Research with the Solar and Heliospheric Observatory (SOHO) spacecraft has revealed the process that may implement the reversal in the direction of the Sun's magnetic field that is known to occur every 11 years. This newly recognized factor in the Sun's magnetic flipping is the cumulative effect of more than a thousand huge eruptions called Coronal Mass Ejections (CMEs).

The CMEs blast billions of tons of electrified gas into space, carrying away the Sun's old magnetic field and allowing a new one with a flipped orientation to form.

This sequence from the Large Angle and Spectrometric Coronagraph (LASCO) instrument on SOHO shows a magnetic structure called a streamer emanating from the Sun. It is a twisted magnetic loop rising from the south pole of the Sun that has trapped hot, electrified gas within, which is glowing brightly. Four days later, a coronal mass ejection erupts from the same region (white and red area near the bottom of second image). After the CME is gone, the streamer has disappeared as well (bottom image). Credit: NASA and the European Space Agency

Reversal of the solar magnetic field is a major event in the Sun's 11-year cycle of stormy activity, when the Sun goes from quiet to active and back again, and the study is the first evidence linking the reversal to CMEs. Since CMEs occasionally disrupt satellites, radio communication, and power systems, solar scientists hope this link will eventually help them better forecast the powerful eruptions.

"The Sun is like a snake that sheds its skin," comments Dr. Nat Gopalswamy of NASA's Goddard Space Flight Center, Greenbelt, Md., lead author of the new report, which appears in the Astrophysical Journal. "In this case, it's a magnetic skin. The process is long, drawn-out and it's pretty violent. More than a thousand coronal mass ejections, each carrying billions of tons of gas from the polar regions, are needed to clear the old magnetism away. But when it's all over the Sun's magnetic stripes are running in the opposite direction."

"This analysis of nearly eight years of CME data is a big step forward in making sense of space weather," said Dr. Joseph Gurman, NASA Project Scientist for SOHO at NASA's Goddard Space Flight Center, Greenbelt, Md. "By identifying the solar origin of these events with CMEs of different speeds and appearances, and at different latitudes, it improves our capability to predict space weather that can affect the Earth, at different phases of the solar activity cycle."

Apparently random CMEs turn out to be signs of the Sun's diligent housekeeping. It keeps sweeping away, out into space, untidy magnetic fields created by sunspots and other contortions in its atmosphere. The climax comes in a busy period of "spring cleaning" after the count of sunspots has peaked, every 11 years. It leaves the Sun with its main magnetic field completely overturned, and its north and south magnetic poles swapped around.

The Astrophysical Journal report published by Gopalswamy and his colleagues takes stock of seven years of observations of such events by the SOHO spacecraft. It also compares them with the mass ejections recorded in 1979-85 by a US Air Force satellite, P78-1. Helping the scientists to decipher the events seen by the spaceborne telescopes are data from ground-based instruments at Kitt Peak, Ariz., and Nobeyama, Japan.

What emerges is a systematic pattern in the outbursts, according to the new research. It changes during the sunspot cycle, as the numbers of dark sunspots seen each day on the Sun's bright surface first increases and then diminishes again. The mass ejections are often directly associated with the sunspots, which always lie in the Sun's equatorial belt or at mid-latitudes.

Other mass ejections occur near the Sun's poles, far away from any sunspots. These events are most frequent at the peak of sunspot activity, but they can continue for a while as the count of sunspots begins to decline. By getting rid of the magnetic remnants of previous activity, the high-latitude outbursts groom the polar magnetic fields in a new configuration, according to the team.

When SOHO began its watch early in 1996, the Sun was quiet. There were very few sunspots and CMEs happened less than once a day. But during the most intense solar activity, 1999-2000, there were more than five a day, on average twice as many as scientists expected. What's more, the average speed of the ejected clouds of gas doubled, from 990,000 to 1,980,000 kilometres per hour (about 610,000 to 1,200,000 miles/hr.).

When the sunspot count passed its peak in July 2000, CMEs continued at a high rate. They did not reach their own peak in frequency until October 2002. Events followed a different timescale in the two polar regions of the Sun. A flurry of high-latitude mass ejections occurred near the solar north pole, completing the magnetic reversal there by November 2000. The south polar region lagged behind, and its new magnetic pole was not "clean" until May 2002. In effect, the solar snake shed the magnetic skin first from its head and then from its tail.

The scientific team includes Gopalswamy, Dr. Alejandro Lara and Dr. Seiji Yashiro of the Catholic University of America, Washington, DC, and Dr. Russell A. Howard of the Naval Research Laboratory, Washington, DC. SOHO is a project of international cooperation between ESA and NASA.