Sun, 15 Mar 2015 - Notable meteor spectrum captured by Bill Ward
Bill Ward captured this particularly notable meteor spectrum at 05:08 GMT on 2015 March 15th :
Bill notes that although the meteor was not particularly bright or spectacular, it does give an interesting insight into the mechanics involved.
The spectrum shows bright emission at 557.7nm from OI emission (neutral oxygen). This is sometimes seen in fast bright meteors in the brief train right behind the meteor for a couple of seconds.
This particular emission line is only seen for fast meteors that start to ablate well above 100km altitude. It is a meta-stable state and it takes the oxygen atom 0.74 seconds to emit the photon. If the atom bumps into another atom before this time, the energy is robbed due to collisions so the emission doesn’t occur. Hence the emission only occurs at high altitudes where, with the atmosphere being very thin, the mean free path is long enough such that any given excited atom won't bump into another before emitting its photon.
Bill adds that this was the brightest OI emission he’d seen in a while and so the meteor must have been travelling very fast - maybe in the high 60's km/sec or 70 km/sec (i.e. a velocity similar to that of the Leonids).
Added by: Tracie Heywood