Last Updated on February 25, 2025 by John Berry
Scientists have written so much stuff about the Earth’s magnetic field or magnetosphere. I’ve added a description and a model here to expand on what’s available. My aim is to show how the magnetosphere affects radiowave propagation as experienced by radio hams.
We can describe the Earth’s magnetic field using the analogy of a bar magnet. The magnet sits within the Earth and lies roughly north-south. Like all such magnets, it sets up a field around it. The lines of flux describing the field emanate from one pole, and link round to the other. The flux lines extend out from the Earth’s core through its surface and out into space, weakening with distance from the Earth. I’ve described the form in the diagram below.
Bar magnet analogy
We know that the north end of a compass needle points to the north geomagnetic pole located in the Arctic. Since opposite poles attract, the north geomagnetic pole represents the south pole of the Earth’s magnetic field.
(Earth by WEBTECHOPS LLP from Noun Project (CC BY 3.0))
At any point on the Earth’s surface, the Earth’s magnetic field can be represented by a vector. We can describe this vector by a magnitude and two angles.
The first angle is that between the line to the Earth’s geographic north pole and the line pointing to magnetic north. We term that angle the declination or magnetic variation.
The second angle is that existing between the line to the horizon and the line describing the direction of the field lines. On the surface of the Earth at the geomagnetic equator, this angle is zero. The field lines point to the horizon. At the geomagnetic north pole, the angle is 90°. And the lines of flux at that point plunge vertically towards the south pole. We term this the inclination or dip angle and it varies with latitude.
So, at any point on the Earth’s surface we would describe the Earth’s magnetic field by the vector triple {magnitude, declination, inclination}. The Earth’s magnetic field magnitude is least at the geomagnetic equator and greatest at the poles. It also varies across the Earth’s surface.
Earth’s magnetic field and propagation
Science tells us that a magnetic field rotates the polarisation of a radio wave propagating in a medium.
We know the effect as Faraday rotation, discovered by Michael Faraday in 1845. It’s caused by interaction between the magnetic field set up by the wave in the medium and the Earth’s magnetic field.
The polarisation of the radio wave in the ionosphere is rotated to align with the Earth’s magnetic field (if not already aligned). The degree of rotation depends on the wavelength of the wave, the strength of the field, and the electron density of the ionosphere. The result is calculated by integrating the effect over the distance for which the wave is in the ionosphere and under the influence of the field.
For radio amateurs, the Earth’s magnetic field has greatest effect when the direction of propagation and the field direction align. Hence north-south and south-north paths through the ionosphere are most affected. The effect manifests as polarisation distortion resulting anything up to 30dB signal loss.
Added complexity
The Earth’s magnetic field extends out into space. There it encounters the solar wind. The solar wind is a stream of charged particles emanating from the Sun. The charged particles of the solar wind modify the Earth’s magnetic field to create the magnetosphere.
I’ve shown the resulting magnetosphere diagrammatically adjacent with the big angry Sun on the left and the little Earth in the middle right. The distance out to the edge of the magnetosphere on the Sun-ward side is about 60,000km. The distance to the end of the magnetotail is about 1,500,000km. For reference the Earth’s diameter is about 12,000km.
The Earth’s magnetic field deflects most of the solar wind and its plasma of charged particles. But some plasma is carried down on the field lines at the poles and into the ionosphere.
Once in the ionosphere, positive ions drift westward and negative ions eastward. The result is a ring current around the geomagnetic equator. This current reduces the strength of the Earth’s magnetic field vector at the equator and gives rise to anomalous equatorial propagation.
The plasma descent into the ionosphere and its subsequent transit within the ionospheric regions causes geomagnetic storms. Those storms give us the sporadic and anomalous propagation throughout the ionosphere that all radio hams enjoy and detest in equal measure!
The intensity of the geomagnetic storms is determined by solar activity and is highly variable. Intense solar activity from sunspots results in shock waves from coronal mass ejections when the solar wind is hugely strengthened. These shock waves cause major trauma in the ionosphere, resulting in auroras.
Causal links
Here’s my attempt at identifying all the causal links in ionospheric propagation: an attempt at a single model. A line with an arrowhead indicates a positive influence, one concept on another (A positively influences B). A line with a fletching near the departure end (and no arrowhead) indicates a negative or degrading influence (A degrades B).
Not all concepts are explained. Search above for more.
Ultimately, the Sun promotes and sustains the ionosphere. All other things being equal, we’d have a nice calm ionosphere that can be modelled in line with the heating effect of the Sun. As a result, path performance would be predictable between any two points on the Earth’s surface. That’s the green arrow route.
Simply, for good propagation we need stability. And the Sun, and its effect on the magnetosphere is often anything but stable.
But all things are not equal. As I’ve shown, there are three major routes that show disturbance in the ionosphere from time to time(red, blue, and purple). And then there are three minor routes that provide some anomalous propagation that’s quite fun (brown, orange, and cyan).
So the normal state is about the Sun, and the quirky state is about the how the magnetosphere affects the normal.
This diagram is a work in progress. Feel free to suggest any changes to better explain.