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.
To see the Northern Lights you need dark skies and from early-April until late-August, the Aurora may be blazing across the Arctic firmament but it is visible only to scientific equipment, as the skies are just too light for the human eye to see the show.