Recently, I wrote about how our magnetic field effects skywaves polarization, turning everything elliptical. Let’s dig deeper.
Three things you need to know. First, free electrons in the ionosphere get captured by and spin around geomagnetic lines of force. You will find that the rate of spinning is called the gyrofrequency, which varies as 1.2 MHz ± 0.5 MHz. This is a natural resonance in the ionosphere, similar to but different from the critical or plasma frequency, which determines when and if transmitted signals bounce back.
Second, the magnetic force lines change wave polarization, as described earlier. Our vertical or horizontal skywaves take on elliptical polarization, and remain so all the way back to earth.
But third, and this is a really neat part, the gyrofrequency effect creates two modes of wave propagation. Our signal is split into ordinary and extraordinary waves, each with different characteristics. You will find the math for this is really scary, but the basic function is f ± fG cos θ, where f is your frequency, fG the gyrofrequency and θ is the angle of incidence between the signal ray and the magnetic force line.
As a result, the ionosphere becomes birefringent, which means that each mode has a different index of refraction.
Enough heavy stuff. Basically, each of the O and X waves follow different paths over different distances, and experience different attenuation. Our ordinary wave best approximates what would happen in the absence of the magnetic field.
Magnetic Field Effects Skywaves – And More
You may not have noticed, but over the past few years, the geomagnetic north pole has been shifting from Canada to Siberia at an increasing rate. This is probably due to shifting molten blobs in the Earth’s core.
But, as a result, all of the standard maps of gyrofrequency variation are out of date. And this effects the results of propagation and ray tracing software.
(Incidentally, the gyrofrequency actually creates 3 or more modes of propagation, but these aren’t relevant at HF.)