Mick Stutler W8BE gave an excellent program recently that walked us through the relationship between solar activity and HF propagation.
He covered everything from how the Sun’s EUV radiation creates the ionosphere to the key indices every ham should know: SFI, SSN, Kp, and Ap. The presentation also breaks down the ionospheric D, E, and F regions, explains how solar flares and coronal mass ejections can cause radio blackouts, and covers NOAA’s Space Weather Scales (R, S, and G).
Whether you’re chasing DX or just trying to figure out why 20 meters went dead, this is a solid primer on the science behind the bands.