A sudden burst of Solar activity and charged particles travel in all directions from the sun. Twenty-Four to Thirty-six hours later some of those particles enter the Earth's atmosphere and may react with the Earth's magnetic field and produce a Visible or Radio Aurora, visible if the event takes place after dark.
The Visible Aurora is flora at the E-Layer. This Aurora can be a curtain of ions capable of refracting radio waves at frequency ranges above 20Mhz. While at the same time the D-Layer is absorbing the lower HF radio waves. This is the time when amazing things can happen at the VHF range.
Aurora propagation returns a signal that is some what diffused and modulated (rapid fluttering) so CW is the preferred mode but there is SSB activity usually at the 50Mhz the SSB signal maybe difficult to understand at higher frequency but a Aurora contact on 144mhz can be a great thrill while the HF bands are virtually dead.
Favored times are late afternoon and early evening. Late evening through early morning, and early afternoon in that order. Major Aurora often start in early afternoon and carry through to early morning. Note: both the transmitting and receiving antenna must be directed towards the polar region.