Radio waves propagation & ionosphere

[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?

 

[Born2bwire]

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.

 

[ctapobep]

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..

 

[jtbell]

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.

 

[chrisbaird]

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.

 

[ctapobep]

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.