It Was the Proton Arc

From the photographer: “As one of the auroral particles, a proton has bigger mass than an electron, and therefore protons are less drifted by the magnetic field of the earth. As a result, proton arcs are usually observed at lower latitudes, hundreds of kilometers south of the aurora area. Aurora storms are predictable by observing solar flares on the sun, which happens from time to time, Proton arcs are much less commonly seen and hard to predict. On August 16th 2015 around 1am PDT, a strong proton beam hit the earth right after an aurora storm, forming a bright strip across the sky – it was the proton arc. It appeared in the southern part of sky. We were observing from Vancouver BC, a city in sub-auroral region.”