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.
Your best chance to see the northern lights is during the cold, clear winter nights between September and April. The prime time to see them in all their glory in usually after midnight. Be spontaneous, brave the cold, and embrace a late night adventure to see the stunning natural phenomena Alaska is known for.