SOHO VIRGO and MDI SCIENCE PROPOSAL Study of Solar Irradiance Variability and Solar Magnetic Activity Based on the SOHO/VIRGO and MDI Measurements Main Investigator J.M. Pap Co-Investigators T. Appourchaux, P. Brandt, G.A. Chapman, D. Crommelynck, W. Finsterle, C. Frohlich, A. Hanslmeier, H. Hoeksema, J. Kuhn, P. Scherrer, M. Steinegger, M. Turmon, R. Ulrich, C.H. Wehrli Objectives The main goal of the proposed research is to identify and characterize the physical properties of the various components of the changes observed in total solar and spectral irradiances. The SOHO/VIRGO experiment will provide measurements of solar irradiance in the entire spectrum and at various wavelengths. The PMO6-V and DIARAD active cavity radiometers will perform measurements of total solar irradiance. The solar spectral irradiance will be measured by three-channel SunPhotometers (SPM) at 402, 500, and 862 nm. Since the VIRGO instruments will observe the Sun as a star, analysis of spatially resolved data is required to study the observed irradiance changes in detail. The analysis of the full disk irradiance observations by VIRGO will be supplemented by detailed spatially resolved information from LOI and the flux budget product and magnetograms of MDI. These space-borne images will be compared and calibrated with high resolution solar images obtained by ground-based observatories (Mt. Wilson Observatory, National Solar Observatory at Sacramento Peak, the San Fernando Observatory at Northridge and the Solar Observatory of the Institute fuer Astronomy at Kanzelhohe, Austria). Scientific rationale Study of changes in solar total and spectral irradiances has been of high interest for a long time because of their astrophysical significance and their relation to the terrestrial atmospheric and climate system. Synoptic observations of solar irradiance from space within the last one and half decades led to the discovery of changes in the solar energy flux from minutes to the 11-year solar cycle. Although considerable information exists about the variations in solar irradiance, the underlying physical mechanisms are not well understood as yet. The current empirical models of total solar irradiance developed from the sunspot blocking function and proxy data for bright magnetic features (facula, plages and the magnetic network) disagree with the observations at the time of solar maximum. It has also been found that a considerable remaining variability exists in both total solar and UV irradiances after removing the effect of sunspots and bright magnetic features over a broad range of periods. It is not clear whether this residual variability is caused by additional effects, such as large scale structures, radius changes and/or temperature changes. Proposed Research Task (1): Study the energy budget of active regions The main goal of this research effort is to study the phase difference between irradiance deficits related to sunspots and excess flux associated with bright magnetic elements (facula/plages and the magnetic network). Since a phase shift has been recognized between the full disk solar magnetic flux and solar total and UV irradiances, a detailed comparison of irradiance values with high resolution solar images and magnetograms observed by the SOHO/MDI experiment will be carried out. The spatial resolution data from LOI will also allow us to deconvolve effects of different photospheric features on solar irradiance. These spatially resolved data from space will be compared to solar images available from several ground-based observations for calibration purposes. We propose to study (a) the change of the contrast of sunspots as a function of their evolution; (b) the umbra/penumbra area and intensity ratios during the evolution of sunspots; (c) the area and intensity of facula/plages and (c) the magnetic field of sunspots and plages as a function of their evolution. This research task will also include the classification of sunspots and plages due to their evolution. Analysis of the MDI images, together with high resolution ground-based images, will also make possible to study the evolution of supergranulate convection cells and magnetic network reorganization. Task (2): Study the effect of sunspots and faculae on total solar and spectral irradiance as a function of their heliographic coordinates (longitude and latitude). Former studies indicate that the most complex active regions tend to be separated 180 degrees apart, this problem will be addressed during the proposed investigation. Study of the latitude dependence of active regions may help to clarify the degree of irradiance changes related to active regions and to large-scale structures and/or temperature changes. Advanced statistical techniques, such as wavelet, singular spectrum and multivariate analyses will be used to decompose the solar signals and to estimate the contribution of various solar features to the changes in solar total and spectral irradiances as a function of the evolution and distribution of active regions. The proposed investigations will help to identify the physical mechanism of the re-radiation of the missing energy in the sunspot-related irradiance dips; namely we expect to clarify whether the missing energy is stored in the convective zone or there is a simple re-distribution of the solar energy flux as the active regions evolve and move across the solar disk. Intercomparison of the full disk irradiance data with the MDI and LOI images will clarify the role of temperature changes and other possible large scale changes of convective efficiency. Supporting ground-based observations: (1) Observations at the San Fernando Observatory, California State University at Northridge by the Cartesian Full Disk Telescopes at 672.3 and 472 nm for observing sunspot characteristics and at 393.3 nm for observing plage characteristics; (2) Precision Solar Photometric Telescopes (PSPT) provide full-disk, broad-band photometric observations of sunspots and bright magnetic features; (3) On-going routine CaII K-line observations at Sac Peak and Big Bear Solar Observatories. Current observations at the Mt. Wilson Solar Observatory generate daily spot, plage and quiet-Sun indices based on magnetic field strengths and areas. The current synoptic Mt. Wilson data also include spatially resolved magnetograms taken simultaneously in FeI 525 nm and CrII 523.7 nm lines. Additional high resolution (1 - 2'') white-light photographs are taken at the Solar Observatory of the Institute fuer Astronomy at Kanzelhohe, Austria. Responsibilities and Management The proposed research will be a collaborative effort between the VIRGO and MDI teams as well as participating ground-based observatories. The generation of the PMO6-V and DIARAD total irradiance data will be the responsibility of W. Finsterle and D. Crommelynck. The generation of the SPM spectral irradiance values will be the responsibility of C. Wehrli. J. Pap will be responsible for the overall integrity and management of the proposed research. Analysis of solar images will be carried out with P. Scherrer, T. Hoeksema, G. Chapman, J. Kuhn, P. Brandt, and A. Hanslmeier based on state-of-the-art automated image processing technique currently under development at JPL. Study of the effect of sunspots and bright magnetic features on solar irradiance will be carried out as a cooperative research effort by P. Brandt, G. Chapman, A. Hanslmeier, J. Pap, M. Steinegger, and M. Turmon. Study of the magnetic field observations will be a cooperative work between the MDI team and the group at UCLA. G. Chapman, C. Frohlich and J. Pap will participate in the modelling efforts. J. Kuhn and R. Ulrich will provide their expertise in the interpretation of the results.