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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