I've seen patches and bands of purple before. But over many years of aurora photography, I've only seen this once: a broad purple aurora. The purple wavelength is emitted by lighter gasses high up in the ionosphere, such as helium and hydrogen. The image was captured during a period of intense geomagnetic activity -- during a time when the Kp index hovered between about 5 and 8 for most of a week, resulting in what seem to have been the strongest and most sustained auroras of the last decade. This image is a panorama, made by stitching together six vertical frames captured in very quick succession with a 35mm f 1.4 lens (the extremely sharp, low coma, reasonably priced Sigma Art).