The ionosphere is stratified, or in other words, has layers. It's the E-layer which,
in the course of the day, charged by sunshine, actually absorbs medium wave
frequencies...it's only when the wavelengths are shorter (shortwave) where the
E-layer reflects for daytime skywave propagation. The E-layer, when particularly
dense like during solar maxima, will even reflect VHF frequencies (normally it passes
them day and night). When that happens you'll hear old radio people call it
"sporadic-E", since it's so, well, sporadic in nature - there's really no predicting
when it will happen. But when it does, you'll sometimes find FM stations from 1000
and more miles away coming in on a car radio, so strong sometimes that it'll blow
over a local station which has only direct line-of-sight propagation to get to the
same receiver.

At night, without the sun to ionize the gases there, the E-layer "evaporates", and
then the F-layer comes into play. The F-layer reflects medium wave frequencies
pretty well, except when they're disturbed by a solar flare or other phenomena. The
F-layer actually has two parts (F-1 and F-2, naturally enough). A shortwave station
which will skip 1000 miles during the day will skip twice that or more at night,
since the F-2-layer is higher and is more in play during the night.
--
For direct replies, take out the contents between the hyphens. -Really!-


"Steve Sundberg" wrote in message ...
On 14 Jan 2004 22:18:49 GMT, (Matt Beckwith)
wrote:

(Sid Schweiger) wrote

Radio waves radiate more or less in all directions from an AM tower, meaning
that some of them radiate skyward.

The ionosphere, in the presence of solar radiation (i.e., during the day),
absorbs the radio waves. In the absence of solar radiation (i.e., during the
night), it becomes a reflector, bouncing the signals back to earth hundreds or
thousands of miles beyond the transmitting tower. This can create interference
with stations on the same or adjacent frequencies at night, where no such
interference would occur during the day. To avoid such interference, many
stations must either cut transmitter power at night, or employ directional
antennas...or sometimes both.

Thanks, that's very interesting. Now, why is it that the ionosphere
absorbs radio waves in the presence of solar radiation, but not in its
absence?

That's just the nature of, uh, nature.