Deriving effective sunspot temperatures from SOHO/VIRGO irradiance measurements. A starspot modelling approach

Abstract

A method is developed to obtain mean effective sunspot temperatures from satellite irradiance deficits. Using a formalism derived from a star spot modelling approach, this method only requires accurate positions and areas of sunspots - but not intensities. The method is applied to SOHO/VIRGO spectral solar irradiance (SSI) at 402, 500, and 862 nm as well as total solar irradiance (TSI) measured during the period 16 July to 17 August 1996, the disk passage of active region NOAA 7981. MDI full disk white-light images are used to extract sunspot positions and areas. First, using the spot pixels, which are identified on MDI images as Is < 0.92, we find the mean effective temperature of NOAA 7981 to be ≃ 300 K less than the local photospheric temperature. Because the area is dominated by penumbral pixels, the temperature is close to a value representing the penumbra. Second, separating umbral and penumbral pixels, we obtain a temperature difference of ≃ 850 K for the umbra and of ≃ 200 K for the penumbra. Finally, if a 5% binning of the spot intensities is applied, a temperature deficit of ≃ 1000 K is found for the darkest regions of the spot.