This page will describe how bounded electrons affect EM radiation. These are the type of electrons that are in the troposphere. First we will consider UHF, VHF and EHF radiation. For these frequencies, the pressure, temperature and most importantly, humidity control the value of the index of refraction.
Play the animation, figure 1, to see the effect of an EM wave on a bounded electron.
In this animation, a bounded electron responds to an EM wave. The important point is that the electron moves in phase with the EM signal.The next animation (Figure 2) shows the same effect, but now the EM radiation is shown in wave diagram form, with the bounded electron shown in red responding to an EM wave packet shown in blue. In this and the following animations, the EM radiation is shown as a wave packet, which is a group of waves.
Next we see the reradiation produced by bounded electrons (Animation 3). Water vapor is polar, which means the molecule is distorted with more electrons tending to stay on one side of the molecule.
In this animation, reradiation due to movement of a bounded electron in water vapor is shown on top, and a bounded electron in other common air molecules is shown in the lower part. The initial EM wave packet is shown in blue. The reradiation is shown in red. Water vapor absorbs and reradiates more of the EM signal in UHF, VHF and EHF frequencies than other types of molecules in the troposphere This is because not only is the molecule distorted like other molecules, but its polar orientation also flips, moving the electrons a larger distance. This is why water vapor has a relatively large effect on refraction for these frequencies. There will also be reradiation in the backward direction, but for simplicity the backward reradiation is not shown here or in the following animations.When a charged particle such a bounded electron accelerates, the reradiated EM field is not just in the forward direction. Some radiation goes backwards and some goes at angles above and below the original EM waves. This spreading is shown in animation 4. When an initial EM packet (indicated by the blue waves and phase lines) causes a bounded electron to accelerate, the resulting reradiation (red waves and phase lines) ) spreads out. It is strongest in the same axis as the initial signal, but energy goes in other directions also. The initial radiation is from a distant source, therefore the phase lines are virtually straight and parallel. The reradiation spreads out from the electron, therefore the phase lines are curved.
We will see later how this spreading causes the phase speed to decrease.
The resulting wave that is transmitted through the atmosphere is a combination of the original signal and the reradiation from the electrons. A single electron only absorbs and reradiates a minuscule fraction of the original signal, but many of them act to change the phase speed enough to cause significant refraction. Animation 5 illustrates how this happens. This is a complicated figure and you will need to spend some time understanding what is shown.
In the top panel, three electrons respond to a distant EM radiation source, as indicated by the blue wave and and parallel phase lines. The electrons absorb some of the energy and accelerate. This causes new EM fields (red waves and curved phase lines). For the sake of illustration, only one reradiated wave from each electron is shown. The bottom panel shows a wave diagram of the original wave packet (blue), the effect of all the reradiation (red) and the net effect resultant wave (green), determined by the combination (i.e. addition) of the original (blue) and reradiation (red).
In Figure 5, look at the region where the reradiation waves intersect. The waves that come from the higher electron travel a further distance than the original wave. This means that their phases lines are further back (to the left) than the original phase lines. Notice how the resultant radiation phase lines lag behind the original phase lines. The stronger the reradiation, the greater this lag is. In reality, the reradiation from just three electrons would have a negligible signal, but you can imagine that the huge number of bounded electrons in the troposphere would have an significant effect on the resultant radiation and cause a slowing of the phase speed from the original signal.
We see that it is the interaction between the original signal and the reradiation, that causes the phase speed to slow down from the original signal. The magnitude of this slowing effect depends on strong the reradiation is. The reradiation strength depends on the number of bounded electrons and the bipolar nature of the molecules. The number of bounded electrons is determined by the atmospheric density, which in turn is controlled primarily by atmospheric pressure and temperature. Water vapor is the only significant bipolar molecule in the atmosphere. Therefore the humidity, temperature and pressure control the reradiation, the phase delay and the value of the index of refraction. Regions with vertical gradients (changes) in these parameters cause refraction or bending of radiation on the VHF, UHF and EHF frequency ranges. The vertical gradient of pressure is always quite constant and predictable in the atmosphere. Temperature and humidity constants are highly variable in time and space. Therefore the challenge of understanding and predicting EM propagation in these frequencies boils down to knowledge about how temperature and humidity gradients vary in the atmosphere.
VHF, UHF, EHF
frequencies
The reradiation effects described above depend on the strength of the reradiation,
not on the EM frequency. Therefore within these frequency ranges, the changes
in index of refraction are the same, whatever the frequency. Therefore the bending
of the EM rays does not significantly depend on frequency.
IR, Visible, UV and Higher Frequencies
These frequencies are so great that the bounded electrons are not
able to immediately respond to the fluctuating EM fields. This causes a delay
in the reradiation that is more due to the delayed electron acceleration than
the angle effects described above. The higher the frequency, the more delayed
(relative to the wavelength) the electron acceleration becomes. Therefore in
these frequency ranges, the index of refraction is a function of frequency.
Higher frequencies are slowed down more and have a higher index of refraction.
Therefore, higher frequencies are more affected by index of refraction variations
and refract (bend) more than lower frequencies in these ranges.
These frequencies are also too great for electrons to shift the orientation
of a bipolar molecules, therefore water vapor does not increase the index of
refraction. The reradiation and refraction still does depend on the number of
bounded electrons and atoms in the air, therefore air density essentially is
what controls refraction. This means temperature gradients can create conditions
where IR, visible etc. rays will bend. But unlike VHF, UHF and EHF, the bending
depends on frequency.
HF and Lower Frequencies
These frequencies have wavelengths that are so large that bounded electrons
are not accelerated as much and so their reradiation effects are smaller. Also,
the region of large gradients is usually small compared to the wavelength, which
causes the radiation to pass right through without significant refraction. In
these frequencies ranges, free electrons have a much greater effect than bounded
electrons on the propagation characteristics; these effects will be the subject
of the next page.
We have seen how bounded electrons interact with EM radiation to cause a slowing of the phase speed. Incoming radiation cause the electrons to vibrate, which induces new EM waves. These new or reradiated waves propagate at different angles from the electron sources. The combination of the original radiation with flat phase lines with the reradiation with curved phase lines from the electrons creates a resultant wave that is delayed from the original. This delay increases the index of refraction and causes bending of the rays if gradients in index of refraction are present. These gradients are due to variations in humidity, temperature and pressure.
Questions 1. - 5. relate to VHF, UHF and EHF frequencies only