Detection of Equatorward Meridional Flow and Evidence of Double-Cell Meridional Circulation inside the Sun

TL;DR

This study uses helioseismology data from SDO/HMI to reveal a double-cell meridional circulation inside the Sun, with an equatorward flow in the middle convection zone and a poleward flow below 0.82 R_sun, challenging previous models.

Contribution

It provides the first observational evidence of a double-cell meridional circulation profile inside the Sun based on helioseismic measurements.

Findings

Poleward meridional flow extends to 0.91 R_sun.

Equatorward flow found between 0.82 and 0.91 R_sun.

Evidence of a second poleward flow below 0.82 R_sun.

Abstract

Meridional flow in the solar interior plays an important role in redistributing angular momentum and transporting magnetic flux inside the Sun. Although it has long been recognized that the meridional flow is predominantly poleward at the Sun's surface and in its shallow interior, the location of the equatorward return flow and the meridional flow profile in the deeper interior remain unclear. Using the first two years of continuous helioseismology observations from the Solar Dynamics Observatory / Helioseismic Magnetic Imager, we analyze travel times of acoustic waves that propagate through different depths of the solar interior carrying information about the solar interior dynamics. After removing a systematic center-to-limb effect in the helioseismic measurements and performing inversions for flow speed, we find that the poleward meridional flow of a speed of 15 m/s extends in depth from the photosphere to about 0.91 R_sun. An equatorward flow of a speed of 10 m/s is found between 0.82 to 0.91 R_sun in the middle of the convection zone. Our analysis also shows evidence of that the meridional flow turns poleward again below 0.82 R_sun, indicating an existence of a second meridional circulation cell below the shallower one. This double-cell meridional circulation profile with an equatorward flow shallower than previously thought suggests a rethinking of how magnetic field is generated and redistributed inside the Sun.