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.
Wintertime is the perfect time to go see them, if you're interested in booking a Northern Lights tour ! For a schedule on how active they are each night, you can check the Icelandic Aurora Light Forecast, updated daily, or The Aurora Service Forecast, with an hourly forecast.