Why can auroras peak a while after a solar eruption?

Answer: Particles need travel time

Particles need travel time ✓ — Correct! Solar material does not appear at Earth instantly. After a CME or high-speed solar wind leaves the Sun, it must travel across tens of millions of kilometers before reaching Earth. Depending on the event, that can take many hours or even a few days. That delay is one reason aurora forecasts are often issued in advance and then updated as the solar material approaches.

Earth must rotate into place — Wrong. Earth's rotation matters for who is on the night side when auroras happen, but it is not the main reason for the delay after a solar eruption. The biggest delay comes from the travel time of solar particles moving through space.

Sunlight must charge the air — Wrong. The atmosphere does not need to “store up” sunlight before auroras can occur. Auroras happen when energetic particles collide with gases high above Earth, not because the air was pre-charged by daylight.