Seeing the northern lights, or aurora borealis, typically requires travelling to frosty climes as far north as Norway or Iceland, but this year they have been visible in London, southern Europe and even Mexico. There were two big light shows, in May and October. While the October event was a result of a straightforward solar storm, scientists are puzzled about the storm that caused the May aurora.

“We’re still quite surprised how far south the aurora was seen,” says …