What Determines the Intensity of Solar Flares?
Tuesday, September 11, 2007
Science Update - A look at CfA discoveries from recent journals

The solar corona is the extended, gaseous, outer atmosphere of the sun. The corona is very hot, over a million kelvin, and motions of this hot gas produce twisted magnetic fields that extend upwards through the sun's atmosphere. When these loop-shaped fields suddenly snap free, like stretched rubber bands, they can power coronal eruptions and flares, and hurl charged particles towards the earth with destructive consequences. Scientists have long been trying to better understand and be able to predict these solar flares.

Writing in a paper in last week's Astrophysical Journal, a team of five SAO astronomers led by a student, Yingna Su, describe a different approach to the problem, and reach some important conclusions. The team analyzed satellite data on thirty-one solar flares in wavelengths across the spectrum including X-ray, optical, and ultraviolet images. They studied eighteen of these events in great detail, analyzing the intensities of the flares and comparing them with six key parameters that describe the magnetic field, including the field strength and measures of its twist.

The scientists note that there is a lot of energy stored by the twisted field, just as twisting a rubber band can store energy in the same way that stretching it can. By statistically comparing all of the various parameters against the flare strength, they find clear evidence that the intensity of a flare depends not on the total amount of field energy, as had been often supposed, but specifically on the amount of energy that is released from the twisted fields. The results are significant not only because they help to identify what determines the intensity of a solar flare, they also imply that predicting the magnitude of a flare is more difficult that had been imagined: even if the field energy can be measured, the amount of twisted field energy that will be released is much more difficult to estimate.