Three space-weather words get used interchangeably and describe three different things. The sequence, not the synonym, explains what actually reaches the ground.

Kp is a three-hour magnetism index that aurora chasers borrowed. It tells you whether tonight is worth staying up for, and almost nothing about whether you will actually see the northern lights.

A solar flare mostly scrambles radio and GPS. The real grid threat is the coronal mass ejection behind it. Here is how a geomagnetic storm damages transformers.

NOAA’s Space Weather Prediction Center has issued a G2 geomagnetic storm watch for July 3 after the sun’s biggest flare of the week triggered radio blackouts across North America.