Why does the sun affect radio wave propagation?

[Tech2025]

I noticed HF radio waves travel farther and easier when the sun is out rather then during the night. Why?

 

[ZapperZ]

I noticed HF radio waves travel farther and easier when the sun is out rather then during the night. Why?

Wait, isn't "the night" is also when the "sun is out"?

Zz.

 

[Gene Naden]

One factor is the "skip." Radio waves can be reflected back down towards the Earth by the ionosphere. The ionosphere is ionized by radiation from the sun.

 

[gleem]

High frequency radio wave tend to move in a straight line so if it was not for the ionosphere their range would be limited to the height of the transmitting and receiving antennae. The ionosphere helps propagate the radio waves many thousands of miles depending on the power of the station.

 

[Bystander]

ionosphere

..., or any other discontinuity, density/temperature/compostition/whatever.

 

[jasonRF]

What specifically have you observed? What part of the HF band have you noticed this for?

The behavior actually depends a LOT on where you are in the HF band, which covers 3-30 MHz, as well as the specific ionospheric conditions. At night the ionosphere is less dense so the higher frequencies simply escape into space, so are not refracted back down to the ground at all. On the other hand, there is much less loss in the lower ionosphere (the D region) at night, so if you are in the lower part of the HF band you will have longer ranges at night than during the day. Then there is the issue of skip zones; if you operate at the same frequency for night and day, at night your minimum range at which you can receive signals via sky-wave will often be larger (an exception may occur if you have a dense layer of ions in the lower ionosphere - "sporadic E" - which can be long-lived at night). Ionosphere propagation at HF is not simple ...

jason

 

[sophiecentaur]

I noticed HF radio waves travel farther and easier when the sun is out rather then during the night. Why?

Several factors at work here. With no cosmic rays, there would be no ionisation. At low intensity level, the ionisation is just at the high layers and, because the gas is low pressure, the ions spend a long time before recombining and the free electrons behave a bit like the electrons in a metal. They reflect radio waves. As the intensity of cosmic rays goes up (daytime) the lower levels become ionised and the electrons which the radio waves set in motion, collide with air molecules (more of them ) and lose the energy from the wave. (Poor propagation) The high ionisation layers take longer to recombine so the higher levels hang around longer after dark (and the sun 'sets' later up there too.)
Then there's the frequency effect. Lower frequencies are reflected at lower levels and, in the day, they will be absorbed. Higher frequencies penetrate further and can pass right through without reflection. Also the angle is a factor. VHF skywave signals get right through the ionosphere (except for occasionally at low elevation)
Medium Frequency skywaves never get anywhere in the daytime so the service area of a transmitter is all ground wave. The network is well behaved. At night, they get through the low layers where the ions have all recombined and make it to the higher levels where they can be reflected and travel thousands of miles (nasty nasty) and interfere with distant ground wave transmissions.
Depending on which frequency you are dealing with, the angle range for which the reflection works will be different. High frequencies are no good for short hops because the escape at high elevations. Lower (HF) frequencies can get to nearer skywave service areas but can be absorbed on longer paths worse than Higher frequencies. You have to pick your frequency band just right for the time of day and wanted range.
The HF spectrum used to be fought over intensely because it's a very international resource.
Interestingly, in the cold war, the BBC used to transmit to Eastern Europe at the highest frequencies available at any time because the low elevation signals from UK could bounce down to the ground in Hungary etc, but the signals from local jamming stations were high elevation and would go straight up and not bounce back to receivers in nearby countries.
PS that's repeating a lot of what @jasonRF said.

 

[DrClaude]

Wait, isn't "the night" is also when the "sun is out"?

I take "the sun is out" to mean it is shining, so not at night.

 

[davenn]

With no cosmic rays, there would be no ionisation.

No, that isn't correct.
Ultraviolet radiation from the sun is the main cause of the ionisation of the Earth's ionosphere …
hence the difference in propagation seen between day and night

The Ionisation caused by cosmic rays occurs below ~ 30 km, which is well below the height of the ionospheric layers

an interesting article on cosmic ray ionisation in the lower atmosphere

https://www.researchgate.net/publication/225150402_Ionization_of_the_Earth%27s_atmosphere_by_solar_and_galactic_cosmic_rays
Dave

 

[davenn]

I take "the sun is out" to mean it is shining, so not at night.

I suspect Zz was having a play with words …. maybe?

 

[sophiecentaur]

No, that isn't correct.
Ultraviolet radiation from the sun is the main cause of the ionisation of the Earth's ionosphere …
hence the difference in propagation seen between day and night
. . . . . . .
Dave

That's right of course - I forgot the details of ionisation causes. I should have used the term "ionising radiation" and kept on the safe side!
It makes good sense about cosmic rays. There are relatively few and their effect can only be significant where there is a dense atmosphere. If there really were enough to have the same effects as UV from the Sun (which must have an energy density of many W/m²), we would surely all be frazzled.

 

[sophiecentaur]

I suspect Zz was having a play with words …. maybe?

When I leave a room, I turn on the Dark Switch.

 

[davenn]

That's right of course - I forgot the details of ionisation causes. I should have used the term "ionising radiation" and kept on the safe side!
It makes good sense about cosmic rays. There are relatively few and their effect can only be significant where there is a dense atmosphere. If there really were enough to have the same effects as UV from the Sun (which must have an energy density of many W/m²), we would surely all be frazzled.

no probs

an interesting thing with cosmic rays is that during solar minimum, a lot more of them penetrate Earths atmosphere
Solar max keeps more of them at bay

D