Auroral Propagation
Auroras are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere aand are referred to as polar auroras but this is not true sine the occur in a ring just bounded by the artic and antartic cirles.
In northern regions the effect is known as aurora borealis but more commonly
called the northen lights. It is only visible in the sky from the Northern Hemisphere, with the chance of visibility increasing with proximity to the North Magnetic Pole. Auroras seen near the magnetic pole may be high overhead, but from further away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the Sun were rising from an unusual direction. The Aurora Borealis most often occurs near the equinoxes.
Its southern counterpart, the aurora australis or the southern polar lights, has similar properties, but is only visible from high southern latitudes in Antarctica, South America, or Australasia. Australis is the Latin word for "of the South."
The sight of aurora in the sky at night can be awe inspiring, taking the form of beautifully coloured glows gracefully changing sky. The colours are usually greens and reds, although on occasions bluish tints can be seen. To many people an aurora is a beautiful sight but for radio amateurs it could mean degraded performance on the HF amateur radio bands, while at VHF it can give the opportunity for a unique form of radio propagation. taking the form of beautifully coloured glows gracefully changing sky. The colours are usually greens and reds, although on occasions bluish tints can be seen. To many people an aurora is a beautiful sight but for radio amateurs it could mean degraded performance on the HF amateur radio bands, while at VHF it can give the opportunity for a unique form of radio propagation. taking the form of beautifully coloured glows gracefully changing sky. The colours are usually greens and reds, although on occasions bluish tints can be seen. To many people an aurora is a beautiful sight but for radio amateurs it could mean degraded performance on the HF amateur radio bands, while at VHF it can give the opportunity for a unique form of radio propagation. taking the form of beautifully coloured glows gracefully changing sky. The colours are usually greens and reds, although on occasions bluish tints can be seen. To many people an aurora is a beautiful sight but for radio amateurs it could mean degraded performance on the HF amateur radio bands, while at VHF it can give the opportunity for a unique form of radio propagation.
The onset of an aurora is bad news for HF amateur radio users as band conditions are most likely to be badly affected. All that can be done is to wait until the radio propagation conditions recover, but it can take up to a week before the HF amateur radio bands are back to the state they were before the storm.
For VHF amateur radio operators the onset of an auroral event brings exciting possibilities of DX with the possibility of amateur radio contacts being made over many hundreds of kilometres. As the ionisation is concentrated around the poles communication is only possible at certain latitudes. For example in the UK those radio amateurs in Scotland, Northern England and Northern Ireland are best placed, although it is possible for stations in Southern England to use it when there is a large aurora. Interestingly is found that stations in Southern Scotland and Northern Ireland seem to be well placed for making some of the longest distance contacts, although stations further north will see more auroras.
Good antennas are essential when using auroral radio propagation. Directional or beam antennas are required and these should be rotated towards the auroral zone, i.e. to the north in the Northern Hemisphere and to the south in the Southern Hemisphere. Signals are then reflected back, i.e. using back-scatter. This means that the beam heading for the optimum signal will not be in the direction of the station being contacted.
It is found that signals that have been propagated using auroral radio propagation are distorted and this means that voice transmissions can be very difficult to copy. The wider the bandwidth the greater the problem and therefore SSB is the best voice mode to use, although copy is difficult. Naturally Morse is good because it occupies a very narrow bandwidth is very resilient to distortion. However even this becomes distorted, having a very rough tone superimposed onto it. This can vary from one aurora to the next, or even during the course of an event. Typically signals flutter very rapidly because of the changes occurring in the ionosphere This flutter can even be so fast that it appears as a low frequency tone or buzz up to 50 or 60 Hz.
In addition to the distortion on the signal, it is also subject to a Doppler frequency shift. This is caused by millions of plasma particles entering the ionosphere. Each is a minute point for reflection and has a different velocity. This means that the Doppler shift has a spread of frequency shifts, resulting in the very distinctive hissing sound. As a general rule the average frequency shift on the 145 MHz amateur radio band is about 0.5 kHz.
Posted 07 August, 2010 14:19:12 GMT