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.
Have you seen them? The northern lights are spectacular and Iceland is the ideal place to search for this natural phenomenon. Sightings are most likely in the evening hours during the months of September to March. NASA predicts that this winter the northern lights will be their most intense in over a decade. To read more check out this article from The Telegraph.