Northern Lights are usually green, and sometimes red. Those are the colors produced by oxygen when it is excited by electrons raining down from space. On April 22nd, Didier Lindsey of Fox, Alaska, witnessed an apparition of aurora-blue:

    

Hours before he took the picture, a CME struck Earth’s magnetic field, igniting a G2-class geomagnetic storm. “The sky filled with very active auroras,” Lindsey says,”including these rare blues.”

In auroras, blue is a sign of nitrogen. Energetic particles striking ionized molecular nitrogen (N2+) at very high altitudes produces a cold azure glow of the type captured in Lindsey’s photo. Why it rivaled the usual hues of oxygen on April 22nd is unknown. Auroras still have the capacity to surprise.

Any auroras tonight, blue or otherwise, would be a surprise. Geomagnetic conditions are quiet as the solar wind relaxes into a steady breeze of low-speed plasma. NOAA forecasters estimate a scant 10% chance of polar geomagnetic storms on April 29th.