Ground Waves and Sky Waves in Sudden Ionospheric Disturbances (SIDs)

From John at SARA Feb 2025:

Hello,

        The signals that we receive consist of a ground wave and a sky wave, the sky wave being reflected / refracted by the ionosphere. The ground wave is fixed based on the distance between transmitter and receiver, but our receivers see the interference pattern between the sky and ground waves. A solar flare increases the ionisation, lowering the effective reflection region of the ionsophere, altering its path length according to the flare strength. The interference pattern can therefore move through adding or cancelling according to the difference in path lengths sky / ground. The result is either a rising or falling signal / SID.

    I hope that helps,

John.