

What are solar flares?

A solar flare is basically a giant explosion on the surface of our Sun which occurs when magnetic field lines from sunspots tangle and erupt. A solar flare is defined as a sudden, rapid, and intense variation in brightness. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Material is heated to many millions of degrees in just minutes and radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to X-rays and gamma rays at the short wavelength end. The amount of energy released is equivalent to millions of nuclear bombs exploding all at the same time! Solar flares are an often occurrence when the Sun is active in the years around solar maximum. Many solar flares can occur on just one day during this period! Around solar minimum, solar flares might occur less than once per week. Large flares are less frequent than smaller ones. Some (mostly stronger) solar flares can launch huge clouds of solar plasma into space which we call a coronal mass ejection. When a coronal mass ejection arrives at Earth, it can cause a geomagnetic storm and intense auroral displays.

A solar flare as captured by NASA's Solar Dynamics Observatory


The classification of solar flares


Class W/m2 between 1 & 8 Ångströms
A <10-7
B ≥10-7 <10-6
C ≥10-6 <10-5
M ≥10-5 <10-4
X ≥10-4


A & B-class solar flares


C-class solar flares


M-class solar flares


X-class solar flares







High Frequency (HF) radio blackouts caused by solar flares

Bursts of X-ray and Extreme Ultra Violet radiation which are emitted during solar flares and can cause problems with High Frequency (HF) radio transmissions on the sunlit side of the Earth and are most intense at locations where the Sun is directly overhead. It is mostly High Frequency (HF) (3-30 MHz) radio communication that is affected during such events, although fading and diminished reception may spill over to Very High Frequency (VHF) (30-300 MHz) and higher frequencies.

These blackouts are a result of enhanced electron densities in the lower ionosphere (D-layer) during a solar flare which causes a large increase in the amount of energy radio waves lose when it passes trough this layer. This process prevents the radio waves from reaching the much higher E, F1 and F2 layers where these radio signals normally refract and bounce back to Earth.

Radio blackouts caused by solar flares are the most common space weather events to affect Earth and also the fastest to affect us. Minor events occur about 2000 times each solar cycle. The electromagnetic emission produced during flares travels at the speed of light taking just over 8 minutes to travel from the Sun to Earth. These type of 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.

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 an 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.

X-ray radio blackout

Image: NOAA SWPC - D Region Absorption Product.

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