Speaker
Description
The study of cosmogenic radionuclides shows that an event of significantly reduced solar activity such as the Maunder Minimum (1645-1715) was not an exceptional event, but is a recurring phenomenon in the history of solar magnetic activity.
Although various ideas have been proposed to try to explain why Grand Minima occur, there is no satisfactory model yet.
Since the decade of 1980 many astrophysicists believe that the tachocline plays a fundamental role in the generation and storage of the toroidal mag- netic flux that eventually becomes unstable and buoyantly rises to emerge at the stellar surface producing sunspots.
I will address the role of the thermodynamic properties (and more specifi- cally, the entropy stratification) of the tachocline in determining the stability of magnetic structures and, therefore, its capability to store magnetic flux. The entropy stratification is quantified by a dimensionless quantity called the superadiabaticity, δ. Tiny variations in δ (of the order of one part in 104 or less) may determine global properties of the magnetic field at the solar surface. The maximuum field-strength of toroidal flux tubes that can be stored in mechanical equilibrium at the bottom of the convection zone is very sensitive to the value of δ. The connection between temporal vari- ations in δ (which would alter the storage capacity of the tachocline) with the occurence of Maunder-minimum-like episodes will be discussed.
I will briefly address the possible role of stochastic resonance in amplifying weak signals up to a level that can affect the long-term magnetic activity by switching between two states of a bistable system, corresponding to weak- field and strong-field regimes.
The approach would be applicable to any rotating star with a radiative zone surrounded by a convection zone.