Deeply Penetrating Banded Zonal Flows in the Solar Convection Zone.

Helioseismic observations have detected small temporal variations of the rotation rate below the solar surface that correspond to the so-called "torsional oscillations" known from Doppler measurements of the surface. These appear as bands of slower- and faster-than-average rotation moving equatorward. Here we establish, using complementary helioseismic observations over 4 yr from the GONG network and from the MDI instrument on board SOHO, that the banded flows are not merely a near-surface phenomenon: rather, they extend downward at least 60 Mm (some 8% of the total solar radius) and thus are evident over a significant fraction of the nearly 200 Mm depth of the solar convection zone.

Dynamic variations at the base of the solar convection zone

We have detected changes in the rotation of the sun near the base of its convective envelope, including a prominent variation with a period of 1.3 years at low latitudes. Such helioseismic probing of the deep solar interior has been enabled by nearly continuous observation of its oscillation modes with two complementary experiments. Inversion of the global-mode frequency splittings reveals that the largest temporal changes in the angular velocity Omega are of the order of 6 nanohertz and occur above and below the tachocline that separates the sun's differentially rotating convection zone (outer 30% by radius) from the nearly uniformly rotating deeper radiative interior beneath. Such changes are most pronounced near the equator and at high latitudes and are a substantial fraction of the average 30-nanohertz difference in Omega with radius across the tachocline at the equator. The results indicate variations of rotation close to the presumed site of the solar dynamo, which may generate the 22-year cycles of magnetic activity.