Speaker Name: Soumitro Banerjee, Institute of Science Education & Research
 Abstract :
Astronomers have noticed that old sun-like stars rotate slower and have reduced magnetic activity than younger stars. The mechanism of magnetic braking of stars is well known, but it is a very slow process. Observational data indicate that there must be another mechanism that becomes active around the middle of a star's life, which renders the magnetic braking mechanism ineffective. There are indications that our Sun, at the age of 4.6 billion years, is now undergoing that stellar mid-life crisis' because the 11-year solar activity cycle is exhibiting intermittent cycles of dormancy. Using a stochastic delay differential equation model of the mean magnetic field generation process in stars, we show that the phenomenon is caused by the existence of two attractors. As the sun ages, the system can come close to a saddle-node bifurcation point, where the periodic orbit representing the normal magnetic activity coexists with an equilibrium point representing a magnetically dormant state. When the two attractors are sufficiently close to each other, the system noise can intermittently knock the state from the periodic orbit to the equilibrium point and vice versa. These two solutions are possibly involved in the observed bimodal distribution of magnetic cycles in the Sun. On that basis, we argue that the Sun is right now undergoing a bifurcation that will eventually lead to the transition from a magnetically active to an inactive state.  Â
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