HONOLULU -- The surface of the Sun is covered with long-lasting depressions and humps that are very similar to oceanographic features on the Earth, according to researchers at the University of Hawaii. Their detection of tiny mounds and valleys, reported in the June 1 issue of Nature, is the most sensitive measurement ever made of a star's shape. The discovery, made using the NASA/ESA SOHO spacecraft, links the fields of oceanography and stellar astrophysics.

The surface of the sun is covered with 100m high "hills" each separated from the next by about 90,000km. These hills are the solar equivalent to a pattern of terrestrial oceanic "bumps" of only a few centimeters in height. The oceanic hillocks barely move, crossing the atlantic and pacific over periods of months to years (cf. see Chelton and Schlax). The sun's bumps are caused bya phenomenon called Rossby waves which produce a grid of weak cyclones that generate the hills and valleys on the sun's visible surface.

The solar features were measured from space with an experiment called the Michelson Doppler Imager (MDI), a part of the Solar and Heliospheric Observatory (SoHO). This experiment allows, for the first time, tiny changes in the position of the edge ("limb") of the sun to be seen. To detect these hills the MDI experiment measured the changing shape of the solar limb over almost a 3 year period as the sun's rotation carried the Rossby wave hills around the limb. Unlike previous techniques, which have looked for waves by measuring the brightness or velocity of the gas in the solar atmosphere, this experiment demonstrates the feasibility of measuring tiny changes in the position of the sun's atmosphere. The same experiment has addressed longstanding questions like whether the sun's radius and shape change during the 11 year solar cycle.

Professor Kuhn explains that "this new understanding of the solar `mountains` which cover its surface will help us solve some longstanding mysteries, like why the sun rotates more slowly at the poles than at its equator. Rossby waves are a new and sensitive probe of the sun's peculiar interior rotation." Kuhn further says that "we can use their measured properties much like how we have learned about the earth's interior when we study the vibrations caused by earthquakes."

This research was supported by NASA. The principle investigator is Prof. J. R. Kuhn at the Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI, 96822, 808-956-8968, email: The lead scientist for the MDI experiment is Dr. P. Scherrer of Stanford. The MDI experiment was built primarily at Lockheed, Palo Alto under the direction of Dr. A. Title.

The SPHERIS experiment has been designed to capitalize on these new solar astrometric capabilities demonstrated by MDI.


This figure illustrates how solar Rossby hills would appear if we magnify their relative height by a factor of about one million. This experiment detected the presence of the hills at the edge ("limb") of the sun only.


The key signature of the rotating Rossby pattern is a "sawtooth" distribution of power. Near the poles the bumps are travelling slowly and the power is confined to low frequencies. At the equatorial east and west regions the opposite condition exists.