Hi Peter, as far as I understand it, the LUF is a function of the absorption
in the D-layer and the noise level at the receive site. It is all
complicated by the fact that the level of absorption is frequency dependent
even in quiet periods. The prediction programs regard the LUF as a function
of the ionising radiation reaching the D (and possibly E ) layer, so it is
highest at local noon, and will change with the solar cycle. This is all
complicated at LF and VLF because the absorption increase as the frquency
increases. There is a parameter called the gyro frequency (a rotational
frequency dependent on the energy of the ionospheric electrons and the
earths magnetic field) which may determine the level of the LUF (My reading
in this area is a little sparce and I only just starting to try to
understand this factor). In periods of intense ionisation, we get
'reflection' rather than absorption, at LF and VLF, at the D-layer boundary
and we see CFH, for instance, a few hours either side of 1200z.
I believe the 2MHz lowering for a 10dB increase in power is just a figure
that comes from the received noise densities at various frequencies. It
effectivey says "some of your power is going to heat up the ionosphere so
you will need more to reach your target range". I think a parameter is
measured by the sounders from which LUF can be inferred.
Geomagnetic disturbances upset this of course. We know how the LF bands
(really should be called MF bands now 160-80-40m) often 'go out' at the time
of an aurora.
There is a little data in the "New Shortwave Propagation Handbook" published
by CQ Communications (available from the RSGB bookstall ....plug over) But
like all amateur publications there seems to be little said about
frequencies below 2MHz.
Cheers de Alan G3NYK