Auroral activity does not usually become visible until the geomagnetic field becomes unsettled. Auroras are the result of particles ejected by solar flares colliding with atoms of oxygen and nitrogen in the atmosphere. These particles cause the gas in the ionosphere to become ionized to form a plasma. This plasma emits light, which results in the familiar auroral displays. Typical altitudes for an aurora are approximately 80 to 150 km, within the E or upper D layer of the ionosphere. At these altitudes auroras can be seen for hundreds of kilometres. Most auroral activity occurs in a belt around the geomagnetic poles, between 65 and 70 degrees of geomagnetic latitude. These belts are called, predictably, the auroral zones. These zones tend to move toward the equator as geomagnetic activity increases. During a major geomagnetic storm, these zones can migrate as close to the equator as 45 to 50 degrees latitude. The auroral zones contain the electrojet, an area of strong electrical currents. The electrojet is a major factor in the formation of increased magnetic fluctuations that we observe in the auroral zones. Geomagnetic storming always increases these fluctuations. However, magnetic activity is always greater in the auroral zones, even on geomagnetically quiet days.
The auroral zones have considerable influence on radio propagation, both on the HF and VHF bands.
In terms of radio activity, auroras peak between 1600 and 2000 local time. They may be predicted
by an increase in the terrestrial A and K indices. Sustained K indices of 5 or more indicate a
storm condition and increased auroral activity is likely. In general, increased auroral activity
degrades HF signals but can provide exciting paths for VHF operators.
VHF Auroral Propagation
Aurora usually absorbs signals below 20 MHz . . . above 20 MHz they can be scattered or reflected . We start to see this on 10 meters when conditions are right. At the other extreme, on rare occasions amateurs have used auroras for VHF propagation as high as 432 MHz. Successful two way QSOs are possible up to 2000 kilometres. Beam antennas are almost essential, as signals should be directed toward the auroral activity and then rotated east and west to peak signals. In general signal strength drops dramatically as VHF frequency increases. Doppler shift and distortion increase to the point where CW is the only reliable mode. While the focus of this article is on six meters, the phenomenon described applies, to varying degrees, to all frequencies from 50 MHz to 70 cm.
Residing at the low end of the amateur VHF spectrum is the six-meter band. Predictably, VHF DXers wishing to use the ionosphere have turned to 50 Mhz. Unfortunately, even when we experience good HF conditions, long distance 50 MHz signal propagation is not always possible. Attempts to transmit six-meter signals long distances by the same means used for HF do not work because they usually pass through the ionosphere and out into space. Semi-reliable six-meter propagation is obtained by F2 refraction when the MUF exceeds 50 MHz. This happens only in the three years around the peak of the 11-year solar cycle. During the summer, and to a lesser degree in the winter, sporadic-E also propagates 50 MHz signals. Meteor scatter produces brief openings from time to time. Transequatorial spread-F (TE) can provides signal paths under the right conditions. Unfortunately TE propagation is not possible throughout most of continental North America, particularly in the northern U.S. and Canada.
Auroral Scatter on Six Meters
Under the right circumstances, VHF radio signals can be bounced from regions of auroral activity. The earth's geomagnetic field over the auroral zone can deviate by several degrees. The deviation introduces a curve in the dip-angle of the magnetic field that serves as a medium for VHF signals. This curvature, together with the high levels of ionization permits six-meter signals to be scattered by the ionosphere. This process is called auroral backscattering and can be a source for long distance six-meter communications. In a similar manner, forward scattering occurs when signals scatter off the aurora in a forward direction toward the Polar Regions. Two-way auroral communications is called bistatic auroral backscatter.
Scattering is completely different than refraction. It means the radio waves are literally
scattered off of the ionosphere near regions of auroral activity. Signals are scattered
backwards, forwards and in exceptional cases at various other angles relative to the transmitted
signal. Sometimes signals can be scattered a number of times off of multiple auroras to achieve
significant long haul communications. The quality of signals propagated in such a manner is
degraded significantly with each auroral contact. Scattered 50 MHz signals are very distorted
and may be somewhat wider than normal. SSB transmissions under these conditions tend to have a
sputtering motor like sound. CW is more intelligible when distorted by aurora than is voice.
While such conditions produce less than optimal signal quality, especially strong auroras have
produced six-meter QSOs exceeding 2200 kilometres. These strong auroras may transform into a
condition called 50 MHz auroral-E, resulting in increased path lengths and low distortion levels.
When and Where
Six-meter auroral backscatter communications are most probable when auroral activity is visible low on the horizon. However, useful aurora may be as far away as 1000 km and well below the visible horizon. Clearly the more intense the activity, the higher the probability for long haul backscatter communications. The likelihood of auroral backscatter is obviously a function of latitude. Operators closer to the equator do not experience auroral backscatter nearly as often as those who live in middle and high latitudes do.
Backscatter communications have two well-defined daily peaks. As I mentioned above, the first and largest peak occurs in the late afternoon between 1600 and 2000 local time. This peak does not appear to be quite so dependent on geomagnetic activity, although it is sensitive to it. The second peak occurs near local midnight. This second peak is very dependent on geomagnetic activity. Backscattering during this second peak occur only during a major magnetic storm. During quiet magnetic periods, this peak is essentially non-existent, with very rare and isolated incidents of backscatter communications.
It is worth noting that the same phenomenon occurs in the Southern Hemisphere with the Southern
Lights. We tend to think of auroral activity as being isolated to our hemisphere because large
populated landmasses are more prevalent in the north. This is somewhat true because the southern
auroral zone is, for the most part, out of range for operators in that hemisphere.
Long distance six-meter auroral propagation is indeed possible, but requires special conditions before DX QSOs can occur. The best times for auroral DX are in the late afternoon and early evenings. The next best opportunities come near local midnight during major geomagnetic storms. Generally, the likelihood for six meter auroral DX increase with geomagnetic activity. This is in sharp contrast to HF communication, which is seriously deteriorated during periods of high geomagnetic activity.