Aurora borealis, the "northern lights" (and aurora australis in the southern hemisphere): one of nature's greatest shows, and a powerful VHF propagation medium as well. Aurora occurs when charged particles (ions) emitted by solar flares and other disturbances on the sun enter the Earth's atmosphere. The interaction of these ions with the Earth's magnetic field cause huge electrical currents to flow in the upper atmosphere, particularly toward the polar regions. The intense ionization of the auroral curtain is capable of reflecting VHF signals.
Aurora signals are characterized by severe distortion and doppler smear (frequency spreading) due to the highly irregular form and rapidly changing nature of the auroral curtain. At 50 MHz, SSB is quite useable, but usually sounds garbled, or has a buzzing effect. At 144 MHz, SSB sounds like a loud whisper, if useable at all; and CW signals sound like a loud hissing sound. CW is definitely required above 144 MHz.
Antennas should generally be aimed toward the aurora, but this does not elways mean due north. VHF signals are reflected from the auroral curtain in much the same way light is reflected by a mirror: at an equal but opposite angle to the beam of light hitting the mirror. From here in Maine, where most of the activity is to our west or southwest, beam headings from north to west-northwest are generally best. However, VE1 signals often peak with the beam slightly east of north. During a few very instense auroras, southwesterly headings are best for working into the deep south. Of course, stations in the sourthern hemisphere should point in a generally southerly direction. Stations with as little as 10 watts of power and small beam antennas have had good success during good auroras.
The further north one goes, the more prevalent aurora becomes. It is the mainstay of VHF propagation in many northern areas, including the northeastern U.S. However, aurora contacts have been made as far south as Florida and Texas during the intense ones! Aurora contacts over distances of up to 1200 miles are common. Longer distances have been worked on occasion.
Aurora occurs when the goemagnetic field is at storm levels (minor, major, or severe storm). This usually corresponds to a "K" index of 5 or more, although stations to the far north can often work aurora with lower numbers. Although aurora can occur at any time of day, it is most prevalent in the late afternoon through evening hours. Auroras are more prevalent around the equinoxes, and when solar activity is high. Predicting aurora is very difficult. The links below offer guidance, but beware... aurora is fickle stuff. It can show up (or not) any time!
Here are some links to check out for aurora information:
Kiruna Magnetogram. Real time magnetometer data from Kiruna, Sweden.
Space Environment Center. Today's Space Weather gives a brief forecast of expected geomagnetic conditions for the upcoming 24 hour period.
My Realtime Aurora Monitoring Page.
Here are some examples of aurora signals to listen to.
A typical 50 MHz SSB signal, propagated by aurora. Note the "buzz".
Provided by VA6DX (ex-VE6MK)
A typical 50 MHz CW signal, propagated by aurora.
Provided by VE5UF
A typical 144MHz SSB aurora signal. Can you copy this one?
Provided by W2RS
Another good example of 144MHz SSB aurora signal.
Provided by GM4JJJ
A typical 144 MHz CW aurora signal. Notice how there is no "tone" at all. The signal
sounds like a loud hiss, or like noise being keyed on and off. These signals are often several kilohertz wide due
to doppler spreading.
Provided by VE5UF
Another typical 144MHz CW aurora signal.
Provided by GM4JJJ
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