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Sun's Got the BeatScientists discover a 16-month cycle in the Sun's differential rotation 225,000 km below the visible surface. |
April 3, 2000 -- Like blood pulsing in an artery, newly
discovered currents of gas beat deep inside the Sun, speeding
and slackening every 16 months.
Eruptions on the Sun are believed to result from the buildup and rapid release of stress in solar magnetic fields. Just as a twisted rubber band can break suddenly, the solar magnetic field under stress "breaks" to a new, lower-energy configuration, releasing tremendous energy. The frequency and intensity of these eruptions rises to a peak over an 11-year cycle, and scientists believe the cycle is also tied to magnetic activity.
The flows of gas under study occur about 225,000 km (140,000 miles) beneath the visible surface, or almost a third of the way down to the center of the Sun. Here is the supposed dynamo region (tachocline), where the turbulent outer region, the convective zone, meets the orderly interior, or radiative zone. The speed of the gas in this "dynamo" region changes abruptly. Near the equator the outer gas travels around the Sun's axis of rotation faster than the inner gas. The difference in speed between the two layers gradually diminishes as latitude increases, until at the polar regions, the situation is reversed, with the inner gas rotating faster than the outer gas. The news from SOHO and GONG is that the contrast in speed between layers above and below the supposed dynamo region can change by 20 percent in six months. When the lower gas speeds up, the upper gas slows down, and vice versa. In observations spanning four and a half years, from May 1995 to November 1999, these alternations in speed occurred three times. They indicate a heartbeat of the Sun at one pulse per 15 to 16 months in equatorial regions, and perhaps faster at higher latitudes. Scientists are able to probe the solar interior by analyzing ripples on the Sun's surface produced by sound waves reverberating through the Sun. Analysis of solar sound waves is the science of helioseismology, and it opened the Sun's gaseous interior to investigation in much the same way as seismologists learned to explore the Earth's rocky interior with earthquake waves.
Above: At depths down to 37,000 miles, bands of gas parallel to the equator move slightly faster or slower than the average speed for their solar latitudes. This image shows the variation of rotation rate with latitude and time from which a temporal average has been subtracted. Banded zonal flows migrate toward the equator on 2-3 year time scales. Red/yellow denotes faster rotation; green/blue means slower rotation. (Image courtesy NSF's National Solar Observatory) [more information] SOHO is a project of international cooperation between the European Space Agency and NASA. GONG is an international project led by the U.S. National Science Foundation, with the participation of twenty nations. |
Web Links |
Solar Cycle Update -March 22, 2000 Science@NASA headline. SOHO home page -real-time images, screen savers, and more SpaceWeather.com -follow the latest events on the Sun |
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