“The HAARP project directs a 3.6 MW signal, in the 2.8–10 MHz region of the HF (high-frequency) band, into the ionosphere. The signal may be pulsed or continuous. Effects of the transmission and any recovery period can be examined using associated instrumentation, including VHF and UHF radars, HF receivers, and optical cameras.”

– https://en.wikipedia.org/wiki/High_Frequency_Active_Auroral_Research_Program

Cameras. They blast the ionosphere enough that you can see the difference. They must have some pretty serious filters for that.