At relatively shallow depths below the solar surface, the Sun's magnetized plasma flows from the equator to the poles. Since the plasma doesn't pile up at the poles, the flow must turn equatorward at greater depth and form a closed meridional-flow cell. Aspects of presumed meridional flows have informed models of the solar dynamo thought to be responsible for the roughly 11-year cycle of solar activity. Now Stanford University's Junwei Zhao and colleagues have used the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory to map the meridional flow in detail, and the results may send modelers back to the drawing boards. Every 45 seconds, the HMI images the Sun's photosphere and extracts Doppler shifts associated with acoustic oscillations on the Sun's surface. Since acoustic waves propagate into the Sun, bounce off the interior plasma flow, and return to the surface, correlations in the oscillations can yield a map of plasma velocity as a function of depth. An essential feature of the research team's analysis was the removal of a systematic effect noted last year by Zhao and coworkers. The figure shows the key new results. One is that the equatorial plasma flow occurs at a depth of 0.09–0.18 solar radii below the surface—shallower than in many solar dynamo models. But perhaps more remarkable is the feature in red: a second flow cell deeper within the Sun. (J. Zhao et al., Astrophys. J. 774, L29, 2013.)—Steven K. Blau
