Why the Sun decides which band is open
HF long-distance propagation depends on the ionosphere, and the ionosphere exists because solar ultraviolet and X-ray radiation ionise the upper atmosphere. More radiation means more ionisation, which means the ionosphere refracts higher frequencies back to Earth instead of letting them escape.
That is the entire chain, and every number below is a proxy for one part of it.
Sunspot number
How does a higher sunspot number affect HF propagation?
Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies
Note the specific threshold. The bands below 20 MHz work reasonably at any point in the solar cycle; it is 15, 12, and 10 metres that live or die by it.
Which of the following are the least reliable bands for long-distance communications during periods of low solar activity?
15 meters, 12 meters, and 10 meters
And the contrast that makes 20 metres the workhorse band:
At what point in the solar cycle does the 20-meter band usually support worldwide propagation during daylight hours?
At any point.
If you learn one band’s behaviour, learn that one: 20 metres is open somewhere, almost always.
Solar flux index
What is the solar flux index?
A measure of solar radiation with a wavelength of 10.7 centimeters
Measured at 10.7 cm — 2800 MHz — because that emission tracks the ionising radiation well and, unlike sunspot counts, can be measured through cloud by an instrument rather than counted by a person.
Rough reading: below about 70 is a dead solar minimum; above 150 the high bands open reliably; above 200 is exceptional.
The two geomagnetic indices
Solar radiation builds the ionosphere. Charged particles from the Sun disturb it, and two indices track that disturbance.
What does the K-index measure?
The short-term stability of Earth’s geomagnetic field.
What does the A-index measure?
The long-term stability of Earth’s geomagnetic field.
K is short-term, A is long-term. Both go up when conditions are disturbed, which is the opposite direction from solar flux — high flux is good news, high A and K are bad.
K runs 0 to 9 on a three-hourly basis; A is a daily figure derived from the K values. A quiet day is K of 0 to 2 and A under about 10.
Disturbances, and how fast each arrives
Three kinds of solar event, and the timing is the exam’s favourite detail.
Sudden ionospheric disturbance (SID) — a solar flare’s ultraviolet and X-ray burst.
Approximately how long does it take the increased ultraviolet and X-ray radiation from a solar flare to affect radio propagation on Earth?
8 minutes
Eight minutes, because that radiation travels at the speed of light. There is no warning: by the time you see the flare, the effect has already arrived.
What effect does a sudden ionospheric disturbance have on the daytime ionospheric propagation?
It disrupts signals on lower frequencies more than those on higher frequencies
The flare’s radiation supercharges the D region, which absorbs — and absorption is strongest at low frequencies. The low bands go silent while the high bands carry on.
Coronal mass ejection (CME) — an actual cloud of charged particles.
How long does it take a coronal mass ejection to affect radio propagation on Earth?
15 hours to several days.
Particles travel far slower than light, so there is warning. This is why space weather forecasts exist and are worth reading.
How is long distance radio communication usually affected by the charged particles that reach Earth from solar coronal holes?
HF communication is disturbed.
Geomagnetic storm — a temporary disturbance in Earth’s geomagnetic field, which is what arriving particles produce.
How can a geomagnetic storm affect HF propagation?
Degrade high-latitude HF propagation.
High latitude specifically: the particles funnel toward the poles along the field lines, so polar paths suffer first and worst.
And the compensation:
How can high geomagnetic activity benefit radio communications?
Creates auroras that can reflect VHF signals.
The event that ruins HF opens VHF auroral propagation. A geomagnetic storm is bad news on 20 metres and an opportunity on 6.
The 27-day cycle
What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?
Rotation of the Sun’s surface layers around its axis.
The Sun rotates roughly every 27 days, so an active region that faced Earth today faces it again in about a month. If conditions were exceptional four weeks ago, it is worth being on the air.
The dashboard, summarised
| Number | Measures | Good is |
|---|---|---|
| Sunspot number | active regions | high |
| Solar flux index | 10.7 cm radiation | high |
| A-index | long-term geomagnetic stability | low |
| K-index | short-term geomagnetic stability | low |
Check yourself
- You see a large flare on a solar imaging site. How long until the HF bands notice, and which bands first?
- K-index is 6 and solar flux is 180. Good day on 15 metres to Europe?
- Why do the high bands, not the low ones, track the solar cycle?
Answers
- About 8 minutes — the radiation travels at light speed — and the lower frequencies are disrupted most, because the D region absorbs.
- Probably not for a polar path: high flux is promising but K of 6 is a substantial geomagnetic storm, which degrades high-latitude propagation. A southern path may be fine.
- The ionosphere always refracts the low bands. Whether it refracts 21 or 28 MHz at all depends on how strongly it is ionised, which is what the solar cycle controls.