Probing Magnetic Fields at the Base of the Solar Convection Zone with Meridional Flows

Abstract

Solar magnetic fields are responsible for most of the activities on the Sun. Many theories predict that it is generated by a dynamo near the base of the convection zone (BCZ), located at 0.71{R}_⊙ . In this study, we use the solar-cycle variations of the meridional flow to probe magnetic field variations near the BCZ. A helioseismic time-distance method is used to measure the travel-time difference between opposite directions in meridional planes, which reflects the meridional flow at different depths. Two systematic effects, the surface magnetic effect and the center-to-limb effect, are removed. Using Solar and Heliospheric Observatory/Michelson Doppler Imager data, we measure the latitudinal distribution of travel-time difference for different travel distances, corresponding to meridional flow signals in the solar interior down to 0.54{R}_⊙ , over 15 years, including two solar minima and one maximum. The travel-time differences at the maximum and the minimum behave differently in three different depth ranges. The travel-time difference at the maximum is greater than that at the minimum above the BCZ, while it is smaller around the BCZ; both are close to zero below the BCZ. The difference in the travel-time difference between the maximum and the minimum changes about 0.1 s from the region above the BCZ to the region around the BCZ, corresponding to a change in flow velocity of about 10 m s^-1 around the BCZ. We tend to attribute this change in the meridional flow to the variation in the magnetic field from the minimum to the maximum near the BCZ.