For a few rare nights, the sun's restless energy has pushed the aurora borealis southward into the northern United States, offering millions of Americans a glimpse of a phenomenon most only dream of witnessing. Charged particles from an unusually active sun are slipping through Earth's magnetic defenses and igniting the upper atmosphere in curtains of green and purple light. It is a reminder that the cosmos occasionally leans down close enough to touch, and that the boundary between the ordinary and the extraordinary is thinner than we suppose.