Astronomy and astrophysics

Solar cycle: modeling and predicting

by Petri Käpylä



The solar magnetic field exhibits a 22-year cycle manifested by the sunspots. The cycle arises from the dynamo process that maintains the large-scale magnetic field. It is shown that essential ingredients for the solar dynamo are large-scale flows (differential rotation, meridional flows), and turbulence affected by rotation (turbulent alpha-effect). A standard axisymmetric model working in a 2D spherical shell and making use of the equations of mean-field magnetohydrodynamics is presented along with a few representative results. Such models may help to predict the strength of the next solar maximum. Such predictions have become critical in deciding about space missions (for example whether it is advisable to pay for maintenance of the Hubble Space Telescope!) Empirically one knows that the strength of the so-called dipole moment during the declining phase of the previous cycle is related to the strength of the following maximum. Recently solar dynamo models have been used to make predictions about Solar Cycle 24 (maximum due around 2012) with rather different outcomes. These techniques will be reviewed and their respective merits and deficiencies discussed.