On Mon, 7 Dec 2015 01:17:34 -0000, wrote:

The term is "skywave" and it is not known to happen below 500 kHz.

I beg to differ. The current issue of QEX has an article on a WWVB 60
KHz frequency standard:
http://www.arrl.org/files/file/QEX_Next_Issue/2015/Nov-Dec_2015/Magliacane.pdf
On Pg 15 is the section on 60 KHz propagation:
LF radio propagation is substantially different from that
which exists at higher frequencies. Its remarkable stability
and reliability have often led to the belief that 60 kHz
signals propagate great distances over ground wave paths alone.
In reality, a combination of surface wave and D-layer
ionospheric paths are responsible for WWVB signal propagation.
At night, cosmic background radiation supports a level of
D-layer ionization that is sufficient for propagating LF (and
lower frequency) radio signals over long distances. Greater
D-layer efficiencies and increased effective height with
decreased ionization levels contribute to greater signal
coverage during the nighttime hours. etc...

So, here we have propagation via the ionospheric D-Layer which I
believe is considered a skywave. Note that the author talks about
measuring broadcast band frequencies to an accuracy 312 micro-hertz,
where such things as varying path lengths are important.

The wavelength of 60 KHz is about 5 km. The height of the D layer
varies from 60 to 90 km or about 12 to 18 wavelengths at 60 KHz.
That's too big for a waveguide structure, which suggests that the
dominant mode of propagation is skywave, not ground wave.

There is no frequency at which ground wave ceases and skywave takes
over. There seems to be quite a bit of overlap.

Mo
https://en.wikipedia.org/wiki/Very_low_frequency#Propagation_characteristics

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558