For more than two decades, solar astronomers have known that the magnetized plasma at the Sun’s surface gradually drifts from the equator to the poles. It follows, then, that somewhere in the solar interior that plasma must return to the equator—after all, the Sun’s poles do not become more massive with time. But whereas surface velocities can be detected by line-of-sight Doppler shifts, advection of passive tracers, and other straightforward means, interior flows are hidden by the Sun’s opaque photosphere. As a result, the meridional return flow has long eluded definitive detection.
Hints of an equatorward flow can be seen in the motion of so-called supergranules, convection cells that cycle plasma between the Sun’s interior and its surface. The smallest supergranules drift poleward, presumably carried by surface currents. But some larger granules can be seen to move upstream, toward the equator. One theory is that the larger cells penetrate deeper...
