I was getting aurora alerts on my phone and with activity having been raging at Kp=10 all day and evening I couldn't resist driving North of the City and going for a look just before bedtime. I set up the modified Camera on a tripod and took a few 30 second, wide angle shots. One short sequence shot at around 2300UT on May 1st, showed a noticeable change when I flicked through it. There were three red vertical beams where there hadn't been any 90 seconds ago. The glow below these on the actual pictures had a greenish hint to it, showing it may have been green aurora but it was too masked by light pollution for me to be satisfied I was seeing aurora, so I had an idea of a rather more scientific technique of image subtraction. I manually blurred and shrunk the images in paint shop pro and did a subtraction. On enhancing the contrast, and getting rid of a few noise artefacts I got this weird picture. It is a difference picture, so the yellowish cloud is where the cloud was advancing, and the darker blue cloud is where the cloud was. So... I managed to defeat the cloud and light pollution to reveal proof of aurora! What's more I decided to do a rough distance calculation. I estimate the top of the red aurora is 400km high, which is seen at 20º altitude. This leads to a distance 1200km, or a guesstimate of around 800km, allowing for curvature. The green and the red emission of aurorae come from atomic oxygen, but the red is from a higher energy, long lived excited state. This state's energy gets quenched by collisions with air molecules below 100km or so altitude, due to the higher density of the atmosphere.
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