Auroral propagation
| Auroral propagation | |
|---|---|
| What it is | VHF signals scattered from the ionized auroral zone |
| Main bands | 6 m and 2 m; also 10 m, 1.25 m and 70 cm |
| Typical range | A few hundred to more than 1,000 miles |
| Signal sound | Raspy, hissing "buzz"; worse at higher frequencies |
| Operating | |
| Antenna heading | Roughly north (in the Northern Hemisphere), toward the aurora |
| Best modes | CW, then SSB |
| Signal reports | "A" replaces the tone figure, as in 57A |
| Needed | A geomagnetic storm; stronger storms for stations farther south |
Auroral propagation is a way of making VHF contacts by scattering signals off the aurora, the glowing, electrically charged region that forms high over the polar regions during a geomagnetic storm. Two stations, both pointing their antennas toward the aurora rather than at each other, can work across hundreds of miles on bands that normally reach little beyond the horizon. The signals have an unmistakable sound, a rough, raspy buzz that turns a clean CW note into a hiss, and once heard it is never forgotten.
How it works
The aurora forms where charged particles from the sun, steered by Earth's magnetic field, pour into the upper atmosphere in a ring around each magnetic pole called the auroral oval. They ionize the air around 100 km up, the height of the E layer of the ionosphere, in sheets and curtains aligned with the magnetic field lines. VHF signals that reach this ionization are scattered back to earth, and the scattering works best when the signal meets the field lines at right angles.
During a strong storm the oval expands toward the equator. That brings the aurora within reach of more southerly stations, and it is the reason auroral openings follow geomagnetic storms rather than the time of day. The K index, which measures geomagnetic disturbance, is the usual guide. As a loose rule, stations in the northern tier of US states may hear auroral signals when the planetary K index reaches about 5, while those farther south need a stronger storm, often a K of 7 or more. Openings can come at any hour during a storm, but many operators find the afternoon and evening most productive, and storms are somewhat more frequent around the spring and fall equinoxes.
The auroral sound
The ionized curtains move quickly and unevenly, so each part of the reflecting region shifts the signal by a slightly different amount through the Doppler effect. The receiver hears a smeared spread of frequencies instead of a single tone. On CW this comes through as a raspy hiss with the keying still readable; on SSB, voices sound like a hoarse whisper. The effect grows with frequency: it is mild on the 10-meter band, distinct on the 6-meter band, and severe on the 2-meter band and above, where signals may also be shifted slightly off the expected frequency.
Because the tone carries no meaning, auroral signal reports replace the tone figure of the RST report with the letter A, so a CW report might be 57A.
Operating
- Point north. In the Northern Hemisphere, aim the antenna toward the aurora, usually somewhere between northwest and northeast, and swing it to find the loudest heading. Two stations a few hundred miles apart east to west may both point north and hear each other well.
- Use CW where you can. CW survives the distortion best and is the main mode on 2 meters. SSB works well on 6 meters if operators speak slowly and clearly, using the phonetic alphabet for call signs. FM is close to useless, and weak-signal digital modes such as FT8 often struggle to decode the spread signals, especially on 2 meters.
- Use horizontal antennas. Auroral work is done on the weak-signal parts of the bands with horizontally polarized antennas, typically Yagis.
- Expect modest distances. Most contacts span a few hundred to about a thousand miles, limited by the height of the auroral ionization.
On 6 meters an auroral event is sometimes followed by auroral E, clean, sporadic-E-like reflections from dense ionization in the auroral zone. Its signals lack the auroral buzz and can cover longer distances, more like sporadic E.
Watching for an opening
Auroral openings are announced by the sun, sometimes days in advance. A coronal mass ejection aimed at Earth, a rising K index, a geomagnetic storm alert from NOAA's Space Weather Prediction Center or a sudden collapse of conditions on the higher HF bands are all signs to check 6 and 2 meters. On the air, the first clue is often a distant beacon or a DX cluster spot with an "A" report, or simply the sound of buzzing CW from the north. Seeing the aurora is not necessary; the radio aurora often extends beyond the visible display.