Last Updated on May 1, 2025 by John Berry
So what’s up there
But what is the ionosphere? What is up there and how does it reflect, refract, or scatter radio waves? Are ions the big thing, or is there something more subtle going on? And why does the ionosphere behave as it does, sometimes supporting propagation for days or only supporting an opening for minutes? Here’s the answer.
Ionosphere structure
The Sun causes the gases of the ionosphere to ionise. The degree of ionisation and the resulting electric charge density differ with height. Molecules higher up and closer to the Sun get cooked more, and hence produce more ions. In the end, it’s all about charge density. Here’s a description of the ionosphere that will aid understanding.
Reflection, Refraction or Scattering?
There’s huge ambiguity across the radio amateur books, magazines, and sites when it comes to describing how it is that the ionosphere returns radio waves to Earth. The ambiguity is this. Many writers routinely talk of reflection. Others call it scattering. And some refer to the mechanism as refraction. So, which is it? Here’s an argument and conclusion.
Background to ionospheric predictions
A prediction asks the question, ‘What DX can I work today?’ In this page I give the background to ionospheric predictions, showing the engine of the prediction. There are two tools popular in making predictions: Rec. ITU-R P.533 and VOACAP. The old FORTRAN code is typically wrapped to give modern-looking tools. This page references the Recommendation.
The normal Sun
There’s one key variable in the formulas for field strength and Maximum Usable Frequency (MUF). This variable is the R number, also known as the Smooth Sunspot Number (SSN). Every 11 years or so, the Sun cycles from no sunspots to a maximum number of sunspots and back again. Simply, the more sunspots, the better the skywave propagation, so the higher the R or SSN count, the better. Here’s how it works.
The quirky Sun
Skywave propagation variation correlates with ionospheric behaviour. And the ionosphere’s behaviour depends (mainly) on the Sun’s behaviour. The Sun is at any time behaving either normally or quirkily! The Sun is prone to random (quirky) outbursts – outpouring of radiation, particles, and plasma. Those random outbursts interrupt normality. I’ve called the states the normal Sun and the quirky Sun.
Making ionospheric predictions
Making ionospheric predictions about whether HF ionospheric paths (or circuits) will work is straightforward. It involves running a program encompassing the method outlined in my page Background to ionospheric predictions. Here I describe VOACAP Online by OH6BG, HZ1JW and OH8GLV.
Equatorial ionisation anomaly
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. It has significant bearing on trans-equatorial ham radio communications enabling chordal hops of up to 12,000km.
Ionosphere bibliography
A list of the literature and books referred to in writing the various pages about the ionosphere and skywave propagation.