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Thread: What happens when various radio waves interact with the ionosphere?

  1. #1 What happens when various radio waves interact with the ionosphere? 
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    To my understanding, different wavelengths of radio waves interact with the ionosphere differently.
    - Short Radio Waves/Microwaves reflects off the ionosphere
    - Medium Radio Waves passes straight through the atmosphere
    - Long Radio Waves refracted by the ionosphere

    At an atomic level, what is happening when these various wavelengths interact with the ionopshere? What causes some to transmit, reflect or refract?

    Thankyou


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    As you can see above, they are not all reflected, it is just an electromagnetic wave, so mirror and prism effects still apply.

    What you are asking me is quite tough. At a molecular/atomic level they don't teach optics. But i will try none the less.

    The ionosphere contains slightly charged particles of gas, mainly nitrogen. This slight charge has the ability to bend the waves around them. They only need to bend slightly because a slight bend will increase the number of charged air they find, and because the density of this charged air slowly grows, then fades, they usually bend inwards (to earth).

    The differences between short and long waves is again, the odds of hitting an ion. If the wave has the length of the interference path of the ion, they reflection will be near perfect. If it nearly intersects with it, there won't be much reflection.

    Hope i told it right.


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    Quote Originally Posted by jeffreyquach View Post
    To my understanding, different wavelengths of radio waves interact with the ionosphere differently.
    - Short Radio Waves/Microwaves reflects off the ionosphere
    - Medium Radio Waves passes straight through the atmosphere
    - Long Radio Waves refracted by the ionosphere

    At an atomic level, what is happening when these various wavelengths interact with the ionopshere? What causes some to transmit, reflect or refract?

    Thankyou
    At the most fundamental level, what matters is the availability of free electrons. Reflection is better thought of as re-radiation. And for radiation to occur, you need to wiggle charges. Electrons fill the bill nicely.

    Solar radiation is the pump that drives ionisation, which explains why propagation varies throughout the time of day. And, of course, you need something to be ionised in the first place, which explains why propagation depends on atmospheric layers in the first place.

    Whether transmission through, or reflection from, an ionised layer occurs depends on the wavelength (frequency) and electron density. The higher the electron density, the more it acts like a metal (it reflects more easily). There is a critical frequency, known as the plasma frequency, below which reflection dominates. The plasma frequency decreases as electron density increases.

    Above the plasma frequency, the electrons are effectively too sluggish to reradiate, and the ionised layer appears more and more transparent to radio waves. For the most part, the difference between plasma frequency and radio wave frequency explains the main frequency dependency of propagation.

    In addition to transmission and reflection, refraction is certainly important. The refraction (bending) of a wave is dominated by the electron density variation with altitude. As the electron density varies, so does the propagation velocity, This situation is analogous to light propagation through water or glass, with the well-known lens effects seen there. The same bending occurs with radio waves propagating through the ionised layers.

    Volumes have been written on the subject, but hopefully the above gets you pointed somewhat in the right direction. Please do not hesitate to post back if you want further clarification.
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    Quote Originally Posted by tk421 View Post
    Whether transmission through, or reflection from, an ionised layer occurs depends on the wavelength (frequency) and electron density. The higher the electron density, the more it acts like a metal (it reflects more easily). There is a critical frequency, known as the plasma frequency, below which reflection dominates. The plasma frequency decreases as electron density increases.

    Above the plasma frequency, the electrons are effectively too sluggish to reradiate, and the ionised layer appears more and more transparent to radio waves. For the most part, the difference between plasma frequency and radio wave frequency explains the main frequency dependency of propagation.
    I'm sorry but I dont fully understand this. I understand that plasma or critical frequency is the frequency, determined by solar activity, that separates waves that either reflect or pass through.

    In your response it says frequencies below the critical frequency, reflect. And frequencies above the critical frequency pass through. However to my understanding, generally, it is the relatively higher frequencies (microwaves) that reflect and the relatively shorter (medium radio waves) wavelengths that pass through.

    Could you please clarify this? And please correct any mistakes I have in my information. Thanks
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