29 Sep
2012
29 Sep
'12
10:13 p.m.
On Sat, 29 Sep 2012, Tony Duell wrote:
I'm basing this on the fact that the aurora glows red at high altitude and
green at lower altitude. According to what I've read, the green glow
comes from oxygen. Wikipedia (I know, not the most reliable source) says:
>> Oxygen
at a suitable pressure? IIUC at very low pressure it glows red,
>> at higher pressure it glows green and at suffieciently high pressure the
Are you sure that oxygen can be got to give a gree nglow? I asked my
father about this, who has done a _lot_ of work on vacuum systems, and he
claimed it doesn;t. The (foten green) glow you see at very low pressures
is in fac the glass fluorescing. Oxygen is unusual in terms of its return to ground
state: it can take
three quarters of a second to emit green light and up to two minutes to
emit red. Collisions with other atoms or molecules will absorb the
excitation energy and prevent emission. Because the very top of the
atmosphere has a higher percentage of oxygen and is sparsely distributed
such collisions are rare enough to allow time for oxygen to emit red.
Collisions become more frequent progressing down into the atmosphere, so
that red emissions do not have time to happen, and eventually even green
light emissions are prevented.
Now that I think about it, it might be a bit tricky to build a nixie tube
where the mean time to collision is on the order of a second :-\
Alexey