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The sight of an 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 to see but it is also an indication of activity in the skies that can also result in some dramatic changes to radio propagation. For radio amateurs this 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.
In order that radio hams can make the best use of these radio phenomena it is useful to have an understanding of the reasons they occur and the mechanics of how the radio signals are propagated under these conditions. To do this it is first necessary to look at the Sun.
The Sun and its affect on radio propagation
The Sun generates a colossal amount of energy, some of which provides light and heat for us here on Earth. It also generates ultraviolet light and X-rays which have an effect on radio propagation. As a result the ionosphere is formed in the upper atmosphere and this enables radio waves to be reflected, or more correctly refracted back to earth, thereby enabling global radio communications on the HF or short wave bands.
The levels of energy emanating from the Sun are not always constant. This in turn affects the condition of the ionosphere, which in turn affects HF radio propagation. Monitoring the energy from the Sun can give a good indication of the state of short wave radio communications, and this can be used by the users of the HF radio bands including radio amateurs, short wave broadcasters and commercial users.
At times there are major disturbances on the Sun and these can have major effects on radio propagation conditions. Solar flares and other forms of disturbance known as Coronal Mass Ejections can totally change the condition of the ionosphere and give rise to auroral activity.
Of the two types of disturbance, it is now thought to be the CMEs that are the major cause of auroras. These CMEs consist of gigantic eruptions on the surface of the Sun that throw vast quantities of material into space, along with this there is a huge increase in the level of radiation emitted.
Under normal conditions the Sun emits matter and this forms what is known as the solar wind. When CMEs occur, the solar wind significantly increases and this affects the Earth when it arrives.
Effect of Solar disturbances on radio propagation
The way in which the solar wind interacts with the earth is quite complicated. Essentially it is normally deflected by the Earth's magnetic field, although some enters via the areas around the north and south poles where the field enters the Earth. This is normal and no undue effects are noticed.
When there is a solar disturbance and the level of the solar wind increases changes occur. The most obvious sign is that a visible aurora occurs lighting up the northern or southern skies. This occurs because high energy particles enter the Earth's atmosphere along the magnetic lines of force entering the Earth at the poles. As the travel they collide with molecules in the atmosphere releasing positive ions and negative electrons. When this occurs a small amount of light is generated and it is this that causes the Northern and Southern Lights.
The increase in solar wind from the disturbance has a significant effect on radio propagation, and this is naturally of great interest to radio amateurs. It is found that the particles pass through the outer parts of the ionosphere with little effect. However as the altitude decreases they reach the E layer. Here they start to collide with the gas molecules, and this increases the levels of ionisation in these areas to a very large degree. The result of this is that the ionisation reflects signals at much higher frequencies than normal. Communications can be established well into the VHF portion of the spectrum and sometimes reflections have been detected at frequencies up to up to about 1000 MHz. This top figure is somewhat exceptional although the normal maximum for amateur radio communications is around 430 MHz.
Unfortunately for HF amateur radio enthusiasts many of the plasma particles travel on downwards into the D layer where again the levels of ionisation are greatly increased. Here the increased level of ionisation serves to absorb radio waves at much higher frequencies than would normally be affected. In this way much of the HF band communications can be blacked out.
It is found that during the course of a normal auroral event, the polar regions are affected first and for this reason the absorption is often called Polar cap Absorption (PCA). Usually the polar cap absorption is confined to latitudes greater than 60 , although during some of the larger events this will extend further towards the equator.
Progress of an Auroral Event
Although different events will vary widely from one to the next they will have many similarities. Often the event will commence with a number of small flares. These cause the level of solar radiation to increase and this brings an improvement in HF band radio conditions. Coupled to this the solar noise also rises.
These small flares are only a precursor to the solar disturbance which occurs causing a Sudden Ionospheric Disturbance or SID. At this point the HF bands close for ionospheric radio communications for a short while. However they soon recover as there is an increase in solar flux. About 20 to 30 hours after the solar activity the solar wind shock wave hits the earth causing a magnetic storm. Radio communications on the HF bands fail and the full auroral event starts. At this point VHF radio propagation is enhanced and contacts can be made over distances of a several hundred kilometres. Then having reached a peak the aurora ends and the HF bands slowly recover, the low frequencies becoming useable first.