Visible auroras are magnificent to view, but they also give an indication of great activity on the Sun, and they can result in huge changes of the radio propagation conditions.
For the HF operator, these auroras can be the sign of a complete shut-down of the HF spectrum as the absorption levels in the ionosphere rise. For the VHF DX’er they can present an opportunity of an abnormal form of propagation.
What is auroral radio propagation
The visible auroras give an indication of the activity that is occurring in the upper atmosphere which manifests itself as a radio propagation auroral event.
Note on Auroral Propagation:
Visible auroras are a sign of a disturbance occurring in the upper reaches of the Earth's atmosphere. This can result in significant changes to radio propagation conditions. HF radio communications via the ionosphere can be blacked out, but it is possible at VHF to use the ionisation around the poles for communication.
Read more about auroral propagation.
With an auroral event, 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.
Using Auroral propagation on VHF amateur bands
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.
Auroral radio propagation summary
Auroral propagation can be a fascinating and rewarding form of propagation for radio amateurs. It provides an interesting means of making radio contacts and has the advantage that it can be sued at times when the propagation conditions on the HF amateur radio bands are likely to be poor. As no special equipment is required, it makes an ideal way in which to make radio contacts on an occasional basis as the conditions arise.