Friends in Radio Land-
In a recent posting that was rather long and rambling, Thomas Giella,
KN4LF, said some things that are just "flat out wrong", like "These high
energy protons emitted in a solar flare create the aurora we see."
The aurora is created by magnetospheric ELECTRONS which have been
accelerated locally to energies of keV, not MeV that are characteristic of
protons accelerated on the sun in flare events. The electrons are incident
on the atmosphere and their position is along the auroral oval, the trace
of the last closed field lines on the earth's atmosphere. (It should be
noted that solar protons can cause a weak polar GLOW, typically emissions
from nitrogen and oxygen molecules much deeper in the atmosphere and
spread across the entire polar caps, but polar glow aurora are rarely
seen.)
Solar proton events differ markedly in their fluxes and energy spectra.
NOAA distinguishes between proton energies and gives fluxes within a given
category. There are events which are only detected at satellite
altitudes, typically below 1 MeV; others, fluxes above 10 Mev reach the
ionosphere and cause polar cap absorption (PCA) events while events above
100 Mev may reach balloon altitudes and those above 1 Gev may reach the
ground and are terme "GLE" for ground level events. The record energy to
date was found in the solar proton event of February 23, 1956 when solar
protons were detected at the magnetic equator in Peru, with energies above
10 GeV.
Solar protons arrive at locations on the earth and its ionosphere
according to their "magnetic rigidity", i.e. momentum per unit
charge, and their trajectories being sorted out by the earth's magnetic
field. The basic theory was worked out by the Norwegian, Stoermer, back
in the early days of the century. His theory used a dipole field but now
we have to use a model which includes the effects of the solar wind which
produce the magnetotail behind the earth.
Blasts of plasma accompany flare events, both being part of the processes
of coronal mass ejections (CME). The clouds of plasma take a day or two
to reach the earth, give rise to magnetospheric electric fields which, in
turn, accelerate magnetospheric electrons to produce aurora as well as
electrojet current systems at ionospheric altitudes, noted on magnetograms
and described digitally by K- and A-indices. The geomagntic field can be
further distorted in these processes, allowing solar protons access to
lower latitudes and aurora to those regions too.
Solar proton fluxes, per se, have nothing to do with geomagnetic storms,
they simply follow openings, as it were, in the field lines. The K- and
A-indices give a measure of the energy in the solar wind that is incident
on the outer reaches of the magnetosphere. Auroral ionization is the
agent that absorbs 160 meter signal during magnetic storms; ionization
from solar protons "devours" those signals. To see the difference, note
that auroral ionization can give rise to a few dB absorption on 30 MHz
riometers along the auroral zone while solar proton events can give rise
to 20 dB absorption over entire polar caps. Devour is putting it mildly.
I spent my entire adult life going to the Arctic and Antarctic, doing
research on these topics and know whereof I speak. If you want to learn
more about the subjects, see my book:
"The Little Pistol's Guide to HF Propagation
now on a CD offered by Worldradio Books.
73,
Bob Brown, NM7M
Professor of Physics, Emeritus
University of California
Berkeley Campus
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