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.
In the most intense Northern Lights area (notably Alaska, Iceland, Northern Scandinavia and Yukon), the lights are observed from late August to mid April. However, from late September to late March, it is dark after 6pm, and one enjoys maximum chances.