Solar Flares (Radio Blackouts)
Solar flares are large eruptions of electromagnetic radiation from the Sun lasting from minutes to hours. The sudden outburst of electromagnetic energy travels at the speed of light, therefore any effect upon the sunlit side of Earth’s exposed outer atmosphere occurs at the same time the event is observed. The increased level of X-ray and extreme ultraviolet (EUV) radiation results in ionization in the lower layers of the ionosphere on the sunlit side of Earth. Under normal conditions, high frequency (HF) radio waves are able to support communication over long distances by refraction via the upper layers of the ionosphere. When a strong enough solar flare occurs, ionization is produced in the lower, more dense layers of the ionosphere (the D-layer), and radio waves that interact with electrons in layers lose energy due to the more frequent collisions that occur in the higher density environment of the D-layer. This can cause HF radio signals to become degraded or completely absorbed. This results in a radio blackout – the absence of HF communication, primarily impacting the 3 to 30 MHz band. The D-RAP (D-Region Absorption Prediction) product correlates flare intensity to D-layer absorption strength and spread.
http://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts
