Earth’s Side-kick: The Atmosphere

The atmosphere is planet Earth’s number one side-kick. From protecting the Earth from harmful rays from the sun to maintaining a habitable climate, the atmosphere can do a whole bunch of things. One of these is to allow radio waves to travel great distances across the planet. So how is this done? First, we must introduce the many layers of the atmosphere. 


Figure 1: A picture of the atmosphere

The Ionosphere

The atmosphere consists of approximately 5 layers. The distinction between layers is based on the distance from the surface. The layers closest to the surface are the troposphere, stratosphere, and mesosphere which are not important to us for the purpose of this article. What is important is the ionosphere which consists of the three outer layers (a little bit of the exosphere, the thermosphere, and a bit of the mesosphere). Its location (closest to the sun) plays an important role for the propagation of radio waves because the ionosphere consists of a whole bunch of electrons. So let’s talk about electrons and why they are so important for us.


Figure 2: The five layers of the atmosphere.

Electrons! Electrons! And more electrons!

Everything in the universe is made up of atoms. Everything. You, me, the computer screen that you’re reading this article on. The atmosphere is no different. Within the atoms in the atmosphere are electrons. These are one of three components that make up atoms. The other two are protons and neutrons. A visible representation of the atom is provided below. The electron is “attached” to the atom, in that it belongs to it; as opposed to “free” electrons which do not belong to any atom and float around in the atmosphere. Sometimes the amount of free electrons can increase significantly. This is due to the sun. 


Figure 3: A simple representation of the makeup of an atom.

The Battle for electrons

The sun is constantly hitting the earth with radiation which is filled with a whole bunch of stuff (including electrons!).  This constant battering from the sun provides the atmosphere with extra electrons. This process is called ultraviolet radiation. It is just a fancy way to describe the sun’s radiation hitting the atmosphere, going through its many layers, and leaving electrons behind. (What is actually happening is the energy of the radiation is large enough that it breaks up the bonds of atoms, thus shooting electrons out in the process - but our description above is an adequate approximation.) 


Figure 4: Energy from the sun hits atoms and electrons shot out, thus becoming “free”


However, at the same time, the opposite is happening. All these now floating electrons, not connected to any specific atom, begin to attach themselves to atoms. As they are constantly moving, they sometimes meet an atom which will accept them. This reduces the number of free electrons in the atmosphere. This process is called recombination. These two processes are constantly occurring and many factors come into play which decide which of these two events win. Some of these factors include the time of the day, the season, and the distance from the surface. So what do these processes and electrons have to do with radio propagation?


Radio Waves & Propagation

Radio wave are emitted from some sort of antenna towards another antenna, usually many miles away. Since the earth is not flat, radio waves cannot travel strictly horizontally to reach their destination (the receiving antenna). So another method must be used. In this case, the transmitting antenna will emit some of the signal towards the atmosphere. And what happens next is dependent on the amount of electrons in the atmosphere at the time of the collision between the atmosphere and the radio wave. 

Figure 5: The path of radio waves from transmitting to destination


The Ionosphere: The Toll-Booth

The free electrons in the ionosphere are not just sitting around, doing nothing. Most of the electrons, due to external factors, oscillate at a specific frequency (if the topic of frequency is unclear, please visit our article on frequency: "Waves I"). This specific frequency at which the free electrons oscillate is called the critical frequency or plasma frequency. The following equation describes this frequency as: 

 fcritical ≈ 9×√(N)  

where N is the electron density (a value which increases when the amount of electrons increase, and vice-versa). A good example is the toll booth metaphor. Think of the critical frequency as the toll one must pay to get onto a tunnel or bridge. This toll will change sometimes depending on external factors. Now an incoming radio wave, send from the surface, also oscillates at a specific frequency. This frequency is something that the user usually sets up beforehand. Next, the radio wave is emitted and travels towards the ionosphere. If the frequency of the wave does not match the critical frequency, the wave is reflected back to the surface. This is the same in the toll-booth metaphor. If the passenger does not have the exact amount of cash for the toll, then they cannot pass and are sent back. Any amount more than the toll, the passenger is allowed to pass freely. Back to our case, any radio wave with a frequency higher than the critical frequency will pass freely through the atmosphere into outer space; and any radio wave with a frequency lower is reflected back to the surface. 


Figure 6: The reflection of radio waves at the ionosphere


This is where the importance of free electrons come into play. The higher the amount of electrons, the higher the critical frequency. An increase in the critical frequency allows the transmitted radio wave to reflect off the ionosphere and reach its destination with ease. Less electrons equates to a lower critical frequency, and now the same radio wave as before may or may not get reflected this time.  


Our friend: reflections 

We want reflections of radio waves to occur at the ionosphere. The other option allows the radio waves to pass right through, so we are basically losing some part of our signal. So since reflection is our friend, we want as many electrons up there. More electrons means we need ionization to win the battle between recombination. This is a quick summary of how propagation of radio waves are affected by the ionosphere. 











Figure 1: http://news.mit.edu/2016/oxygen-first-appearance-earth-atmosphere-0513

Figure 2: https://www.thinglink.com/scene/596057785884475394

Figure 3: https://www.pinterest.com/pin/123497214756944879/

Figure 4: http://www.medizinischestrahlung.de/fakten-zum-thema-strahlung/was-ist-strahlung/ionisierende-strahlung/

Figure 5: https://sites.google.com/a/britishschool.com.ua/pe-y11-clone/curriculum-resources/science/term-1-week-3

Figure 6: https://coherence.wordpress.com/tag/radio-waves/