led to a model of the sun's differential rotation that was commonly referred to as "constancy on cylinders." The sun in this picture, at least through the convection zone, was supposedly composed of a set of nested cylinders that extended from pole to pole, aligned with the sun's axis of rotation. The inner cylinders, which surfaced at the higher latitudes, rotated more slowly than the outer ones, which met the surface at the more rapidly rotating lower latitudes. This also meant that the angular velocity at a particular latitude should have gradually decreased with depth.

But this picture failed to fit the observations of a number of helioseismologists, including Timothy Brown at the National Center for Atmospheric Research in Colorado and Cherilynn Morrow, then a student at the University of Colorado. Morrow and Brown began to show that the sun's rotation rate at a given latitude actually remains fairly constant down through the convection zone. Past that zone, angular velocities at the poles and equator shift toward the same rate. Halfway into the sun, beyond the convection zone and into the radiative interior, the sun rotates somewhat like a rigid body. These observations confirm the suspicion that the sun's differential rotation at the surface, long a mystery, is somehow generated by convection rather than processes deeper in the interior.

Brown and Morrow's model was sustained and extended by a wealth of new data gathered by Ken Libbrecht. For 6 months in 1986 at Caltech's Big Bear Solar Observatory, located in the center of Southern California's Big Bear Lake, Libbrecht and his students took a Doppler image of the sun each minute, gathering a total of around 70,000 pictures. The team then extracted vibrational modes from these images after some 40 hours of supercomputer time. "We were interested in measuring as many modes as we could," says Libbrecht, "because each mode has its own story to tell about the medium in which it was trapped." Lastly, inversions of these modes, performed by Jorgen Christensen-Dalsgaard of Denmark's Aarhus University and others, mapped the sun's rotation down to a depth of about 450,000 kilometers, 60 percent of the way to the sun's center. Similar sets of images were taken again in 1988, 1989, and 1990 and added to the data base.

"We find that the rotation persists almost independent of radius, down to the base of the convection zone," says Libbrecht. "Then, there is a fairly sharp transition to solid body rotation. This is one of the biggest outstanding questions concerning the sun—why does it rotate in this manner? We couldn't go down to the very core—we went down about



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