This is one of the best posts I’ve read about how the Sun interacts with our atmosphere to affect radio propagation. The whole blog is worth a read, but this part offered some nice utility and an easy guideline to remember:

(Solar flux) of less than 100 is fairly poor propagation, the MUF  will be lower than 15MHz.  An SF of more than 150 is fairly good propagation, the MUF will be greater than 25MHz. A general rule of the thumb is 10M is open when the solar flux is greater than 150.

via W2LJs Blog – QRP – Do More With Less.: Sunspots