Magnetic "Topography" Helps Predict Solar Flares

TUCSON (Dec. 14, 2011) -- Mapping hills and valleys of solar magnetic fields, as seen by a network of telescopes operated by the National Solar Observatory (NSO), is showing the way to improved space weather forecasting.

Using observations by NSO's Global Oscillation Network Group (GONG), a team in Sydney, Australia found a combination of indicators that predicts when a magnetically energized region is most likely to flare.

The new tool, called Flarecast, was developed by Graham A. Steward and Phil J. Wilkinson of the IPS Radio and Space Services and Vasili V. Lobzin, Iver H. Cairns, and Peter A. Robinson of the University of Sydney. They reported their results in the November 2011 issue of Space Weather, published by the American Geophysical Union.

"This is an excellent use of the GONG data archives," said Dr. Frank Hill, director of NSO's synoptic observing program in Tucson, AZ. "The Flarecast team meticulously worked their way through a lot of possibilities to come up with something that is very promising."

GONG is a network of six identical stations positioned around the world to watch the Sun 24 hours a day. NSO operates GONG in concert with international partners, including a station at Learmonth, Australia, and makes the data available worldwide.

As its name suggests, GONG watches up-down motion of the solar atmosphere to reveal activities deep inside the Sun. It also measures the locations and strength of large-scale magnetic fields, the underlying cause of the sunspot cycle and space weather. Solar flares are the result of a magnetic reconnection, sort of a short circuit.

The link between flares and magnetic regions has been known since the 1940s and now is obvious. But reliable forecasts have been elusive even as the increasing fragility of modern technologies has made them more important. So if the obvious doesn't work, poke around in the details.


A GONG magnetogram of the Sun from April 23, 2003. No. 3 of the four active regions is highlighted here by Flarecast. The top right image shows the magnetic gradient contours for north (red) and south (blue) polarity. The yellow lines in the second image show the SPILs (strong-gradient polarity inversion lines), which would trace between the red and blue contours. This area was AR 10338, which produced an M5.1 flare and then three smaller C-class flares that day. The orange dot represents the size of Earth. (From Steward et al) Click here for larger version.

The Flarecast team decided to look at strong-gradient polarity inversion lines, or SPILs, and neutral lines in maps of solar magnetic fields. SPILs are more likely to produce flares. But that is no guarantee. "Such sheets can exist for a very long time without major changes," the team wrote, "then a minor change may trigger a flare." Or the region may quietly fade away.

"The trigger for solar flares is still elusive" and will require understanding a range of phenomena from below the visible surface or photosphere to the corona, the overlying, superhot atmosphere.

The Global Oscillation Network Group (GONG) is a network of six identical stations positioned around the world so that at least two and usually three can always see and monitor solar activity. Units are stationed at Big Bear Lake, CA; Mauna Loa, HI; Learmonth, Australia (shown); Udaipur, India; El Teide, Canary Islands, Spain; and Cerro Tololo, Chile.

At the heart of each is an interferometer based on a design that A.A. Michaelson invented in 1887 in early relativity experiments. The name means "to measure by interference," which the device does by making light waves cancel or reinforce each other to form light and dark patterns. As used in GONG, it lets the unit measure subtle changes in spectral lines as internal sounds make the surface of the Sun oscillate in (red shift) or out (blue shift). This field is known as helioseismology ("shaking Sun") and focuses on 5-minute oscillations that reveal activities inside the Sun.

A polarizing filter can be inserted to measure the strength of a magnetic field along the line of sight. From these measurements GONG produces magnetographic maps in black & white & gray. Black indicates south magnetic polarity going into the Sun, and white is north polarity rising from it. Gray is where the magnetic activity is too fine for the telescope to see.

But even without knowing the trigger, it may be possible to find indicators that something is about to erupt. So the Sydney team looked through GONG magnetograms from 2003, an active year, at regions that did and did not erupt. Every ten minutes, GONG produces a magnetogram, a map of magnetic fields across the face of the Sun. The maps depict south magnetic polarity as black and north as white. Gray areas in between are magnetic, too, but so fine that the north and south signals wash together.

But solar magnetism is more than black and white. Each pixel records the field strength and polarity in a region about a seventh the diameter of Earth. The change in strength from one pixel to the next gives the gradient of the magnetic field, much like a topographic map tells how steep a mountain slope is. Further, the line between north and south represents a polarity inversion asscated wth a neutral sheet where reconnection might occur.

The team used five measurements including steepness of the magnetic field gradients, the length of the neutral line (up to six times Earth's diameter!), and the curvature. Then they tested 44 combinations of those five to see which ones gave the most accurate predictions and an acceptable low rate of false alarms.

The combination that worked best correctly predicted 168 out of 190 flares, an accuracy of 88 percent. Even then, several of the 22 false alarms did occur, but outside the 24-hour warning window that the team set for itself. In addition, Flarecast predicted that 130 regions would not flare. Of those, 31 did erupt, but often as subflares that are of little consequence to space weather.

Overall, the Sydney team rated Flarecast as 83 percent correct and with 10 percent false positives (that is, areas predicted to flare that did not). IPS Radio and Space Services to now is testing Flarecast against the current solar cycle to see if it holds up. They started out with a bang. Flarecast made its first real-time prediction of a major X-class flare on Feb. 15, 2011, using magnetographic data from the GONG station at Learmonth.

"This can be a powerful new tool for space weather forecasters," Hill added. "We're proud that GONG made such a contribution."

This is one of several examples of NSO data being used in space weather forecasting. The NOAA Space Weather Prediction Center in Boulder, CO, has a new, sophisticated forecast model, WSA-Enlil, that substantially improves predictions of space weather impacts on Earth. They use the GONG hourly synoptic maps to forecast solar winds. They back it up with maps from NSO's Synoptic Optical Long-term Investigations of the Sun facility.

  • Steward, G. A., V. V. Lobzin, P. J. Wilkinson, I. H. Cairns, and P. A. Robinson. 2011. Automatic recognition of complex magnetic regions on the Sun in GONG magnetogram images and prediction of flares: Techniques for the flare warning program Flarecast, Space Weather, 9, S11004, doi:10.1029/2011SW000703.
  • Space Weather: The International Journal of Research and Applications is an online publication devoted to the emerging field of space weather and its impact on technical systems, including telecommunications, electric power, and satellite navigation. Space Weather is published by the American Geophysical Union and is co-sponsored by the International Space Environment Service (ISES).