# Radio waves propagation & ionosphere

• ctapobep
In summary: So if someone could explain this topic in a way that is a little more accessible, that would be much appreciated. In summary, the ionosphere reflects short radio waves because of the oscillation of electrons with the same frequency. This oscillation changes the direction of the wave.
ctapobep
I've read that ionosphere mirrors the short radio waves (25-30 MHz). So 2 questions:
1. Why this happens? There is material on the internet, but it's rather complicated for me so far, I've just started to study physics. So I'd appreciate if someone explains this in more or less simple way.
2. Why this is true only for a particular diapason: 25-30 MHz and why other waves go through the ionosphere without problems?

The ionosphere is a plasma. What we find with electromagnetic waves traveling through plasmas is that low frequency waves cannot propagate but high frequency can. The reason is that at low frequencies, the electrons can oscillate in phase with the waves which allows them to cancel out these waves. However, at high enough frequencies the electrons cannot oscillate fast enough to correllate with the waves due to their inertia. So what you see is that above a given frequency, electromagnetic waves can propagate in the plasma but below this frequency they cannot. Thus, waves incident on the plasma (ionosphere in this case) will reflect strongly if they are below the plasma frequency and will transmit into the plasma if they are above this frequency.

Okay, thanks for your answer. Have one further question:
The reason is that at low frequencies, the electrons can oscillate in phase with the waves which allows them to cancel out these waves.
And why the reflecting per se happens? What makes the wave reflecting? Now I understand that the reason is oscillation of electrons with the same frequency, but still don't get why this oscillation changes the direction of the wave..

Oscillating electrons radiate electromagnetic waves. That "new" radiation interferes with the incoming radiation. The effect of the interference (e.g. constructive or destructive) depends on the amplitude and phase of the "new" radiation with respect to the incoming radiation, which in turn depend on the frequency of the radiation and the properties of the plasma.

ctapobep said:
Now I understand that the reason is oscillation of electrons with the same frequency, but still don't get why this oscillation changes the direction of the wave..

The same reason that you can see your reflection in a mirror. The oscillating charges are free to move enough when experiences a force due to the electric field in the impinging electromagnetic wave, that they "catch" the wave before it can travel any further, but then because they are still moving, they "throw back" the reflected wave. I realize these are imprecise terms, but you have to get into the math to understand the proper terms correctly.

Interestingly, the plasma frequency (the threshold frequency below which waves are reflected and above which waves are transmitted) in general is proportional to the average charge density of the plasma. That means that if there are more electrons, then higher frequencies get reflected. Depending on the time of day (more sunlight means more ionization of atmospheric molecules, creating more charged particles), certain frequencies will or will not be reflected by certain layers of the atmosphere.

Guys, thanks a lot for your help. I don't understand the full picture of course, but I understand much more than before :) If you know any good resources where I can find this information, I'd be grateful if you point me there (also interested in this maths that can explain the picture in more depths).
I find that most of literature in the internet is pretty hard to understand, seems like people write articles to look smart, not to share their knowledge.

## What are radio waves?

Radio waves are a type of electromagnetic radiation that can travel through space. They are used for communication and can range in frequency from a few hertz to hundreds of gigahertz.

## How do radio waves propagate?

Radio waves propagate through the air or outer space in a straight line. They can also be reflected or refracted by various objects, such as the Earth's atmosphere or the ionosphere.

## What is the ionosphere?

The ionosphere is a layer of the Earth's atmosphere that is ionized by solar and cosmic radiation. It is responsible for reflecting and refracting radio waves, allowing them to travel long distances.

## How does the ionosphere affect radio wave propagation?

The ionosphere can affect radio wave propagation by reflecting and refracting the waves, causing them to bounce around the Earth's atmosphere. This can lead to interference and changes in signal strength.

## What factors can impact radio wave propagation through the ionosphere?

There are several factors that can impact radio wave propagation through the ionosphere, including solar activity, atmospheric conditions, and the frequency and angle of the radio waves. Changes in these factors can affect how radio waves travel through the ionosphere.

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