What should be done to continue a long-distance contact when the MUF for that path decreases due to darkness?
Why The maximum usable frequency depends on ionization in the F region, which is produced by solar radiation. As the path goes into darkness the ionization decays, the critical frequency drops and the MUF for that path falls, so the band you were using may now pass through the ionosphere instead of refracting back. Moving down to a lower HF band puts your signal back below the MUF and keeps the path open, which is why 20 meters often closes at night while 40 and 80 meters open up.
Watch out Going to a higher band is exactly the wrong direction, since the frequency is already approaching the MUF and a higher one will punch through the layer. Raising the takeoff angle shortens the hop distance, which hurts rather than helps a long-distance path.
Sun goes down, frequency goes down. Less ionization means a lower MUF.