The pulse of the Sun's dynamo
03 Apr 2000Currents of gas deep inside the Sun pulsate like the blood in human arteries, speeding and slackening every 16 months. Solar scientists are astonished by this discovery. It comes from an international team pooling observations from the MDI instrument on the ESA-NASA SOHO spacecraft and from a worldwide chain of ground stations called GONG. Rachel Howe and her colleagues announce their results in the 31 March issue of the journal Science.
An urgent need to understand the Sun's magnetic behaviour, which produces space storms affecting the Earth, gives practical as well as theoretical importance to the discovery. To explain the sunspot cycle, in which the count of dark sunspots and solar storminess peak at intervals of about 11 years, theorists visualize a dynamo inside the Sun. Relative motions between neighbouring 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, and it breaks out to the solar surface, causing sunspots and magnetic explosions. The changes now observed are at the right depth for a dynamo.
"We are excited to see the first evidence of changes close to the location of the solar dynamo," says the lead author, Rachel Howe of the National Solar Observatory in Tucson, Arizona. "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."
The flows of gas under study occur about 220,000 km beneath the visible surface, or almost a third of the way down to the centre 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 changes abruptly. Near the equator the outer gas travels around the Sun's axis of rotation faster than the inner gas. At mid-latitudes and near the poles, the outer gas rotates more slowly.
The news from SOHO's MDI and from GONG is that the contrast in speed between layers above and below the supposed dynamo region can change by 20 per cent in six months. When the lower gas speeds up, the upper gas slows down, and vice versa. In observations spanning 4.5 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 15-16 months in equatorial regions, and perhaps faster at higher latitudes.
Revelation of what goes on beneath the bright surface comes from helioseismology, which analyses motions of the surface due to sound waves reverberating through the Sun's interior. MDI, the Michelson Doppler Interferometer on the SOHO spacecraft, measures the motions at a million points across the visible surface. Similar instruments on the ground, at cloud-free sites in California, Hawaii, Australia, India, Tenerife and Chile, make up the US-led Global Oscillation Network Group, or GONG, and between them they observe the Sun for 24 hours a day. The same pulses are seen in the space-based and the ground-based observations, interpreted by two different analytical methods, which removes any doubt about the reality of the phenomenon.
A European pioneer of helioseismic theory, Jxrgen Christensen-Dalsgaard of Aarhus University, Denmark, participates in both the space and the ground projects. He is one of the co-authors of the report in Science.
"For the very first time we have a handle on important variations deep inside the Sun that are probably linked to the sunspot cycle," Christensen-Dalsgaard comments. "Until now, our attempts to explain the complicated magnetic changes seen at the surface were pure speculation. One day, we may even be able to use observations of the interior to predict the long-term changes in the Sun's outward behaviour that affect the Earth."
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 60,000 km, 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 towards the equator, as the years go by. A similar equator-ward shift has long been observed in the locations of sunspots, as the sunspot cycle approaches its maximum of activity.
The solar heartbeat is the second landmark discovery within a month, from SOHO's MDI instrument. On 9 March came the news that it had used sound waves to see right through the Sun and detect sunspots on the far side. A Californian team led by Philip Scherrer of Stanford University provided MDI, which is the largest of three helioseismic instruments on SOHO. The latest results follow a long series of achievements with MDI, including the detection of sub-surface jet streams and the charting of the bubbling action under the ordinary visible surface and under sunspots.
SOHO is a project of international cooperation between ESA and NASA. The spacecraft was built in Europe for ESA and equipped with instruments by teams of scientists in Europe and the USA. NASA launched SOHO in December 1995, and in 1998 ESA and NASA decided to prolong its highly successful operations until 2003. As a result, the SOHO helioseismologists will be able to see whether the solar heartbeat changes during the decline in activity after the sunspot maximum occurring this year.