Sun's Got the Beat

Scientists 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.

The solar "heartbeat" throbs in the same region of the Sun suspected of driving the 11-year cycle of solar eruptions, during which the Sun goes from stormy to quiet and back again. Scientists are hopeful that this pulse can help them unravel the origin and operation of the solar cycle.

Right: The solar "heartbeat." The data points show variations in the Sun's differential rotation rate at a location 72% of the way from the core of the Sun to its surface. (Image courtesy NSF's National Solar Observatory) [more information]

The discovery comes from an international team pooling observations from the Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric Observatory (SOHO) spacecraft and from a worldwide chain of ground stations called the Global Oscillation Network Group (GONG). Dr. Rachel Howe of the National Science Foundation's National Solar Observatory in Tucson, AZ, and her colleagues announced their results in the March 31 issue of the journal Science.

"We are excited to see the first evidence of changes close to the location of the solar dynamo, the region that generates the Sun's large-scale magnetic field and is believed to drive the solar cycle," says lead author Howe. "It's very surprising to find that the changes have such a short period -- 16 months or so rather than the 11 years of the solar cycle."

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.

To explain the solar cycle, theorists visualize a dynamo inside the Sun, where movement of electrically charged gas generates a magnetic field. Because magnetic fields are produced by moving electric charges, relative motions between neighboring layers of electrified gas supposedly drive the dynamo. As the years pass, so the theory goes, the magnetic field becomes too strong for the gas to hold. As a result, the magnetic field breaks out to the solar surface, creating active regions with sunspots and magnetic explosions. The changes now observed are at the right depth for a dynamo.

Left: Click on the cutaway image for a 3 MB mpg animation that shows how the solar rotation rate varies in time below our star's surface. Light brown tones indicate fast rotation, blue tones slow rotation, and white intermediate rotation. (Image courtesy NSF's National Solar Observatory)

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.

The Science report also raises the question of whether there may be a link between the deep changes and another remarkable phenomenon seen by helioseismologists nearer the surface. 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. Although the effect is subtle, it is very persistent, and the scientists see the bands of fast and slow gas gradually moving from high latitudes toward the equator as the years go by. A similar "equator-ward" shift has long been observed in the locations of sunspots, as the solar cycle approaches its maximum of activity.

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.

Related 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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