Impulsive and Gradual Phases of a Flare

Impulsive and Gradual Phases of a Flare

. The time profile of a solar flare at hard X-ray energies, above 30 keV, is characterized by an impulsive feature that lasts for about one minute. This impulsive phase coincides with the acceleration of high-speed electrons that emit non-thermal bremsstrahlung at hard X-ray wavelengths and non-thermal synchrotron radiation at centimeter radio wavelengths. The less-energetic emission shown here, below 30 keV, can be composed of two components, an impulsive component followed by a gradual one. The latter component builds up slowly and becomes most intense during the gradual decay phase of solar flares when thermal radiation dominates. At even lower soft X-ray energies (about 10 keV), the gradual phase dominates the flare emission. This data was taken on 15 November 1991 with the Hard X-ray Telescope (HXT) aboard Yohkoh. (Courtesy of NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)

Copyright 2010, Professor Kenneth R. Lang, Tufts University