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.
The northern lights usually tend to be very active for two to three nights, then low for four to five nights, in ongoing cycles. Naturally, not everyone can take long trips here and Iceland is a renowned stopover destination, but if the northern lights are on your bucket list we highly recommend you make sure you can take a good long trip here.