In 1619 Galileo declared the northern lights were sunlight reflecting off high vapor clouds. He gave the phenomenon its name and got the physics completely wrong. This episode reveals auroras as visible evidence of an invisible battle between the solar wind, a stream of magnetized plasma traveling around 400 kilometers per second, and Earth's magnetosphere. It explains magnetic reconnection, why a southward-pointing interplanetary magnetic field lets the solar wind grab our field lines, how the stretched magnetotail snaps back like a slingshot, and how the magnetic loss cone funnels particles into the auroral ovals around the poles.

The conversation then decodes the colors: why high-altitude oxygen takes up to two full minutes to emit red light and only survives in near-vacuum, why collisional quenching leaves fast green emission to dominat ...