Solar UV Center-to-Limb Variation of Active Regions


Authors:          Patrick Crane (1), Linton Floyd (2), John Cook (3), Lynn Herring (2),

and Eugene Avrett (4)

Affiliations:     1) Remote Sensing Division, NRL, Washington, DC,

                        2) Interferometrics Inc., Chantilly, Virginia,

                         3) E.O.\ Hulburt Center for Space Research, NRL, Washington, DC,

                        4) Harvard-Smithsonian Center for Astrophysics, Cambridge, MA


The center-to-limb variation of solar radiant flux places important constraints on candidate models of the solar atmosphere.  Time series of solar ultraviolet (UV) irradiances measured by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite (UARS) have been analyzed to describe the center-to-limb behavior of the excess surface brightness of solar active regions over the wavelength range 142--265 nm.  From 27 September 1994 to 4 June 1995, solar UV irradiances in the 168-210 nm wavelength range exhibited strong 13.5-day periodicity, and weaker 13.5 day periodicity elsewhere.  We show that, on average, active regions exhibit weak limb brightening for 142-168 nm, strong limb darkening for 168-210 nm, and weak limb darkening for 210-260 nm.  In the shortest wavelength region, our results agree well with a different time series analysis of SOLSTICE irradiances by Worden, Woods and Bowman (2001).  However, the strong limb darkening found for active regions at intermediate wavelengths differed substantially from earlier quiet sun observations and those derived from the semi-empirical solar atmospheres model described by Fontenla et al. (1999), which included subsequent improvements by Avrett.