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.
Here is a post on exactly this subject http://www.northernlightsiceland.com/best-place-to-see-northern-lights/ In general you can see the lights anywhere in Iceland, however there are couple of factors that are important to look at.nfortunately the Aurora Borealis isn't that predictable, having spent 5 weeks in Iceland and about 4 years in total in Norway, Sweden and Finland, the best time to see this phenomenen is obviously ONLY at night.