Sun Releases a Powerful Burst of Radiation – An X1-Class Solar Flare

NASA’s Solar Dynamics Observatory captured this image of a solar flare — as seen in the bright flash at the Sun’s lower center — on October 28, 2021. The image shows a subset of extreme ultraviolet light that highlights the extremely hot material in flares and which is colorized here in teal. Credit: NASA/SDO

The Sun emitted a significant solar flare peaking at 11:35 a.m. EDT on October 28, 2021. NASA’s Solar Dynamics Observatory, which watches the Sun constantly, captured an image of the event.

Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth’s atmosphere to physically affect humans on the ground, however — when intense enough — they can disturb the atmosphere in the layer where GPS and communications signals travel.

This flare is classified as an X1-class flare.

X-class denotes the most intense flares, while the number provides more information about its strength. An X2 is twice as intense as an X1, an X3 is three times as intense, etc. Flares that are classified X10 or stronger are considered unusually intense.

The classification system for solar flares uses the letters A, B, C, M or X, according to the peak flux in watts per square meter (W/m2) of X-rays with wavelengths 100 to 800 picometres (1 to 8 ångströms), as measured by the GOES spacecraft at the Sun-Earth distance from the Sun of 2.7×1017 km.

ClassificationApproximate peak flux range at 100–800 picometer
(watts/square meter)
A< 10−7
B10−7 – 10−6
C10−6 – 10−5
M10−5 – 10−4
X> 10−4

The strength of an event within a class is noted by a numerical suffix ranging from 1 up to, but excluding, 10, which is also the factor for that event within the class. Hence, an X2 flare is twice the strength of an X1 flare, an X3 flare is three times as powerful as an X1, and only 50% more powerful than an X2. An X2 is four times more powerful than an M5 flare. X-class flares with a peak flux that exceeds 10−3 W/m2 may be noted with a numerical suffix equal to or greater than 10.

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