Wednesday, 12 March 2014

Aurora borealis!

Yeah! I caught a glimpse of the Northern Lights for the first time this solar 'season' on Thurs 27 Mar 22UT. It was not too impressive aesthetically, but lovely to see the lights again. A colourless dim glow was hovering, detached from the horizon and for a time, a shaft of light appeared in the sky just beneath Cassiopeia. The advances in Camera technology (and mine is well out of date) meant an easy capture of the wonderful colours.
Here I've done a stack of 8 x 15s images pointing N, just N of Norwich. I stacked on the ground, not the stars, so they are trailed. The green, lower glow is from atomic Oxygen, i.e. O atoms, emitting during the decay from excited singlet S state to the singlet D state, an allowed transition. The red light, also from atomic O, is only seen at higher altitudes, where the air is much thinner, this would occur at lower altitude but the excited state causing it is knocked back down by collisions with other air molecules. This excited state is the singlet D mentioned above, the 'first' or lowest energy excited state, and it releases its red light with a time constant of about 107 seconds. This means it needs to avoid getting hit for about a minute, in order for it to have a change to spit out its red light. The reason for this slow time is because the transition from singlet D to the 'ground state' (triplet P) is forbidden, as the spin has to change from singlet to triplet, something not allowed by quantum mechanics.

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