Solar flares are large outbursts of electromagnetic radiation from the Sun lasting from minutes to hours. They are caused by magnetic reconnection associated with large-scale eruptions of magnetic flux called “coronal mass ejections” (CMEs). Solar flares occur in a large range of strengths and are classified on a logarithmic scale based on their intensity in the 1-minute averaged NOAA/GOES XRS instrument’s 0.1 -- 0.8 nm spectral band, with the smallest flares being labeled “A” flares, the next (10 times) larger called “B” flares, the next larger “C” flares, followed by the fairly large “M” flares, and finally the largest “X” flares.
The history of this lettering scheme is believed to be that C flares are fairly “Common” during solar sunspot activity, M flares are “Medium” in size, and X flares are “Extreme” in size. A and B categories were added below C flares to recognize that smaller flares occur very frequently during any given solar magnetic cycle.
X-rays and extreme ultraviolet light from solar flares ionize the Earth’s atmosphere, causing an enhancement of the lower part of the dayside (Sun-facing) ionosphere which blocks radio signals that normally are reflected off of the ionosphere. Reflection of radio waves off the ionosphere allows long distance radio communication without having clear line-of-sight between the transmitter and receiver. When the enhanced ionosphere absorbs the radio waves, no radio communication is possible. This creates conditions referred to as “Radio Blackouts”.
Practically speaking, a Radio Blackout is the absence of a capability to communicate on High Frequency bands in the 5 to 35 MegaHertz spectral range, but lower frequency radio communications may also be significantly degraded during a Radio Blackout event.
Radio Blackouts are classified by a five-level NOAA scale based on the solar flare X-ray scale. The table below provides the link between Radio Blackouts, Solar Flares, Solar radiant energy flux in Watts per square meter, and designated Severity of the event.
Radio Blackout ....X-Ray Flare..... Flux (W/m2)......Severity
R-1........................ M1.............. 0.00001............. Minor
R-2........................ M5.............. 0.00005.............. Moderate
R-3........................ X1.............. 0.0001 .............. Strong
R-4........................ X10 ........... 0.001 ................ Severe
R-5........................ X20............ 0.002 ................ Extreme
For more information on Radio Blackouts, see the NOAA Space Weather Scales
Image courtesy of NASA