The Twilight Zone
Steve Nichols G0KYA
E-mail: [email protected]
Originally written in May 2001
Intro: Steve Nichols G0KYA, of the RSGB’s Propagation
Studies Committee, believes that propagation around sunrise and sunset is not
fully understood. Here he outlines the mechanisms behind grey line and other
twilight propagation modes and a research project to help us understand them.
Worldwide
communication using the HF bands is dependent on radiation coming from the sun.
In general, and to grossly oversimplify reality, at LF
(160, 80 and 40m) we need a night-time path between
the two stations. At 28MHz, a daylight path is
generally needed. But twice a day, at sunrise and sunset, the ionosphere undergoes
dramatic changes, giving enhanced propagation in some directions.
In
terms of radio propagation, the D and E layers are responsible for most of the
absorption of radio waves that pass through them, but the absorption is
frequency dependent. The D layer can completely absorb signals on 160, 80 and
40 metres during the day, and can attenuate signals on 20m
too. Hence the reason you don’t hear much, if any, DX on the low bands during
the day as sky-wave signals are absorbed before they can reach the E and F
layers.
The
ionosphere undergoes a dramatic change in ionisation at the transition from day
to night. The electron (and ion) density in the E-layer decreases by a factor
of 200 to 1 and the F1 by nearly 100 to 1 (see
graph). At sunset, the D layer disappears rapidly.
Around
the other side of the world other regions that are entering into daylight have
yet to form any significant D layer and the E layer has not built up from its
night-time low. Therefore, for a short period propagation between two regions
simultaneously experiencing sunrise and sunset can be highly efficient. Signals
on the lower bands can theoretically travel over great distances with little
attenuation.
This
is well documented with many examples of grey line propagation being logged on 160
and 80m over the years.
Many
amateurs will be familiar with this so-called grey line propagation (the term
was coined in 1975 - see Ref 1) – propagation that occurs along a line
separating night from day. The line is called the terminator but it is diffuse,
due largely to the earth's atmosphere that scatters the light over a large
area. In radio terms, the radio terminator is not the same as the visual one.
The latter refers to the point when we see the sunrise or sunset at ground
level on the earth and the period of visual twilight that either precedes or
follows. The former refers to the way the sun illuminates the ionospheric D, E and F layers.
For
example, the PC program Geoclock defines the point at
which the sun starts/stops illuminating the D-layer as being offset from the
visual sunrise/sunset by 6.596 degrees longitude. As the earth rotates 15
degrees per hour this could be as much as 24 minutes before or after sunrise or
sunset, although the actual figure will depend upon the time of year and latitude
(see diagram).
The
HF “twilight” zone –the region on earth between the loss
of the D layer and where the sun starts/stops illuminating the F layer (roughly
defined as being offset from sunset by 14.165 degrees longitude) can therefore
be almost one hour before and after sunrise and sunset.
E
layer illumination starts/finishes somewhere in between these two, but the
average height is much closer to that of the D layer.
To
confuse matters, these values are based on average D- and F- layer heights and
the apparent heights of these can change too. So it is no good looking for grey
line DX exactly at your visual sunrise/sunset – you could be out by up to
an hour depending on the band, your respective locations, and the time of year
(see diagram).
And
even worse, for signals at an angle to the terminator we are interested in
where the first ionospheric refraction or hop
actually occurs once you radiate a signal, which is likely to be many hundreds
of miles to the east or west of you – where the sun may still be
illuminating the F layer. This is well illustrated on page 93 in the book “HF
Antenna Collection” by Erwin David, G4LQI – see refererence
7.
Most
books relating to HF propagation give a brief description of grey line
propagation, and how and why it works. What they don't tell you is the actual
frequencies affected, other than a vague idea that 80/160m
are definite bands for grey line, and "some" HF bands also exhibit
grey line enhancements.
Either
way, all these books tell you that grey line enhancements occur along the
terminator. That is, when both stations are at the sunrise/sunset condition.
John
Devoldere’s book "ON4UN's Low-Band DXing". suggests that his own
experience shows paths perpendicular to the terminator may enjoy the greatest
signal enhancement. That is, on the low bands, as sunset occurs at the
receiving station, you may get grey line enhancements at right angles to the
terminator in the direction towards the dark side of the earth - not along the
terminator.
He
also points out that the width of the terminator will vary according to the
season and your position on the earth, and cannot be thought of as a fixed
entity - the grey line will be narrower at the equator and wider at the poles.
So the time-span available for grey line conditions will also vary depending
upon the time of year, and the locations of the two stations, which is what I
proved earlier.
Likewise,
the width of the grey line will depend upon frequency as D layer absorption is
frequency dependent - you may still be able to work DX on 40m
24hours a day in mid-winter, while DX on 160m will fade out quite quickly after sunrise to the
greater D layer absorption.
But
what of HF? There appears to be little hard research of grey line
propagation at higher frequencies. The vague suggestion in most books appears
to be that grey enhancements can and do occur on 20m.
Ten metres is theoretically too high for the effect to appear as D layer
absorption is virtually non-existent normally at these high frequencies,
although I have read more than one article about how to work grey line on 10m! See the graph of frequency -dependent D-layer
absorption predictions at http://www.swpc.noaa.gov/products/d-region-absorption-predictions-d-rap
My
own studies show that enhancements on 10m do occur.
