With the MDI observations we have been able to measure the average poleward flow in the solar interior for the first time. The measurements show that in the top 15,000 miles (at least) there is a poleward motion in both northern and southern hemispheres, with a flow speed of about 55 mph.
Frames 62-71 illustrate the effect of the poleward flow. Flow lines overlay the rotation rates at the Sun's surface. (The false colors again represent speed: red material is rotating fastest, blue slowest.) The flow lines are the paths that would be followed by individual blobs of solar plasma.
As material rotates, it is also flowing toward the poles at a relatively slow velocity of about 50 miles per hour. The lines represent how this motion would appear if you could stand on the surface of the Sun about 30 degrees from the equator, and move with the same speed as the material there. If you were at this position in the northern hemisphere, material closer to the equator would appear to move to the right of the image as it flowed north, because it is rotating faster. Material closer to the north pole would appear to move to the left as it flowed north, because it is rotating slower. The cutaway on the left of the image represents the observed polar flow 15 thousand miles beneath the surface and a hypothetical, slower moving return flow from the poles to the equator, estimated to be 120 thousand miles beneath the surface.
The combination of differential rotation and poleward flow has been the explanation for the stretched-out shapes of magnetic fields when these regions migrate to the poles and get pulled to the left. These new observations demonstrate for the first time that the poleward flow is a property of at least 12 percent of the convection zone. It is not just a surface phenomenon.
We have not yet detected a return flow deeper in the convection zone, but expect that it must be present at some very slow speed.
These images were developed from data taken using the Michelson Doppler Imager (MDI) instrument on the NASA/ESA Solar and Heliospheric Observatory (SOHO) spacecraft over a period of 12 months ending June 1997. The poleward flow measurements were made be Peter Giles and Thomas Duvall using a new technique called "Time-Distance Helioseismology", developed by Duvall.
(Photo Credit: Stanford University)