Sudden ionospheric disturbance

When a solar flare occurs on the Sun a blast of ultraviolet and x-ray radiation hits the dayside of the Earth after 8 minutes. This high energy radiation is absorbed by atmospheric particles raising them to excited states and knocking electrons free in the process of photoionization. The low altitude ionospheric layers (D region and E region[?]) immediately increase in density over the entire dayside.

Earth's ionosphere reacts to the intense x-ray and ultraviolet radiation released during a solar flare. produces Shortwave Fadeout on the dayside of the Earth as the result of enhanced X-rays from a solar flare

The Effects on Radio Waves Short wave radio waves (in the HF range) are absorbed by the increased particles in the low altitude ionosphere causing a complete black out of radio communications. This is called a Short Wave Fadeout[?]. These fadeouts last for a few minutes to a few hours and are most severe in the equatorial regions where the Sun is most directly overhead. The ionospheric disturbance enhances long wave (VLF) radio propagation. SIDs are observed and recorded by monitoring the signal strength of a distant VLF transmitter.

• You can view data and learn how to build an antenna for monitoring SID's at http://www.aavso.org/committees/solar/sid.stm