The main activity of the group is the investigation of the irradiance properties of individual sources in the solar photosphere that contribute to solar irradiance changes: sunspots, pores, faculae and network. Their relation with the magnetic features underlying them is essential to understand the mechanism that produces the irradiance variations. Active regions can be considered as individual sources and as a conglomerate of single sources in evolution.

Irradiance and magnetic field measurements from the VIRGO and MDI instruments on board the Solar and Heliospheric Observatory (SOHO) are being analyzed for the study of magnetic activity influence on the solar radiation emission. The time interval 1996 through 1998 offers an opportunity to investigate the sources of solar irradiance variations and to relate them to structural changes in the solar interior, if they are not strictly superficial as some theory predicts (fig.1 )

Fig. 1: Composite of the evolution of the total solar irradiance for the period 1979-2000, as measured by instruments on board different satellites and probes. The period in which each radiometer has taken measurements is indicated.

During this period of time the Sun activity cycle evolves from a minimum of activity, at the end of cycle 22, in 1996, to the start of the full fledged activity by the end of 1997 and through 1998. Both, the study of individual sources (sunspots, plages, the network) and the study of active regions as integral sources, are being pursued. Figure 2 shows an example of the angular distribution of the excess energy radiated by an active region in a particular wavelength.

Fig. 2: Three-dimensional rendering of the angular distribution of the excess irradiance emitted at 500 nm by an active region at two stages of its developement. The brightening of the facular emission at the latter stage corresponds to a more extended active region. The surfaces under the distributions are MDI magnetograms at the time of the central meridian passages of the active region.

The group is also involved in present and planned measurements of the total solar irradiance, the so called "solar constant", an essential component of Earth's climate. It can only be measured from space. Its variation with the solar cycle of activity has been observed for two cycles. Future research requires continuity of the synoptic measurement and improvement of solar radiometry over presently flown instruments by about one order of magnitude in accuracy.