Last Updated on January 20, 2026 by John Berry
Why a radio aurora occurs is complicated. I’m not clear yet on all the mechanisms. But here’s my assessment so far as an auroras overview. The adjacent pages expand.
Auroras are caused by the interaction between the solar wind, the molecules in the upper atmosphere (ionosphere), and the interplanetary magnetic field (magnetosphere). This follows an ejection of high-energy helium and hydrogen ions and electrons from the Sun heading earthward at speeds of between 200km/sec and 1,500km/sec. Auroras occur around three days after the ejection (the time it takes for the plasma to reach Earth).
Radio auroras
Visual auroras occur following the release of energy from recombinations of electrons, and ions. This occurs in the ionosphere from 90km to 400km above the Earth.
Radio auroras are caused by increase in electron density in the E Region of the ionosphere at around 100km. The ionosphere already has a heightened electron density following excitation from the Sun’s rays.
Visual auroras are the appearance of colours in the night sky. And it needs to be dark to see them well. Red is caused by oxygen (O+) recombinations at around 300km. Violet-blue is caused by nitrogen (N2+) recombinations at 100km. But O+ recombinations also cause white-green colour at 200km and green at 120km.
Annually, about 30-40 visual auroras occur on the 60∘N isochasm (line of equal frequency) spanning from Oslo to Aberdeen to Newfoundland. Occurrence reduces with reducing latitude. Further north, the aurora is visible almost every night.
Birkeland currents
Auroras are caused by Birkeland currents (after Kristian Birkeland, Norwegian physicist) that start in early afternoon on the sunny side of the globe and continue to flow to midnight. Radio auroras start in early afternoon at about 60∘N and continue for an hour or so and may rarely descend to about 50∘N. They can reoccur the same day at 60∘N at around 21:00 onwards and descend to about 50∘N at 01:00UTC. There is correlation between the Birkeland currents and the occurrence of radio auroras but one does not cause the other and radio auroras do not necessarily occur with visible auroras.
Birkeland currents flow along field lines connecting the Earth’s magnetosphere and the Earth’s atmosphere and through the ionosphere in the polar regions. They are huge: 100,000 to 1,000,000 Amps. Their strength depends on the energy in the solar wind. Importantly, they cause heating of oxygen and nitrogen and other atmospheric molecules promoting ionisation. As noted, this increases electron density in the E Region at around 100km. They rotate axially and hence cause particle rotation. Increased electron density and rotation are key points in understanding radio auroras.
As noted above, Birkeland currents build from mid-day and peak. The result is a delay of around three hours in the start of the radio aurora . I’ve noted elsewhere that the strongest radio auroras typically start in late afternoon.
Auroras overview
Only one in ten or so visual auroras develop into radio auroras. Radio auroras are caused by sub-storms off the main auroral activity and require considerably more energy.
And of course, the active auroral oval must be to the north of the participating stations.
The intensity of effect varies. Radio auroras vary in their ability to support communications. They can be weak affairs enjoyed only by radio amateurs located in the far north, or they can be strong affairs descending and enjoyed by radio amateurs in locations down to 50∘N. Around one in five radio auroras are intense enough to support SSB QSOs. I’ve noted that most are weak, supporting weak signal modes like CW – I really must learn Morse!. The result is one or maybe two strong radio auroras per annum towards the sunspot maximum.
The adjacent pages expand on these ideas. And I’ll update as I understand more.