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.
It is also not unheard of to see the lights as early as mid-August, once the final traces of the midnight sun summer are gone. Second most important factor is the length of time you choose to stay in Iceland. To get the best odds of seeing the lights, it is recommended you stay a minimum of seven nights in the country.