Last Updated on February 10, 2025 by John Berry
The equatorial ionisation anomaly is the appearance of heightened ionospheric electron density and associated heightened Maximum Usable Frequency (MUF). It’s found on the daylight side of the world at around 20 degrees north and 20 degrees south of the geomagnetic equator. The core cause is an electromagnetic anomaly; hence the focus is the geomagnetic equator and not the geometric equator or the tropics. I’ve drawn this as a stylised image below based on a Mercator projected world map.
Ionosondes
The equatorial ionisation anomaly was discovered when ionosondes around the world measured the vertical incidence MUF for the ionosphere’s F2 region, foF2, against date and time. Charting the loci of points having the same MUF in this data gave a map like that below with iso-MUF contours. The MUF peaks are represented by the horizontal lozenges.
Outcomes of the anomaly
Under the above twin-MUF-peak scenario, two stations, A in the UK and B in South America, are likely to be able to communicate easily over a path of 12,000km. They do so by trans-equatorial chordal ionospheric propagation – the transmitted wave enters the ionosphere and remains trapped. It propagates between regions of heightened electron density and exits some significant distance away from point of entry.
The same would apply, for example, between stations in UK and Australia, at UK dawn and Australia evening.
Losses over free space, as defined by the terms Li and Lg in the path loss equation, increase with number of hops. Single hop chordal propagation therefore yields big benefits. It’s worth reading my Background to ionospheric predictions for more on this equation. I saw a significant example of this in October 2024 when SSB phone QSOs were possible on 50MHz between the UK and Argentina. Operators reported 5/9 signals – some 30dB above receiver threshold – and far greater than that likely from a three-hop path.
Several variables must be in concert for this enhancement over normal to occur. It must be daytime at both ends of the path. And there must be a high Smooth Sunspot Number (SSN) resulting in high MUFs, and there must be a stable magnetosphere signalled by low Kp.
Equatorial ionisation anomaly
The root cause of the equatorial ionisation anomaly is the Appleton anomaly where fountains of plasma rush from lower to higher ionospheric regions at the equator. But the phenomenon is not completely understood. There are still questions on the ionosphere and magnetosphere interaction that causes it. This interaction is key to why the MUF peaks occur where they do. In common with other ducting phenomena, it’s also not clear why the wave propagates as if trapped in the ionosphere. And it’s not clear why the wave exits where it does at the other side of the twin peaks.
In interpreting this phenomenon, it’s worth remembering that although I have drawn single rays, no propagation is ever that simple. Transmitted energy spreads to infinite rays, and signals received are the vector sum of all arrivals.
You can probe this and other effects by looking at the normal ionosphere using a path prediction tool such as those embedding VOACAP or Rec. ITU-R P-533.