The Sun's Large-Scale Magnetic Field and its Long-Term Variations

Author: Yi-Ming Wang
Naval Research Laboratory

Variations in cosmogenic isotope abundances are widely used to infer long-term changes in solar activity and solar irradiance. However, the cosmic rays that produce cosmogenic isotopes are modulated by the heliospheric magnetic field, whose variation is not the same as that of the closed fields (in the form of dark sunspots and bright faculae) that control the total irradiance. We discuss the relationship between the Sun's open and closed magnetic flux, and show how both components can be modeled over the 11-year solar cycle. The model includes the effect of the Sun's meridional circulation, which plays a major role in reversing the polar fields and in determining the cycle period and amplitude. We use the model to predict the evolution of the solar magnetic field over the last 100 years and during the Maunder Minimum, and we confront the predictions with 10Be and geomagnetic activity records.