Satgen202  A New HF Propagation Mode Pt2  by GM4IHJ           6th Feb 93
BID of this msg is SGEN202  Always use that BID if you retransmit the msg

Satgen200 reported NM7M's question " Why am I hearing RS12 29.408 MHz when
it is the other side of a sunless north pole", and it gave a summary of
answers available at that time. Unfortunately a leaky BBS somewhere near
the GM/G border lost sgen200. So to get the message to VK ZL that we
wanted them to listen for South Polar propagation in their winter June to
Sept 93. Satgen200 extra was issued on 4th Feb to bridge the gap. As
reported there NM7M discovered that his professional colleagues had
information which suggested that conditions in the Interplanetary Magnetic
Field could at times cause injection of dense plasma into the F layer
above the polar oval. What was not known, was, when where or how ?
Nature UK science magazine has now partially answered these questions (
Vol 361 No.6411 4Feb93 p407 and 424 ). A review article describes how
merging at the Magnetopause can result in intense precipitation of ions
and electrons into the F layer of the Polar oval.  This effect should be
distinguished from the quite different Solar Wind activity which causes
Polar Aurora. At times F layer precipitation is much more frequent than
auroral activity, and the F layer ionisation provides forward oblique
incidence refraction propagation, quite different from the auroral
reflection and backscatter mode.
In the Nature paper which accompanies the review article the results of a
radar experiment ( EISCAT UHF RADAR Tromso Norway ) are concurrently
compared with a DMSP F10 satellite experiment which is looking down to
read electron density and ion temperature. The paper tables comparitive
results as the satellite overflew an F layer area south of Svalbard whilst
it was illuminated by the EISCAT radar. Thereby revealing electron density
of more than 1 million electrons per cm^2, whilst the radar revealed the
northward movement of the selfsame plasma.
So we have firm evidence for why we get these high maximum usable
frequencies over the pole in mid winter darkness. Further monitoring of
both South and North polar satellite passes will reveal just how often we
can expect these polar propagation events, but what good does that do. The
cynic will ask "Who wants to talk to the Polar Bears or Penguins. Few of
them are licensed ?".
That question misses the point. For years IHJ has been asked at DX 
lectures about otherwise weird results. Eg the well known GM YL DXer who
found she could join an Antipodeal YL net by beaming from Scotland over
the North Pole ie miles away from the short or long path great circle
bearings. These DX contacts are much easier to explain now we know that
conditions above the pole can refract 10m, 15m and 20m signals when the
propagation forecast say "impossible". Once we get across the pole
ordinary propagation modes can take our signal further and the F layer
polar path avoids a great deal of absorption which otherwise bars the
higher HF signals. Yes there is also sometimes a non great circle aspect
to this type of propagation because the ionosphere is non linear in almost
all planes not just the vertical. Which may help explain one well known DX
feat when G3IOR accessed satellite RS12 whilst it was over the Equatorial
Pacific by beaming just west of North , and then used the satellite for
the first ever UK / New Zealand satellite QSO.
Northern European HF Users should therefore be aware that about 4 hours
either side of local noon , useful paths may sometimes exist to their
north, where none have been predicted before.  73 de GM4IHJ @ GB7SAN