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This link is for a SID - VLF receiver project and a great article explaining Sudden Ionospheric Disturbances. It is from the “Backyard Astronomy” site by Joel Gonzalez.

Sudden Ionospheric Disturbances are indirectly monitored via receivers which measure the signal strength of VLF transmitters. Solar flares, for example, can cause great disturbances in the Ionosphere, which affects the amplitude or signal strength of transmissions as they travel through the atmosphere.

 

The following videos from NASA show solar activity which can greatly affect VLF transmissions in the Ionosphere:

This link is an example of a VLF transmitter station. The signal strength can be recorded by a VLF receiver tuned to 24 KHZ. This is an indirect method of knowing when Sudden Ionospheric Disturbances occur. This station is the most powerful VLF station in the world, with a power of 2 million watts (2 megawatts). The following web site tells the history of the transmitter and other details about its construction.

Courtesy of Nick England, K4NYW, US Navy Radio Communications web site