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.
You can see the Northern lights in Iceland as early as August, as soon as the night skies get dark enough and weather conditions are favourable. It helps if it's cool and crisp outside, and a small moon helps make them easier to spot.