In my 28 December 2001 posting, I said:
> Those pmap pictures referenced by W4ZV, et al, only
> tell us where visible aurora is likely to occur (low energy
> precipitating electrons).
In W4ZV's 29 December 2001 posting, Bill challenged that statement with:
> That's not what NOAA says about the plots, Carl
and then he quoted the text at the www.sec.noaa.gov/pmap/UsePlots.html
Based on the actual data measured by the satellite, my statement is
correct. The detector onboard the satellite measures precipitating
electrons with energies between 50eV to 20KeV. Electrons in this energy
range get down to about 200km and 95km, respectively, and are the
electrons that cause visible aurora. This was the intent of the
satellite - to measure the low energy electrons that make light. The
electrons that get to the lower E region and the D region where
nighttime and daytime absorption occurs, respectively, are not counted.
But as the text that Bill quoted says, the oval can give us a
"best-guess" estimate of where radio propagation paths may be degraded
because of increased absorption. That would be where the number of dots
perpendicular to the satellite track is very high (indicating that there
may be even higher energy electrons precipitating down to the heights
we're interested in) and where the solid bars are longest (indicating
possible significantly increased ionization). Where this statistically
occurs is generally at the equatorward and poleward edges of the oval.
The entire oval is not full of ionization that degrades our RF.
Thus the oval doesn't really tell us about the electrons that can cause
us problems, because it is based on measurements that exclude these
electrons. But the oval does give us a good idea of where to look.