Meridional circulation, which transports solar materials between low and high latitudes inside the Sun, is a fundamental property of the Sun, but was poorly known. It is widely believed that the circulation plays an important role in redistributing solar angular momentum and transporting magnetic flux, setting up 11-year solar activity cycles. Previously it was thought that the circulation had a single-cell structure with a poleward flow near the surface and an equatorward flow located near the bottom of the convection zone.
Through analyzing the unprecedented high-quality helioseismic data obtained from SDO/HMI by using a helioseismic analysis technique called time-distance helioseismology, the solar physicists found that the solar meridional currents have at least two circulation cells in each hemisphere, with an equatorward flow located between about 40,000 and 80,000 miles below the surface. That is, the equatorward flow is roughly in the middle of the convection zone. This result provides a new insight into the dynamics of the solar interior hidden from direct observations, and requires a reexamination of the theories of solar magnetic cycles.
|Schematic plot showing the double-cell meridional circulation structure inside the Sun, discovered by Zhao et al. (2013).|
|Helioseismic inference of the solar interior meridional-flow speed, covering the period of May 2010 through April 2012. Red color indicates the flow is northward (up) and blue color indicates the flow is southward (down).|
|Illustration of a previous assumed scheme of meridional circulation, based on near-surface measurements from SOHO/MDI (Solar and Heliospheric Observatory / Michelson Doppler Imager) and released in August 1997, showing a single-cell meridional circulation profile in each hemisphere, in contrast with the new results.|
|Animation illustrating the meridional circulation structure inside the Sun. The Sun in this movie, shown in its chromosphere, was observed by SDO/AIA (Solar Dynamics Observatory / Atmospheric Imaging Assembly).|
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