SID
(Sudden Ionospheric Disturbance)

This system allows you to detect solar flares, during daytime hours, without a Ha-telescope and the associated clear weather precondition. The working principle is to monitor the behavior of the ionosphere. The D-layer existence is a direct function of solar X-rays (~ 0,1 nanometer). If there is an excess flux, e.g. from a solar flare, the ionisation  increases accordingly. With a suitable radiosignal from some distant (no groundwave present) transmitter, one can identify solar flare events by measuring and following the signal level reflected by the D-layer. When it comes to choosing a transmitter, the field is wide open for experiments.

A "long wire" aerial picks up the signal outdoors. My "black box" contains several OP-Amps tuned for the frequency proper and a rectifier. The signal is plotted with a pen-recorder. I detect 60 kHz from Rugby, UK.   Nowadays one uses a A/D-converter hooked up to a computer.