Image: Stanford University
A couple years ago researchers announced that they had discovered traces of water on the sun. A team at Stanford University has just gone them one better, discovering entire rivers on the sun. These are not rivers in the familiar sense, of course; rather they are huge, snaking flows within the white-hot plasma (electrically charged gas) that makes up the sun. The unexpected finding is helping astronomers fill in their picture of the processes that cause periodic and sometimes dramatic changes in solar activity.
Monitoring the circulation of the sun is a difficult process. There are no clouds to follow, no landmarks to reckon by. But the Solar Oscillations Investigation group at Stanford has a powerful tool at its disposal: the Michelson Doppler Imager on board the Solar and Heliospheric Observatory (SOHO). Each minute, this instrument can measure the up-and-down motion of roughly one million points on the sun. Scientists then analyze that data to infer motions occurring not only on the sun's surface but also deep inside. This process of reconstruction is called helioseismology, by analogy to the way that geophysicists derive information about the earth's interior from studying seismic waves.
Last year Philip H. Scherrer of Stanford and his colleagues used data from SOHO to pin down the location of the magnetic dynamo that drives the sun's 11-year activity cycle. At times of "solar maximum," sunspots and flares are more abundant. These phenomena can disturb satellite communications and lead to power outages. The solar cycle is also associated with a tiny variation in the brightness of the sun; some scientists suspect that more significant fluctuations occur over longer stretches of time. Scherrer's results confirmed the prevailing theory of the origin of the solar cycle and demonstrated that the turbulent motions responsible are centered on a zone some 210,000 kilometers (135,000 miles) below the solar surface.
The new results offer further insight into the sun's complex motions. The riverlike flow occurs "totally inside the sun," Stanford's Jesper Schou explains. "It cannot be seen at the surface." But the Michelson Doppler Imager nailed down the flow, revealing it to follow a ring-shaped course around the sun close to its poles (at about 75 degrees latitude). This flow may be connected with the beginnings of a new solar cycle. Scherrer makes an analogy not to a river but to the earth's jet streams, which follow a somewhat similar form, although "they are immense compared with atmospheric jet streams on the earth," he notes.
Another newfound circulatory pattern--this one broadly similar to the earth's trade winds--shows up at the sun's surface. Overall, the sun rotates faster at the equator than at the poles (something that astronomers have known for centuries by following the march of sunspots across the disk of the sun). But the SOHO data show a more elaborate pattern. Six belts of plasma, each about 65,000 kilometers (40,000 miles) wide, stretch across the solar surface, each moving slightly faster than the surrounding areas. (For comparison, the entire sun is 1.39 million kilometers, or 864,000 miles, across.) The speed difference is small, only about 15 kilometers per hour (10 miles per hour), but it seems significant. Craig DeForest, also of Stanford, points out that sunspots tend to form at the boundaries of the belts. He speculates that the small velocity imbalances may produce significant magnetic effects in the sun's electrically conductive material.
Although astronomers had some inkling of the existence of such flows for more than a decade, it took SOHO's instruments to determine their true extent. The Stanford team has discovered that the solar trade winds reach at least 18,000 kilometers (12,000 miles) below the surface. Moreover, they do not remain in place. Over the course of an 11-year cycle, the plasma belts migrate from middle latitudes toward the solar equator. Under the very different conditions that prevail on the earth, trade winds remain fairly steadily in place, but the overall motions are broadly similar. This circulatory pattern is also reminiscent of winds that create the promiment belt patterns on Jupiter.
Yet another peculiarity emerged from the current round of SOHO investigations. The outer layer of the sun circulates as a whole: everything down to a depth of at least 25,000 kilometers (15,000 miles) steadily drifts from the equator to the poles. At its rate of about 80 kilometers per hour (50 miles per hour), that solar drift carries material at the equator to the poles in about one year. Oddly, the direction of the movement is opposite that of the sunspots and the trade-wind flows, which seem rooted to a different circulatory pattern. As a result, "sunspots act something like ships streaming against the wind," DeForest says. "When they break up, we can see debris that is caught in the polar flow and carried away."
These latest discoveries are unlikely to have immediate practical implications, but they do signal a rapid improvement in understanding of the once mysterious inner workings of our star. Such knowledge is already proving valuable in the emerging field of "space weather"--the ability to forecast interplanetary magnetic squalls and so protect satellites, communications equipment and power supplies from the vicissitudes of the sun.
And not a moment too soon: the next solar maximum occurs in 1999.