Greyline propagation between the UK and New Zealand
Steve Nichols G0KYA, of the RSGB’s Propagation Studies Committee, outlines propagation between the UK and New Zealand in October and explains the concept of “Greyline propagation”.
Worldwide communication using the LF and HF bands is dependent on radiation coming from the sun. 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 reflective (more correctly, refractive) 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. At sunset, the D layer disappears rapidly.
Around the other side of the world, as the sun sets in the UK, New Zealand is entering into daylight and has yet to form a significant D layer, plus the E layer has not built up from its night-time low. At UK sunrise the reverse happens.
Therefore, for a short period propagation between the 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 such 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) – 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 is related to the way the sun illuminates the ionospheric D, E and F layers.
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.
There is an alternative way of looking at grey line conditions, which I favour, and which is connected with the critical frequency (fof2). At frequencies above f0f2 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 an angle of 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 end up travelling almost parallel to the earth, giving a very long first skip distance. This is the condition for the Pedersen or critical ray, discovered in 1927 and 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 the critical frequency passes through 80m and could account for long distance communications under grey line conditions.
For example, the critical frequency above the UK on 18th October 2012 (as measured by the Chilton ionosonde) at 06:00UTC was 3 MHz, but by 06:20UTC it had risen to 3.950 MHz. This suggests that the right conditions for a long-range Pedersen Ray between the UK and New Zealand might have existed at some point in those 20 minutes.
In 1913, contact between the UK and New Zealand was made initially at 06:15 UTC and signals were lost at 07:30 UTC.
Therefore, on a critical frequency basis I would say that the optimum time for a contact between G and ZL on 80m on 18 October 2014 is somewhere between 06:00 – 06:30 UTC (0700 – 0730 BST). At UK sunset the critical frequency was around 7.5 MHz, so I wouldn’t expect to see any Pedersen Ray enhancement at sunset.
Alternatively, let’s look at the sunrise/sunset times for the UK and New Zealand on 18th October 2014 – the 100th anniversary of the historic contact between G2SZ in England and Z4AA in New Zealand.
In the UK the sun will rise (in Birmingham) at 06:39 UTC and set at 17:06 UTC. Note that we will still be on British Summer Time (BST) so the local times will be 07:37 UTC and 18:06 UTC.
You may have to adjust this slightly for actual location – sunrise in Lowestoft in the east is actually 15 minutes earlier and in Penzance it is 26 minutes later.
At Christchurch, on the South Island of New Zealand, sunrise is at 17:35 UTC and sunset is at 06:54 UTC.
So, in effect there is no period of mutual darkness between the UK and Christchurch at the UK sunrise. But at the UK sunset there is about 30 minutes of mutual darkness.
On this basis, I would say that the optimum time for the morning (long path) contact is around the UK sunrise, +/- 30 mins, say 06:00 – 07:00 UTC.
At the UK sunset (short path) it might be worth looking between 16:45UTC to 17:30 UTC (17:45 BST – 18:30 BST), but I am less optimistic about a contact being made at UK sunset for the reasons I mentioned earlier.
These figures appear to be backed up by the W6ELProp propagation prediction program, which is usually more accurate than VOACAP-based programs for 80m predictions. In fact, VOACAP is unlikely to show the path as being open on 80m at all.
This is a long and difficult path with all signals going through the polar regions, which will be affected by solar conditions. So ideally we need a low K/A index, representing quiet geomagnetic conditions.
Steve Nichols G0KYA
Chairman, RSGB Propagation Studies Committee
Grayline 1.2 by PA3CGR
VOAProp by G4ILO