An interesting article.
> http://www.qsl.net/w2vtm/grayline.html
>
> For those of you new to a solar cycle, an interesting form of working DX
> is called "working gray line." This simply means working 15m or 10m
> during twilight hours. [Take notes, this is on the exam!]
>
> Here's what happens:
>
> During the day, solar radiation collides with the molecules in our
> ionosphere, ripping off electrons. These electrons are called "free
> electrons" because they are not attached to an atom or molecule. All of
> these free electrons cause the density of the ionosphere to increase. The
> more dense the ionosphere, the higher the frequency that is reflected back
> to earth. Our electron density is what determines the maximum usable
> frequency (MUF), and the action of solar radiation separating electrons
> from the molecules is called ionization.
>
> During the day, solar radiation causes ionization to stratify, that is, to
> form distinct layers. The layer closest to the earth is called the
> D-Layer. It does not reflect signals generally, but does absorb some of
> the energy, and hence the D-Layer is often called the "absorption layer."
> Higher up in our ionosphere, we find the E- and F-Layers. These layers do
> reflect the signals back to earth if they are below the MUF, and is
> exactly what causes "skip propagation." So during the day, the sun is
> ionizing the D, E and F layers (there are actually two F layers, called F1
> and F2). Your 10m signal must travel through the D-Layer, getting
> attenuated, then bounces back from the E or F layer to some exotic DX
> spot, passing through the D-Layer for more absorption again. But since
> solar radiation has to travel the farthest to get the D-Layer, absorption
> is usually fairly minimal. So far, during the middle of the day, we have
> moderate absorption, and good skip propagation.
>
> AT SUNDOWN ... solar radiation no longer strikes our ionosphere right
> above our heads, and ionization stops. This means there is no solar
> radiation to form free electrons. In fact, without this solar radiation,
> these free electrons tend to get attracted back to recombine with their
> host molecules. This is called "recombination" (gee, how original!).
> Recombination, when it starts to get dark, causes the electron density to
> go down, forcing the MUF to go down as well, which is why by total
> darkness, 10m (and a bit later 15m) are completely dead. The MUF is far
> below 28 MHz.
>
> The D-Layer is the first layer where ionization stops, since the sunlight
> no longer reaches near the surface of the earth, but is still illuminating
> (and ionizing) the ionosphere far above our heads. (For the same reason,
> we can see satellites pass overhead in the early evening ... it's dark on
> the ground, but the satellites are still being illuminated.) As the
> D-Layer goes into recombination, the electron density goes down, and the
> absorption does down. This is why signals appear stronger at night,
> because there is less absorption by the D-Layer at night.
>
> BUT DURING TWILIGHT ... OR IN THE "GRAY LINE" ... the D-Layer suddenly
> causes little absorption to signals passing through it, while the E and F
> layers are still being ionized by sunlight. This makes for about 45-60
> minutes of interesting operating, especially for QRPers (low power
> operators). There is almost no signal attenuation, but the MUF is still
> very high, so long-distance skip is still possible. However, when the sun
> quits illuminating the E and F layers, the MUF can drop dramatically ...
> sometimes with only a few minutes of warning, sometimes between
> heartbeats. So when you establish a contact, get the QSL info fast!
>
> One other advantage of gray-line DX is that your signals tend to reflect
> off the edge of the ionized portion of the upper layers. This means
> propagation will often be in a southerly direction, bouncing along the
> shadow, or terminator, between sunlight and darkness. This is good for
> working into South America and the South Pacific. Your signals can also
> bounce northward along the terminator, bending around the pole, and down
> the morning terminator across eastern Europe, the Middle East, and into
> Africa (depending on the time of year). So gray-line DX also affords an
> opportunity to work portions of the world not usually accessible during
> the day, where propagation tends to be more east-west circuits.
>
> The same principles apply at sunrise. The upper ionosphere begins to
> become ionized, while the D-Layer is still dark and offers low absorption,
> although, the MUF in the morning generally does not support propagation on
> 10m, so most people enjoy gray-line work on 20m or 15m (if open). Morning
> gray line can even be eventful on 80m and 40m, due to the low absorption
> before the sun starts heating the D-Layer.
>
> And remember, 10m and 15m (and often down to 30m) are not generally
> bothered by a geomagnetic storm. So even during major geomagnetic storms,
> the higher bands may be open and fairly quiet. And even if a bit noisy,
> the short period of gray-line operating can still produce a couple of good
> QSO's.
>
> Hope this helps to explain the "gray line" phenomenon, and hope it helps
> you snag a few new ones.
>
> From an e-mail posting by Paul Harden, NA5N, to the "Low Power Amateur
> Radio Discussion E-mail List"
> 30 December 1998 (12:01:49 AM)
>
> By Paul Harden, National Radio Astronomy Observatory, Socorro, New Mexico
> Reprinted without explicit permission on the assumption that Paul is OK
> with the idea or he wouldn't have posted it to the Internet, and there's
> no copyright notice.
>
>
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