The Classification of X-ray Solar Flares
or "Solar Flare Alphabet Soup"
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A solar flare is an explosion on the Sun that happens when energy stored in twisted magnetic fields (usually above sunspots) is suddenly released. Flares produce a burst of radiation across the electromagnetic spectrum, from radio waves to x-rays and gamma-rays. [more information]

The following image is a magnetogram of the sun from the SOHO (SOlar Heliospheric Observatory) data.  The white rings in the image represents a RHESSI solar flare with the each rings representing different energy levels.  The black dots in the picture are sunspots.  In brief, sunspots are basically a temporary concentration in the magnetic field on the sun, where convection of hot matter from the sun's interior is inhibited, resulting in a cooler, darker area on the photosphere of the sun.  Believe it or not, those tiny black dots in the picture are each roughly the size of Earth.  The average sunspot is about the same diameter as the Earth.  As previously mention, solar flares occurs around twisted magnetic fields (i.e. sunspots).

A magnetogram of the sun from the SOHO (SOlar Heliospheric Observatory) data from April 3, 2001.  The X and Y axes are measured in arcsecs, from -1000 to 1000.

Scientists classify solar flares according to their x-ray brightness in the wavelength range 1 to 8 Angstroms. There are 3 categories: X-class flares are big; they are major events that can trigger planet-wide radio blackouts and long-lasting radiation storms. M-class flares are medium-sized; they generally cause brief radio blackouts that affect Earth's polar regions. Minor radiation storms sometimes follow an M-class flare. Compared to X- and M-class events, C-class flares are small with few noticeable consequences here on Earth.

This figure shows a series of solar flares detected by NOAA satellites in July 2000:

A graph depicting a series of solar flares detected by NOAA satellites.  The X-axis reflects Universal time, from July 12th to July 15th 2000.  The Y-axis reflects watts in m-2.  The solar flares occur where the data spikes.

Each category for x-ray flares has nine subdivisions ranging from, e.g., C1 to C9, M1 to M9, and X1 to X9. In this figure, the three indicated flares registered (from left to right) X2, M5, and X6. The X6 flare triggered a radiation storm around Earth nicknamed the Bastille Day event.

Peak (W/m2)between 1 and 8 Angstroms
 I < 10-6
 10-6 < = I < 10-5
 10-5 < = I < 10-4
 I > = 10-4

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