Comparative Magnetic Minima: Characterizing quiet times in the Sun and stars Proceedings IAU Symposium No. 286, 2011 C. H. Mandrini & D. F. Webb, eds. c  International Astronomical Union 2012 doi:10.1017/S1743921312005017 Semi-empirical modelling of stellar magnetic activity Adriana Valio† CRAAM, Mackenzie University, Sao Paulo, Brazil email: avalio@craam.mackenzie.br Abstract. Since Galileo, for four hundred years, dark spots have been observed systematically on the surface of the Sun. The monitoring of the sunspot number has shown that their number varies periodically every 11 years. This is the well-known solar activity cycle that is caused by the periodic changes of the magnetic field of the Sun. Not only do spots vary in number on a timescale of a decade, but the total luminosity and other signatures of activity such as flares and coronal mass ejections also increase and decrease with the 11-year cycle. Still unexplained to the present date are periods of decades with almost an absence of activity, where the best known example is the Maunder Minimum. Other stars also exhibit signs of cyclic activity, however the level of activity is usually thousand times higher than the solar one. Obviously, this is due to the difficulty of observing activity at the solar level on most stars. Presently, a method has been developed to detect and study individual solar like spots on the surface of planet-harbouring stars. As the planet eclipses dark patches on the surface of the star, a detectable signature can be observed in the light curve of the star during the transit. The study of a different variety of stars allows for a better understanding of magnetic cycles and the evolution of stars. Keywords. stars: activity, stars: spots, stars: rotation 1. Introduction Sunspots are regions of strong magnetic fields on the surface of the Sun, of the order of hundreds to a few thousands Gauss. The magnetic field suppresses the overturning motion of the convective cells and thus hampers the flow of energy from the stellar interior outwards to the surface. Therefore, the region becomes cooler than the surrounding photosphere and thus appears darker. Very likely, all cool stars with a convective envelope like the Sun, or even fully con- vective, will have spots on their surfaces. This is not a new idea, in 1667, the French astronomer Ismael Boulliau (1605-1694) introduced the concept of a starspot to explain the periodic light variability of Omicron Ceti, which turned out to be a Mira variable star, but nevertheless the concept was introduced. In the 1940s, starspots were observed by Kron in the eclipsing binary AR Lacertae as a significant light variability outside the eclipse (Kron 1947). Starspots are observable tracers of the internal dynamo activity, and their study provides a glimpse into the complex internal stellar magnetic field. As dark spots cross the stellar disk due to rotation they modulate the total brightness of the star. The periodic variation generally follows the rotational period of the star. There exists an observational bias toward young stars that are fast rotators (need less observing time) and are also more active. Moreover, their light curve variability is stronger. This is the case of CoRoT-2, the second star discovered by the CoRoT satellite to harbour a planet (Alonso et al. 2008). † aka Adriana V. R. Silva or Adriana Silva-Valio 307 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921312005017 Downloaded from https://www.cambridge.org/core. IP address: 54.161.69.107, on 09 Jun 2020 at 02:09:58, subject to the Cambridge Core terms of use, available at