Auroras & Radio Propagation including Auroral Backscatter

An auroral event can give a fantastic light show in the sky, but also have a major impact on radio communication causing blackouts for HF ionospheric propagation & auroral backscatter at VHF.

Auroral propagation tutorial includes:
Auroral propagation

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.

The Northern Lights: these are a visual indication of the ionised particles entering the Earth's atmosphere through the magnetic polar regions

For short wave radio communications and broadcasters this can mean that the HF radio bands are severely degraded as the propagation is degraded, while at VHF it can give the opportunity for a unique form of short-lived radio propagation.

Whatever one's interest in aurora or whether using radio communications that might be affected by the phenomenon, it is useful to have an understanding of the reasons it occurs 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 and its activity - the way it produces both radiation and ionised particles.

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.

The Sun showing its sunspots — The Sun with sunspots visible
*Image Courtesy NASA*

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.

Under quiet conditions, if that can ever be a term applied to the sSun, there is a steady stream of plasma (highly charged particles travelling at high speed), that is emitted, and this is called the solar wind.

The solar wind travels at high speed - anywhere between about 300 to 800 km/s. The solar wind travelling towards the Earth is normally diverted by the Earth's magnetic field, which is actually distorted by it.

Under steady conditions some of the particles will enter by the poles and can give small levels of auroral lighting.

At times there are major disturbances on the Sun and these can have major effects on the levels of plasma emitted as well as the levels of radiation emitted and hence the various forms of radio propagation.

There are several forms of disturbance which are often mentioned:

Solar flares: Solar flares can be described as explosions on the Sun. They are sudden eruptions of energy in the solar atmosphere that can last anywhere from a few minutes to hours.

Both radiation and particles are emitted from these flares. Flares are classed to give a quick description of their size classes with names: A, B, C, M, and X, with A being the smallest and X being the largest. Each category has nine subdivisions, e.g., M1 to M9, and X1 to X9. As these flares can emit additional plasma they can give rise to some auroral activity.
Coronal Holes: Coronal holes are formed when a region of the corona extends. These holes appear as dark areas in the solar corona when the radiation is viewed or measured using just extreme ultraviolet (EUV) or soft x-ray. They appear dark because they are cooler, and less dense when compared to the surrounding plasma.

Another important factor is that they are regions of open, unipolar magnetic fields. As a result of this the plasma is able to escape more easily, and this results in extra streams of relatively fast but low density solar wind.
Coronal Mass Ejections, CMEs: Coronal Mass Ejections can be major occurrences on the surface of the Sun. They were only discovered relatively recently because they could not be detected until modern space satellites were able to image the Sun in a way to detect them.

Essentially they are an outflow of plasma from or through the solar corona. They are often, but not always, associated with erupting prominences on the Sun, disappearing solar filaments, and/or flares. CMEs vary widely in overall size, structure, density, and velocity. Large and fast ones can emit plasma with velocities of up to 2000 km/s. If the plasma from a CME is in the direction of the Earth, it can result in a significant geomagnetic storm. CMEs are the main reason for large geomagnetic storms on Earth.

It typically takes between about 20 and 40 hours for the plasma stream from a CME to reach the Earth, and with modern detection systems, it is possible to have good warning of their occurrence. Various news outlets as well as Aurora watch feeds on websites and social media are ideal for this.

These are some of the main disturbances on the Sun which affect radio propagation, and they can also have a major effect on aspects such as visible aurora.

Auroras and the Earth

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 magnetic 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. There is often some lighting of the skies in regions near the magnetic poles but a CME will considerably increase this.

The lights around the poles occur 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.

Sadly, these lights are only really viable in regions towards the poles. Normally they can only be seen at latitudes greater than about 55° north or south, but on occasions when there are very large geomagnetic storms they can be seen further away.

The visible displays can appear as coloured glows consisting mainly of green, blue, purple and red, although green is the most common.

it is found that several different colours can be generated:

Reddish colour: This can be generated during periods of intense solar activity by excited oxygen atoms at altitude above about 240 km.
Green: This is probably the most commonly seen colour and it occurs as a result of excited oxygen atoms below about 240km - green is emitted at this lower altitude instead of red because there is a higher concentration of oxygen atoms.
Purple: This results from excitation of nitrogen atoms above altitudes of around100km.
Blue: This results from the excitation of nitrogen at altitudes up to 100km - again the colour change results from the greater level of nitrogen at lower levels.

Effect of auroras on radio propagation

The increase in solar wind from the disturbance also has a significant effect on radio propagation.

There are two main effects, namely those affecting the HF portion of the radio spectrum and those affecting VHF and above.

• Auroral effects at HF

For HF radio communications users it is found that any aurora will have a very major impact on the state of the ionosphere and hence radio propagation conditions.

Many of the plasma particles travel on downwards increasing ionisation levels int he ionosphere.

There are two phases, namely the positive and negative storm phases.

Positive storm phase: In this phase, ionisation levels in the upper atmosphere are increased, and not only in the polar region but at lower latitudes as well. This is because the ionisation is carried by winds set up by the heating effect of the aurora at high levels of the atmosphere, along with the force applied by the electric fields present. These tend to transfer the ionisation across the globe. This phase gives an enhancement in ionospheric radio propagation for a short while.

Negative storm phase: This phase is rather more complicated, but D region levels start to increase as well as the higher regions experiencing a deficit of free electrons to refract the signals.

Accordingly those signal that reach the F region will pass straight through and into space In many ways the effects of an aurora are similar to those of a short wave fade, SWF caused by a solar flare, but the difference is that where an SWF lasts from several minutes to several hours, the negative storm phase storms caused by an aurora can last from several hours to several days, although there is a gradual recovery during daytime hours.

• Auroral effects at VHF and above

In terms of VHF and UHF radio communications, an auroral event can give rise to a relatively short lived form of radio propagation called auroral backscatter.

This arises because of increased ionisation levels at lower altitudes. 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 significantly increases the levels of ionisation in these areas. 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 about 500 MHz is more common.

The transmitted signals are directed toward the auroral region and not any station with whom contact may be established. In this way the signals are "back-scattered" or reflected back in a similar or other direction and the receiving station can pick this reflected signal up.

The optimal beam headings can only be found by trial and error, and they may vary as the auroral event proceeds.

It is also found that the signals are very badly distorted because of the movement of the streams of particles. As the ionised particles precipitate rapid fading as well as the movement of them causes a rough low frequency buzz of around 50 to 60 Hz or so to be superimposed on the signal.

In addition to this, the signals are roughly Doppler shifted as the electrons stream down. This means that the signals are shifted in frequency by about 1 kHz at around 150 MHz and correspondingly more or less at higher or lower frequencies.

• Auroral effects on GNSS

Geomagnetic storms can have a major impact on satellite navigation systems. During these storms the positional accuracy can be degraded in addition to the possibility of damage to the satellites themselves in major events.

The presence of the additional the increase in the influx of ionised particles as a result of the large increase in solar wind gives rise to phase fluctuations. These can degrade the positional accuracy of any GNSS system as the electron density changes in the various regions of the ionosphere over the period of the storm.

Typically the errors are much greater in the magnetic polar regions where the ionised particles from the solar wind are entering the Earth;'s atmosphere.

Auroral events are absolutely amazing to see and once one has been seen it will never be forgotten. They are such an unusual and amazing sight that brings to light the wonders of the universe, its power and its magnificence.

But these amazing auroral events have a major impact on radio conditions, even outside the zones where the aurora can be seen. So whatever ones, interest, a watch can be kept on the various aurora social media feeds for the possible onset of an exciting auroral event.