heliospheric Observatory (SOHO) spacecraft since its launch in 1995. The SWAN instrument was designed to observe the entire sky in H Ly-α in order to obtain a global view of the variable interaction of the solar wind with the neutral interstellar hydrogen streaming through the solar system. From its viewpoint at the L1 Lagrange point between the Earth and Sun it obtains an unparallel view of the Sun, its large extended corona, and the entire sky. For a more detailed description of SWAN, see Bertaux et al.