Title: Study of Solar and Spectral Irradiance Variations via MDI and VIRGO on SOHO
Author: Michael Turmon
Affil: JPL
Email: turmon@jpl.nasa.gov
Authors: Judit Pap, Kacie Shelton
Affils: University of Maryland BC
Abstract: Several instruments on SoHO facilitate study of the Sun's variability. Since the VIRGO experiment observes the Sun as a star, it is necessary to examine high-resolution spatially resolved data to account for the variations in irradiance related to solar magnetic features, and to understand to what extent the observed irradiance variations are related to global effects. The use of MDI data, which are free from atmospheric distortion, allows consistent identification of finer facular and network features than currently possible with terrestrial observations. Also, the regular cadence of the MDI images allows study of active region evolution in great detail, thereby clarifying its role in irradiance variations. We have applied statistically-based methods for automatically identifying the various surface structures on the Sun which may contribute to observed changes in solar total and spectral irradiance. These structures are divided for our purposes into three types: quiet sun, faculae, and sunspots (umbra plus penumbra). Statistical models characterizing the properties of the region types are found either from expert identification of a sample set of regions, or automatically by unsupervised clustering. This allows objective and automated classification of a large set of images. We report results from a series of MDI images from 1996-2000, and the influence of the resolved structures on observed total and spectral irradiance from VIRGO.