What are radio blackouts?

Radio blackouts are caused by bursts of X-ray and Extreme Ultra Violet radiation which are emitted during solar flares and affect the sunlit side of the Earth. Radio blackouts primarily affect High Frequency (HF) (3-30 MHz) communication, although fading and diminished reception may spill over to Very High Frequency (VHF) (30-300 MHz) and higher frequencies. These effects occur on the sunlit side of the Earth and are most intense at locations where the Sun is directly overhead. These blackouts are a consequence of enhanced electron densities caused by solar flare emissions. These emissions ionize the sunlit side of Earth, which increases the amount of energy lost as radio waves pass through the upper atmosphere. Radio blackouts are the most common space weather events to affect Earth. Minor events occur about 2000 times each solar cycle. Radio blackouts are by far the fastest space weather event to impact our planet. The electromagnetic emission produced during flares travels at the speed of light taking just over 8 minutes to travel from the Sun to Earth. Radio blackouts can last from several minutes to several hours depending on the duration of the solar flare. How severe a radio blackout is depends on the strength of the solar flare.

Highest Affected Frequency (HAF) during a solar flare

The Highest Affected Frequency (HAF) during an X-ray radio blackout during local noon is based on the current X-ray flux value between the 1-8 Ångström. The Highest Affected Frequency (HAF) can be derived by a formula. Below you will find a table where you can see what the Highest Affected Frequency (HAF) is during a specific X-ray flux.

GOES X-ray class & flux Highest Affected Frequency
M1.0 (10-5) 15 MHz
M5.0 (5×10-5) 20 MHz
X1.0 (10-4) 25 MHz
X5.0 (5×10-4) 30 MHz


NOAA uses a five-level system called the R-scale, to indicate the severity of a X-ray related radio blackout. This scale ranges from R1 for a minor radio blackout event to R5 for a extreme radio blackout event, with R1 being the lowest level and R5 being the highest level. Every R-level has a certain X-ray brightness associated with it. This ranges from R1 for a X-ray flux of M1 to R5 for a X-ray flux of X20. On Twitter we provide alerts as soon as a certain radio blackout threshold has been reached. Because each blackout level represents a certain GOES X-ray brightness, you can associate these alerts directly with a solar flare that is occurring at that moment. We can define the following radio blackout classes:

R-scale Description GOES X-ray threshold by class & flux Average frequency
R1 Minor M1 (10-5) 2000 per cycle (950 days per cycle)
R2 Moderate M5 (5×10-5) 350 per cycle (300 days per cycle)
R3 Strong X1 (10-4) 175 per cycle (140 days per cycle)
R4 Severe X10 (10-3) 8 per cycle (8 days per cycle)
R5 Extreme X20 (2×10-3) Less than 1 per cycle

The image below shows the effects of an X1 (R3-strong) solar flare on the sunlit side of the Earth. We can see that the Highest Affected Frequency (HAF) is about 25 MHz there where the Sun is directly overhead. Radio frequencies lower than the HAF suffer an even greater loss. Image: NOAA SWPC - D Region Absorption Product.

X-ray radio blackout

Polar Cap Absorption Events

Radio blackouts also occur at Arctic latitudes during space radiation storms. These are known as Polar Cap Absorption Events and can last for days. These events are indicated by the S-scale which is used for space radiation storms. Polar Cap Absorption Events are not to be confused with radio blackouts that are caused by bursts of X-ray and Extreme Ultra Violet radiation which are emitted during solar flares. Polar Cap Absorption Events are caused by protons that enter Earth's atmosphere above the polar regions during space radiation storms.

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