Northern Lights

This image of the Northern Lights over Yellowknive, Canada, sent to me by Stan who of course spent many years admiring the phenomenon from Alaska. A sad truth is that during my early courtship of Stan's daughter, Sonnet and I failed to visit her home-state when it would have been convenient, ie, from San Francisco (London it's a 20+ hour indirect voyage + Stan and Silver now live on the Western Slope). As for the mechanism: Auroras are produced by the collision of charged particles, mostly electrons but also protons and heavier particles, from the magnetosphere, with atoms and molecules of the Earth's upper atmosphere (at altitudes above 80 km). Most originate from the sun and arrive at the vicinity of earth in the relatively low-energy solar wind. When the trapped magnetic field of the solar wind is favourably oriented (principally southwards) it reconnects with that of the earth and solar particles then enter the magnetosphere and are swept to the magnetotail. Further magnetic reconnection accelerates the particles towards earth.

The collisions in the atmosphere electronically excite atoms and molecules in the upper atmosphere. The excitation energy can be lost by light emission or collisions. Most aurorae are green and red emission from atomic oxygen. Molecular nitrogen and nitrogen ions produce some low level red and very high blue/violet aurorae. Unfortunately, I'm not able to credit the image which is part of an unmarked series.