Variations in Solar Motion
Frames 72-81 show a different view of the Sun's rotation: a change in speed of motion rather than rotation rate. Previous images depicted the general variation in rotation rates from the equator to the poles. Here instead, we computed the smooth variation of differential rotation in the Sun's surface, and subtracted that from the value at each latitude and depth. [For those who care, the term subtracted is of the form A + B*sin^2(latitude) + C*sin^4(latitude) where A = 450.78, B = -48.55, C = -67.76 all in nano-Hz] As before, red-yellow is faster than average, and blue is slower than average. These depictions highlight the smaller scale variations in the change in rotation from the equator to the poles.
On the left side of the image, the light orange bands are zones that are moving slightly faster than their surroundings. The new observations indicate that these extend down approximately 12,000 miles into the sun. Sunspots, caused by disturbances in the solar magnetic field, tend to form at the edge of these bands. SOI scientists speculate that this may be due to the difference in speed at the edge of these zones which tend to "twist" the magnetic field generated by the moving hot, electrically charged plasma gas. The cutaway on the right side of the image reveals speed variations in the interior of the Sun. Only the outer 30 percent of the Sun's interior, where the variations are more certain, is shown. The red ovals embedded in the green areas at the poles are the newly discovered polar plasma "jet streams". They move approximately ten percent faster than their surroundings, and each is about 17,000 miles across, large enough to engulf two Earths.
Similar bands were detected on the surface about 15 years ago by Robert Howard and Barry LaBonte at Mt Wilson Observatory, California. They reported alternating fast and slow bands which moved from high latitudes towards the equator during the 11 years of the solar sunspot cycle. In fact they saw several bands in each hemisphere, and showed that the sunspots erupt near the poleward side of the faster streams. The streams move to the equator during the cycle, following (or at least connected with) the movement to the equator of the locations at which new sunspots appear.
The new observations just recently published (Kosovichev and Schou, "Detection of Zonal Shear Flows Beneath the Sun's Surface from F-Mode Frequency Splitting", ApJ letters, accepted, preprint) show that these bands penetrate at least 15,000 miles into the Sun. Detailed comparisons with the Mt Wilson surface observations (continued by Roger Ulrich of UCLA) show that these interior bands are indeed the same as the surface bands seen before. We now know that these bands are real and deeply rooted, not due to some obscure systematic observing error in the surface data. We can now go back and re-evaluate the Mt Wilson data to learn more about the solar cycle.
These images were taken using the Michelson Doppler Imager (MDI) instrument on the NASA/ESA Solar and Heliospheric Observatory (SOHO) spacecraft. They are a result of computations using observations taken continuously from May 1996 to May 1997. Again, the false color represents differences in speed between various areas on the Sun, both at the surface and in the interior. Red-yellow is faster than average and blue is slower than average.
(Photo Credit: Stanford University)