Solar flares are energetic explosions in the low solar atmosphere which can heat the surrounding material to millions
of degrees in just a few seconds or minutes. Solar flares typically occur near sunspots where magnetic field is
concentrated in the active regions on the photosphere. Flares emit radiation in several bands of the electromagnetic
spectrum (white light, untraviolet, x-rays, gamma rays) and are observed by ground based and space based telescopes.
In addition to emitting large amounts of radiation, solar flares also accelerate particles which are ejected into space.
The left image below shows a magnetically continued loop of material heated by a solar flare off the right limb of the
Sun observed by EIT on the SOHO spacecraft. The image on the right shows a hot flare loop imaged by the TRACE satellite.
Click on each image to view a larger one.
X-Ray Flare Classification
Solar flares are classified based on their x-ray intensity. A series of letters and numbers is used to
classify the energy level of an x-ray flare. The letters used are A, B, C, M and X, with A being the
weakest and X being the strongest. The full classification of an x-ray flare is composed of a letter
followed by a number, C3.2 for example. The number indicates the specific intensity of the flare.
X-ray flare intensity is measured in units of power per area, or Watts per meters squared. Each letter
(A, B, C, M or X) represents a certain numeric value and the number which follows the letter in the
flare classification multiplies that value. The numeric values of the letter classes are listed below:
A = 1.0x10-8 (W m-2)
B = 1.0x10-7 (W m-2)
C = 1.0x10-6 (W m-2)
M = 1.0x10-5 (W m-2)
X = 1.0x10-4 (W m-2)
To determine the intensity indicated by a flare's classification multiply the number in the x-ray classification
of that flare by the value of its class listed above. For example, a C5.9 flare would have an intensity of
5.9x10-6 W m-2.
Flare x-rays can occur as short impulses, gone in minutes, or brightenings that decay over hours to days. The
impulsive events usually produce a stream of high energy particles leaving the flare site. The slower events are
often accompanied by a CME.