During the G2-class geomagnetic storm, auroras were sighted across Canada and Alaska, in multiple northern-tier US states, and 35,000 feet above the Southern Ocean. STEVE made an appearance, too.
What caused the outburst? It's simple: Auroras love equinoxes. Around the beginning of spring and fall, cracks open in Earth's magnetic field--a phenomenon called 'the Russell-McPherron effect'. Solar wind pours in to fuel geomagnetic storms. An unusually large crack opened on March 20th, supercharging the display.
The same phenomenon could multiply the effect of the CME expected on March 23rd, turning a glancing blow into a nice show.