Dolores Beasley
Headquarters, Washington, DC
March 30, 2000
(Phone: 202/358-1753)
Susan Hendrix
Goddard Space Flight Center, Greenbelt, MD (Phone: 301/286-7745) RELEASE: 00-50
SUN'S HEARTBEAT MAY HELP UNRAVEL MYSTERY OF SOLAR CYCLE 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. 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 announce 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
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-2- 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. The flows of gas under study occur about 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. 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. More information and images are available on the Internet at: http://pao.gsfc.nasa.gov/gsfc/spacesci/sunearth/sunearth.htm#pulse -end-
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