Topband: PCA Events

[Robert Brown]

Friends in Radio Land -

In recent postings, Tom, KN4LF, spoke of "proton absorption" and how it
would affect 160 meter propagation adversely.  In his last posting, he
called attention to a proton event that was almost at an end. I went to
the website and looked at the data, a magnified version of what I saw
earlier on Space Weather.

The data was from the 10 MeV detector on the spacecraft and the flux at
the present time is 2 pfu (proton flux units or protons/sq cm/sec/ster.)
Now the energy of the detector's threshold is one thing, the flux is
another.  When combined with a range-energy curve for air, the energy
determines the depth of penetration of protons into the atmosphere (in
gm/sq.cm.)

But for propagation purposes, the flux in pfu is extremely important.  Let
me give you some examples from my year-long study of long-path
propagation from April '91 to March '92.

There were 14 PCA events in that study, with fluxes ranging from 10 pfu to
3,000 pfu.  An event with 52 pfu gave rise to 2.2 dB absorption on a 30
MHz riometer at Thule, Greenland while the event with 3,000 pfu gave rise
to 17 dB absorption.  See my original long-path book for details.

The PCA event in the first week of November was a real "bomb" with a peak
flux of 31,000 pfu.  I have no data on the absorption but am confident it
was in excess of 20 dB, the dynamical range of typical riometers. 

But the current proton flux is 2 pfu and any absorption would be a small
fraction of a dB, hardly at threat to DXing.

The point of all this is the threshold energy (in MeV) of a satellite
detector is not enough to give in discussing propagation - the particle
flux is the physical variable that determines the rate of ionization.
Beyond that there is a day/night effect for PCA events, with 4-5 times
less absorption in full darkness,  So "160 MeV" is only a small part of
the story as the magnitudes of the fluxes must be considered too.

73,

Bob, NM7M