What is a SID?
SID is an abbreviation for Sudden Ionospheric Disturbance.
After a solar flare on the Sun, it takes about 8 minutes for the ultraviolet (UV), x-ray and (sometimes) gamma ray radiation to hit the side of the Earth facing the Sun. The radiation is absorbed by the atmosphere, knocking electrons free. The D and E layers increase in density, which has the effect of causing complete or partial blackouts of short wave radio communications. Interestingly, the same effect that causes short wave fading enhances
longer wavelength propagation (like VLF). So, you can 'see' a SID by monitoring distant VLF transmitters.
For example, here is NAA (24kHz), with a small SID around 15:15 UTC (circled in red):
The graphs below allow monitoring for SIDs by looking for unexpected slope changes in the graphs. When it is observed in more than one graph, it reduces the likelyhood that it is an artifact limited to a specific VLF transmitter.
For more information:
Sudden Ionospheric Disturbance
The D Region
The E Region