On many occasions I have heard signals from Indian, Indonesian and other
stations on 10m just after their local sunset - these
stations were not audible before. I have also worked a Brazilian (PT2GTI) station on 10m just after his
local sunrise, receiving a 59+ report using just 10 Watts into an indoor
dipole. I have also heard a station in Puerto Rico (KP4NU) at 59, one
hour after his local sunrise in November. Both stations were still audible
later that morning but at reduced signal strengths – down 10-15db.
These
are not grey line paths, but there were definite enhancements.
Reports
of sunset/sunrise enhancements at 50MHz over long
distances have also been logged, notably between the UK and USA. One suggestion
(see Ref 6) is that that this is due to E or Es
enhancements as the E layer increases in altitude at sunset.
The
increase in altitude of the E layer needs further explanation. As the sun sets
the lower regions of the E layer are not illuminated so the effective height of
the reflecting layer appears to increase. Likewise, at this time we can imagine
the radio ionosphere as being tilted as it is being illuminated at an angle.
This is probably the vehicle for the enhanced propagation at 28MHz and 50MHz – D layer
absorption probably has nothing to do with it.
If
the theory holds, look for enhanced signals during local daytime in G from
stations along their terminator – from the west at their local sunset and
from the east at their local sunrise. The signals should be strongest at
roughly right angles to the terminator – the same as ON4UN’s prediction
of propagation on LF, but from the illuminated parts of the globe, not dark.
There
is an alternative way of looking at grey line conditions on 7MHz
and 10MHz connected with the critical frequency (fof2). At frequencies above fof2
a radio wave travelling vertically upwards would pass through the f2 layer into outer space. Below f0f2
it would be reflected back to earth. Now imagine a radio wave hitting the
ionosphere at about 75-85 degrees to the earth - a near vertical incidence wave
(NVIS). Below the critical frequency it would be returned. If it is some way
above fof2 it will pass into space. At some frequency
close to fof2 it could be refracted through a large
angle and could end up travelling almost parallel to the earth, giving a very
long first skip distance. This is the condition for the Pedersen (see Ref 2) or
critical ray, discovered in 1927, characterised as being high angle, long
distance and close to and probably above the fof2
frequency. As there would be no intermediate ground hops the signal strength
could be very high indeed.
It
is likely that these conditions exist around local sunset/sunrise as fof2 passes through the two bands and could account for
long distance communications under grey line conditions on 7MHz
and 10MHz – see http://www.spacew.com/www/fof2.html
for predictions of fof2.
Either
way, there is more to grey line and twilight propagation than meets the eye.
The effects are different on every band, and the mechanism behind the
propagation is probably different too. What we can say is that twilight
propagation is not always best along the terminator and there may not be any
enhancement at all on some bands. Some books would have you believe that you
can just tune up on 20m at sunset and work ZL 59+20dB every day – if you can I would like to hear
about it!
I
am currently doing some research into twilight propagation on many of the
amateur bands, starting with 10m. The early results
show that we can and do see enhancements from signals originating from areas
experiencing sunrise/sunset.
The
graph of the beacon SV3AQR on 28.182MHz is typical. This
was produced using SpectrumLab software connected to
the audio output from a Yaesu FT-920. With the AGC
turned off, the vertical scale indicates signal strength while the horizontal
scale shows time. You can quite clearly see a 10db
increase in signal strength around the time of sunset at the beacon’s Greek
location. The effect has been seen on other beacons, but like all ionospheric effects, it doesn’t occur every day and is
virtually impossible to forecast.
More
monitoring work needs to be done before we can write the definitive guide to
grey line and twilight propagation and this is where I need readers’ help. If
you have a PC with a soundcard, can run the SpectrumLab
software (http://www.qsl.net/dl4yhf/spectra1.html/)
have a very stable receiver (the software needs stability in the order of a few
Hz), and can leave your system monitoring for an hour or more at a time then I
would like to hear from you. As part of the Propagation Studies Committee’s
work I plan to systematically look at twilight propagation on all the HF and LF
bands using known, quantifiable signal sources such as beacons and broadcast
stations. This is not a five-minute job though, but is essential if we are to
finally clear up what has been a grey area of propagation research for a long
time – every pun intended!
Steve Nichols G0KYA QTHR
Useful Software
Beacon Time Wizard by Taborsoft
(www.taborsoft.com)
GeoClock
(www.geoclock.com)
Grayline
1.2 by PA3CGR
References:
1. Dale Hoppe, K6UA et al, "They Grayline
Method of Dxing," CQ, Sept 1975, P27.
2. J. G. Lee, "An Introduction to Radio Wave
Propagation", Babani BP293, 1991.
3. Ian Poole, G3YWX, "Your Guide
to Propagation," RSGB, 1998.
4. Jacobs, Cohen and Rose, "The New Shortwave
Propagation Handbook", CQ Communications, 1995.
5. John Devoldere, ON4UN,
"Low-Band Dxing", ARRL, 1999.
6. Ken G4IGO, "E layer and
Sporadic E - Two Modes of Propagation at 50MHz",
UK SMG web site (www.uksmg.org).
7. E David, “HF Antenna Collection”, RSGB, 1991
ENDS