Last Updated on May 28, 2024 by John Berry
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! I’ve called these Sun states the normal Sun and the quirky Sun.
If the Sun was ‘normal’, everything would be predictable. We could use R12/SSN, the 12-month Smooth Sunspot Number, to give an estimate of the best frequency and likely received field strength for any path between any two points on the globe. And we could even use better indices to optimise our predictions, like R5, the five-day rolling average sunspot number, in place of R12. But everything is not normal. We must instead account for the quirky Sun.
The Sun is a ball of nuclear fusion. I’ve described how the surface layer of the Sun changes over an 11-year cycle, revealing first no sunspots, then a maximum sunspot count. Then back to zero again. While the sun is cooking up sunspots, it’s being more active. And that activity adds semi-periodic effects – effects that correlate roughly with the sunspot number.
The quirky Sun
The main protagonists in this quirkiness are:
a) the incidence on the ionosphere of heightened electromagnetic radiation causing D region attenuation;
b) the incidence of high-energy solar particles causing polar cap absorption and;
c) the solar wind impact on the magnetosphere causing ionospheric storms and auroral effects.
The image below shows this graphically.
I’ll discuss these in turn.
Electromagnetic radiation
HF blackouts occur because heightened electromagnetic radiation from the Sun following flares increases D region absorption. They occur during daytime.
For those not yet familiar with the D region, it’s the ionospheric region closest to the Earth. If it absorbs radio waves from Earth, those waves won’t have a chance to be refracted by higher regions. Hence those higher regions will never have chance to support radio communications.
The effect of these blackouts is greatest at the lower end of HF. The maximum frequency of blackout depends on the magnitude of the electromagnetic radiation. The image below shows the effect. There are two parts to the image. On the left is a map showing the focus of the absorption – the Sun’s area of effect on Earth. On the right is the excess attenuation in decibels (dB) caused by D region absorption against frequency.
Flares and blackouts are reported on SpaceWeatherLive.com. D-region absorption is available from NOAA/SWPC. It’s interesting to receive the flare report, then the blackout report, and then to observe the effects on the HF bands. Fades don’t affect the whole of the Earth’s surface, so it’s possible to receive a blackout report, but there be no local effect.
High energy solar particles
The Earth’s magnetic field causes plasma – charged particles travelling on the solar wind – to be directed to the poles.
On arrival at the polar regions, the particles cause local D region absorption. Simply, the greater the particle energy, the greater the polar cap absorption. This affects any skywave propagation expecting to exploit reflection points over the poles.
The action of high energy solar particles on the polar D region of the ionosphere is independent of auroral effects.
Geomagnetic interaction
The magnetic fields of Sun and Earth interact. When the Sun is at its most quirky, this interaction can cause storms in the Earth’s magnetosphere.
The solar wind results from high energy material escaping from the Sun’s gravity, then attracted toward Earth by its gravity. Energy escape follows solar flares and coronal holes.
The first outcome is a disturbed (stormy) magnetosphere and disturbed ionosphere. This disturbance is transmitted on winds above the E region. The resulting diminished F region electron density gives associated reduced support for HF propagation.
The second is a locally enhanced lower ionosphere. I’ve discussed on the pages about auroral communications how the plasma creates E region anomalies from the poles towards the equator. Whether or not a solar flare gives rise to an aurora depends on the energy in the solar wind. It takes two to three days for the plasma to reach Earth, and for a magnetic storm to develop from it.
The degree of disturbance of the Earth’s magnetic field (magnetosphere) is measured by the worldwide Kp index (reported in indices from 0-9). The corresponding local effect is reported in dst, the disturbance storm time (reported in nT, from 0 to about 500, against time) from local magnetometers.
A quiet Kp of 1 or 2 means that the ionosphere will perform normally and support HF communications. Anything more is disturbed with reduced support.
A high Kp and a sudden peak in dst to, say 500nT, bodes well for auroral communications via the E region. This is dependent on existing excitation of E region in the auroral oval. Useful radio auroras typically occur following a heightened dst occurring at mid-day.
Conclusion – the quirky Sun
If there were no quirky effects, the Sun, and hence the ionosphere, would behave normally. All would be predictable.
But there are quirky occurrences. The Sun is prone to random outbursts – outpouring of radiation, particles, and plasma. Those random outbursts interrupt normality.
Those outbursts are probabilistic. Those quirky effects correlate with the sunspot number, so Kp correlates with R12/SSN. That’s no surprise since the number of sunspots is a measure of general solar activity. We can say that quirky behaviour is more likely at higher SSNs. We can say a quirky effect is imminent because we can observe the Sun and sense flares and the like. But we can’t forecast quirkiness with any foresight or accuracy.