Aurora and Geomagnetic Storms
Geomagnetic storms — caused by coronal mass ejections and solar wind disturbances — are both the bane and the fascination of ham radio operators. A strong geomagnetic storm can shut down HF propagation for days, ruining carefully planned DX attempts and EmComm operations alike. But the same storms produce aurora, and aurora produces a unique propagation mode on VHF that allows contacts of hundreds to thousands of kilometres with a distinctive buzzy, distorted audio quality that is immediately recognisable. This guide covers how geomagnetic storms affect HF propagation and how to exploit aurora for VHF operating.
Storm effects on the ionosphere
When a coronal mass ejection reaches Earth, the magnetised plasma interacts with Earth's magnetic field and deposits energy into the upper atmosphere, particularly at high latitudes. This energy heats and disturbs the ionosphere — increasing absorption at HF frequencies, lowering the MUF, creating highly variable and patchy ionisation, and in severe storms virtually shutting down all long-distance HF propagation. The higher HF bands (15m, 12m, 10m) are most severely affected. The lower bands (40m, 80m) are less impacted but still show degradation. Paths through polar and auroral regions are disrupted first and most severely.
Storm phases and recovery
A geomagnetic storm typically begins with an initial phase (1–2 hours of slightly enhanced conditions as the solar wind pressure compresses Earth's magnetosphere), followed by the main phase (12–48 hours of degraded conditions as the storm develops), and then a recovery phase (1–4 days as the ionosphere gradually returns to normal). During the recovery phase, HF conditions often improve dramatically and temporarily above pre-storm levels — a phenomenon sometimes called the "storm bounce" where operators who weathered the storm are rewarded with exceptionally good conditions as the ionosphere recovers. Check K-index trends rather than the absolute value to catch the recovery phase.
How aurora propagation works
During geomagnetic storms, energetic charged particles from the solar wind stream into the upper atmosphere along Earth's magnetic field lines, producing the visible aurora borealis (northern hemisphere) or aurora australis (southern hemisphere). The ionised curtain of the aurora acts as a moving, irregular reflector of VHF signals. Operators point their antennas towards the aurora (roughly north for operators in the northern hemisphere mid-latitudes) and use the aurora as a scattering medium to reach other stations who are also pointed at the same aurora region. The geometry allows contacts of several hundred to several thousand kilometres between operators who would normally be far outside each other's VHF range.
Aurora signal characteristics
Aurora signals have a very distinctive sound — a raspy, harsh, distorted quality sometimes described as sounding like a chainsaw or a waterfall of static. The distortion is caused by the rapid, irregular motion of the aurora ionisation region, which Doppler-shifts the reflected signal continuously. SSB phone signals are typically unintelligible through aurora — the voice is so distorted it cannot be understood. CW is the primary mode for aurora contacts because the simple on-off keying of CW survives the distortion and remains recognisable as Morse code even with significant audio degradation. Some digital modes also work through aurora.
Check for aurora conditions
Monitor the K-index at swpc.noaa.gov — K-index 4–5 or higher indicates potential aurora propagation at mid-latitudes. Check the DX cluster for aurora spots and reports on 144 MHz. The NOAA aurora oval forecast at swpc.noaa.gov/products/aurora-30-minute-forecast shows the predicted extent of the aurora oval — if it extends to your latitude, aurora propagation is possible.
Point your antenna north
In the northern hemisphere, beam your antenna towards magnetic north (slightly different from geographic north — account for your local magnetic declination). If you do not have a directional antenna, aurora propagation is still possible with an omnidirectional antenna, but a yagi pointed north concentrates your signal towards the aurora curtain and produces stronger signals at the other end.
Tune across the band in CW
Tune slowly across the 144 MHz CW portion of the band (144.000–144.100 MHz is the primary aurora CW region). Aurora signals have the characteristic raspy quality — once you have heard it you will immediately recognise it. When you hear a station, respond with your callsign in CW at a moderate speed (15–18 WPM maximum — faster is harder to copy through aurora distortion).
Complete the contact
An aurora CW contact is typically brief — both callsigns and a signal report (typically given as "A" for aurora — e.g., "59A" meaning 59 aurora). Signal strength via aurora varies significantly with the activity of the aurora itself. Contacts are often made quickly because aurora conditions can change or end rapidly. Log the contact promptly.
How do I know if aurora propagation is happening right now?
Check DX clusters filtered to 2m for aurora spots — experienced operators post aurora spots when they hear the characteristic raspy signals. NOAA's aurora oval forecast shows whether the aurora extends to your latitude. Some VHF propagation monitoring groups on social media post real-time aurora reports. The most direct check is to tune 144.000–144.100 MHz in CW and listen for the distinctive buzzing, raspy signals that indicate aurora activity.
Can I work aurora with a vertical antenna?
Vertical antennas can work aurora but are less effective than a horizontal yagi pointed north. Aurora propagation on 2m uses horizontal polarisation primarily because the aurora reflects signals in a way that favours horizontally polarised signals. A horizontal dipole pointed north is a reasonable aurora antenna if you do not have a yagi. A good yagi with 7+ elements pointed north gives significantly better results.
Does aurora affect 70cm (432 MHz) the same as 2m?
Aurora propagation on 432 MHz is possible but much less common and typically weaker than on 144 MHz. The higher the frequency, the less effectively aurora scatters the signal. 144 MHz is the primary aurora band for most operators. Some strong aurora events produce workable signals on 432 MHz — experienced aurora operators monitor both bands during strong K-index events.
How long does aurora propagation last?
Aurora propagation can last from 15 minutes to several hours depending on the geomagnetic storm's intensity and evolution. A rapidly rising K-index often produces brief, intense aurora propagation. A sustained storm can maintain aurora propagation for hours with varying intensity. Aurora conditions can improve and worsen repeatedly over the course of a night as the storm evolves. Operators who stay on the band through quieter periods are rewarded when activity picks up again.