Re: What was the blue glow in the atmosphere after the ABM test 07-07-00?

Trying to explain somebody else's observations a month after the fact and 
300 miles away is sometimes a little difficult, but I'll give it a try...

I observed something fairly similar a couple of years ago when I saw a 
Titan IV launch from Cape Canaveral as I was driving to K-Mart, about 50 
miles from the launch site.  After thinking about it for a couple of 
minutes, I realized what I was observing...

As a rocket is launched, it produces exhaust which is guided by the motor 
nozzles so that it provides the maximum forward thrust.  The effluent from 
the nozzle is pushed on by the atmosphere so that, as the rocket moves 
forward, most of its velocity is in the aft direction -- that is, it 
spreads out laterally, but not nearly as much as it is blown backward from 
the rocket.

As the rocket goes to higher and higher altitude, the atmosphere becomes 
less and less dense so that the lateral pressure holding the exhaust 
stream vanishes and it is propelled (and diffuses) outward much more 
rapidly than in the lower atmosphere.  The net result of this is a vacuum 
exhaust or plume that broadens widely in the lateral directions and, 
because of diffusion, may actually move forward to appear to start to 
envelop the motor and rocket (although some of the apparent envelopment 
is, I think, an illusion created by the fact that we're viewing the 
spreading exhaust cloud from below).  The net result is that the actual 
exhaust plume in the vacuum of space gets really huge -- that's the part 
about you seeing something that was really, really big and I'm almost 100% 
sure that this is the correct explanation for the size of what you saw.

As far as the blue-green glow is concerned, it could be from a couple of 
things.  My best guess, given that it was just after sunset, is that the 
particles in the exhaust vapor (mainly water vapor, carbon dioxide, and 
residual fuel/oxidizer that may not quite have gotten burned) scattered 
some of sunlight that probably illuminated the vehicle and exhaust at the 
flight altitude -- but which didn't illuminate you because the sun had 
gone down at sea level in Santa Monica.  The when sunlight scatters off 
atmospheric gases and particulates, the blue spectral component is 
preferentially scattered -- that wavelength dependence is characteristic 
of the process, and is why the sky appears blue (rather than black) during 
the day.  Another possibility is that some of the residual oxidizer (which 
could have been oxygen, although I don't know for sure what launcher was 
used) could have either left some excited oxygen or some oxygen which was 
excited by the sunlight at that altitude (thereby exciting it) so that the 
so-called auroral oxygen spectral lines were generated when the molecules 
decayed back to the ground state.  Not surprisingly, given the name, this 
process is what produces some of the blue-green colors in the aurora 
borealis (or australis, if you're in the southern hemisphere).

Beyond that, I haven't a clue as to what could have produced the blue 
glow.  It must have been pretty cool looking, however, particularly 
because it sounds like it both surprised and amazed you.

Keep looking up! Ain't science amazing?  (And it's all free... except, of 
course for the gozillion bucks the government spent on the launch).

Steve Guch