Aurora Event Propagation
HF operators cringe when they hear the words solar flare or coronal mass ejection.
They know that a large burst of charged particles is going to hit the ionosphere and disrupt their bands, oftentimes generating the Northern Lights.
What many Hams don't know is that these flares can create fascinating opportunities for QSOs on 6 and 2 meters, and even 70 centimeters in extreme cases.
Contacts 500 miles away are possible utilizing aurora.
The sudden ionospheric disturbances or SIDs associated with aurora events are one instance of a class of SIDs that the general class license test teaches us "disrupt signals on lower frequencies more than those on higher frequencies".
Further it teaches us about the solar flares that give rise to these disturbances and how quickly they disturb our communications.
Specifically "it takes 8 minutes for the increase ultraviolet and x-ray radiation from solar flares to affect radio propagation on earth".
Methodology: 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 completely 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.
The general class license test points out that it takes 20 to 40 hours for the charged particles from a CME to affect radio propagation on earth.
When there is a solar disturbance the level of the solar wind increases, together with the charged particles that comprises that wind.
Many of these charged particles penetrate the weakest parts of the geomagnetic field near the polar regions.
This is because the geomagnetic field lines guide these charged particles into these regions.
High energy particles entering the Earth's atmosphere tend to follow the magnetic lines of force to the poles.
At these polar regions extreme ionization can result at altitudes up to 1000km.
The most obvious sign of the event is that a visible aurora occurs lighting up the northern or southern skies.
As the charged particles 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 light that causes the Northern and Southern Lights.
The high energy particles pass through the outer parts of the ionosphere with little effect.
However as they penetrate to lower altitudes they reach the E layer.
Here they start to collide with the gas molecules, and this increases the levels of ionization in these areas.
The ionization reflects signals at much higher frequencies than normal.
Communications can be established well into the VHF portion with usable MUFs typically in the 70cm band or lower.
Unfortunately for HF 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 ionization 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. A weaker event will aid lower frequencies, but only for higher latitudes. Timing is an important factor in utilizing aurora events. 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.
HAARP has been at the forefront of research into studying the properties and behavior of the ionosphere at high latitudes utilizing aurora.
Utilization: Most aurora events effect primarily the northern latitudes above 60 degrees.
If the event is stronger then opportunities extend further toward the equator.
The key to working an aurora opening from the contiguous 48 states is to point your antenna to the north, as you know if you took the advanced license test.
The northern lights are an indicator as to how far the ionization is occurring at the poles; the larger the flare, the greater the disturbance, and the farther away from the poles aurora openings will occur.
By pointing your antennas north, you are using the edge of the aurora as a reflector of your radio signal.
The auroral layer may reflect radio waves from the HF-band (3-30MHz) all the way up to and including the entire UHF-band (300-3000MHz).
However, due to its very irregular shape and constant movement, heavy fading (QSB) is common in the reflected radio signals.
This QSB can also result from multiple reflections within these auroral layers, causing rapid phase shifting.
An auroral signal is often recognized at 30MHz as a bubbling sounding modulation or "under-water-like" modulation.
Finally, because of the extreme and sudden phase shifts, narrow band modes such as CW and digital are the most reliable modes for DX contacts.
Also, the surface of the aurora isn't smooth.
As a result, your transmitted signal hits the surface of the aurora and scatters in several directions simultaneously, more or less back toward you.
This results in a very distorted signal.
On 6 meters, you can usually understand an SSB signal enough to make a QSO.
On 2 meters, the distortion is so severe that voice QSOs are often impossible.
CW is the most effective mode for aurora QSOs on 2 meters and up.
The extra class license exam states more generally that CW " is the best emission mode for aurora propagation" .
Even then, the distortion is so pronounced that a pure CW tone, which normally sounds like a BEEP, will be almost whisper-like, sounding more like a PFFFT.
However, openings of hundreds of miles are possible under these conditions.
Rapid fluctuations of strength and distortion are the norm for Aurora propagation.
Prediction: Because the solar wind that produces the aurora arrives well ahead of the aurora formation this phenomenon can be predicted.
NOAA provides a resource at NOAA aurora forecast.
The HAARP project, now run by University of Alaska, provides a forecast at HAARP aurora forecast.
A forecast of the geomagnetic indices that are associated with an aurora event can be found here
geomagnetic indices.
A related general class license question states that a benefit of high geomagnetic activity is " auroras that can reflect VHF signals " .
There is a an element of periodicity to aurora events.
Just as Earth rotates on its axis making a complete rotation every 24 hours, the sun spins on an axis making a complete rotation in 27 days.
Solar phenomena on the surface of the sun distribute high speed plasmas that often result in increased auroral activity on earth.
These phenomena may be sunspots, coronal mass ejection, filaments, or a prominence.
As the sun rotates, the solar phenomena and resulting areas of high-speed plasma are likely to reoccur every 27 days until the phenomena dissipate.