S wave -- a shadow zone appears when the velocity suddenly decreases 103-180 degrees from the epicenter. Describe the differences between P and S waves. P waves expand and contract, can move through liquid solid and gas. S wave moves up and down or side to side moves through only solids.
Optimal conditions to see the northern lights in Iceland is when skies are clear and dark (away from the city lights, in cold temperature and ideally from mid-September to mid-April.
Any of those months is as likely as the next one to see some aurora activity, it just depends on your luck with the weather. (See also the largest selection of Northern lights tours in Iceland, or check out these Northern lights holidays and trips). Actually the Northern lights are active all year long but we can only see them in darkness, when the skies are clear.