Deriving effective sunspot temperatures from SOHO/VIRGO irradiance measurements^*

A starspot modelling approach

¹ King Saud University, Department of Physics and Astronomy, PO Box 2455, Riyadh, Saudi Arabia
² Kiepenheuer-Institut für Sonnenphysik, 79104 Freiburg, Germany e-mail: pnb@kis.uni-freiburg.de
³ Institut für Geophysik, Astrophysik und Meteorologie, 8010 Graz, Austria
⁴ Sonnenobservatorium Kanzelhöhe, 9521 Treffen, Austria
⁵ Physikalisch-Meteorologisches Observatorium Davos, WRC, 7260 Davos Dorf, Switzerland

Corresponding author: Z. Eker, eker@ksu.edu.sa

Received: 1 March 2002
Accepted: 21 March 2003

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 , 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.