Why Does the Sun Have Kilogauss Magnetic Fields?

Abstract

Magnetic fields in the solar photosphere are concentrated in flux tubes with kilogauss field strength surrounded by nearly field-free plasma. Observations show that the flux tubes are located in convective downdrafts where the temperature is lower than average. We assume that the convective downdrafts extend to large depths in the convection zone, and that flux tubes follow the downdrafts to these depths. We develop a model for the magnetic field strength B(z) in the flux tubes as a function of depth z below the surface. Our calculations reveal that epsilon, the ratio of magnetic pressure to gas pressure, has a large depth variation: at the base of the convection zone where epsilon ~10^{-5} (B ~10^5 G), while at the top epsilon ~1, in broad agreement with solar observations. Thus the model can explain why the field strength at the photosphere is around 1 KG.