“The Sun, and the Ionosphere”
The sun is a gigantic, continuously running nuclear fusion reactor, 93 million miles away. It is so far away it takes the light from the sun eight plus minutes to reach Earth. Yet, that light from the sun can still cook the skin right off your shoulders if you are not wearing sunscreen. That energy also can create complex chemical reactions in the ionosphere, that allow radio waves to be refracted, absorbed, or passed, thus allowing us to work DX, or closing the bands. Clearly the sun is delivering quite a large energy punch to the earth’s upper atmosphere, and just as clearly that energy punch is changing how the ionosphere refracts radio waves.
Just how much energy? There is a number scientists call “total solar irradiance”, that number is a measure of how much energy the sun delivers to the top of the earth’s atmosphere, in watts per square meter, with the sun directly overhead. A square meter is about three feet by three feet. In a single square meter, the latest NASA satellite missions have measured total solar irradiance at around 1360 watts. For comparison purposes, your microwave oven delivers around 1000 watts into the cooking chamber. Now imagine how many square meters there are across the top of the atmosphere– that is a lot of energy being pumped into the ionosphere. How much energy? Around 52, 000 terawatts…
That 52,000 terawatts, (yes terawatts), of energy, being sprayed across the entire sunlit side of earth’s upper atmosphere, is what causes the ionosphere to form up each morning. During the day, the upper atmosphere is absorbing vast amounts of energy from the sun,via IR, UV, X-Ray, Gamma Ray, and the solar wind. That amount of energy is enough to start knocking electrons from the outer shell of any number of different compounds all up and down the upper atmosphere. Those knocked off electrons are called free electrons, and hold a net negative charge, while the atoms that have had electrons knocked off, are called Ions, and hold a net positive charged. If you remember from Part I, free electrons are responsible for the bending, or refraction, of RF signals. Usually a downward refraction causes Skip, or DX, and an upward refraction launches our signals into space.
At night, the sun is no longer dumping those terawatts of energy into the upper atmosphere, and the chemical reactions that begun during the day, start to undo themselves due to reduced energy in the upper atmosphere. Remember the atoms that lost electrons, and became ions during the day? Those atoms have a net positive charge, and are still floating around the upper atmosphere, looking for ways to become neutral. How does an ion become neutral? One way is to absorb or eject an electron, or two, to get to net zero charge.
Back to what happens at night– as the sun sets, the energy feeding the upper atmosphere starts to decline, when the energy level is low enough, the negatively charged electrons are able to reattach themselves to the positively charged ions, and they recombine, forming a net neutral atom again. This process is called recombination. The process described above, is why the ionosphere forms up during the day, when large amounts of energy are stripping electrons off atoms, and dissipates at night, when the free electrons can recombine.
Looking at the image to the left, you will see a red line running vertically. That red line indicates electron density, (increasing electron density is to the right, decreasing to the left), verses altitude.
You can see that the electron density changes with altitude. When the density increases rapidly, and then decreases, that is called a layer. The ionosphere consists of four layers, the D, E, F1, and F2 layers. Why different layers and densities? Different chemistry at different altitudes, combined with varying amounts of energy at different altitudes, all changing as the day progresses conspire to create the ionosphere.
Each layer behaves differently. Each layer can, refract, absorbe, or pass a radio wave at some specific frequency and approach angle. As an example let’s look at the D layer, and 80 meters. The D layer absorbs radio waves at 3.5 Mhz., or 80 meters, so when the D layer forms up during the day, you lose refractive sky wave propagation on 80 because the D layer absorbs the radio waves at 80 meters. At 20 meters however, the D layer is transparent, so radio waves at 20 meters pass right through the D layer, and are able to hit the more ionized F1 and F2 layers where they are refracted back to the earth’s surface, and appear as DX.
Conversely at night, (when the D layer recombines and dissipates), that absorption on 80 meters ends, and the 80 meter radio wave is now able to travel higher into the atmosphere, and see the F layer, where they are refracted back to the earth from a higher point, which allows the signal to travel further, thus DX on 80 “goes long” in the evenings. This is why you will hear short range signals, (30 to 200 miles), during the day on 80, and longer range signals, (200 miles and longer), at night.
The D layer is the lowest layer, and forms up only during the day. It is 35 to 70 miles above the earth’s surface, and has several sublayers, each being affected by different energy events. The D layer forms up at sunrise, and dissipates at sunset.
D layer from 40 to 50 miles
Starting at 40 miles above the earth’s surface, is a 10 mile wide band, between 40 to 50 miles. This band is primarily ionized by galactic cosmic rays.
D layer from 50 to 55 miles
Between 50 to 55 miles high represents a 5 mile thick band of ionization. This band is formed primarily by Lyman Alpha line radiation.
D layer above 55 miles
Yet another band about 5 miles thick exists starting at 55 miles above the surface of the earth, ionization in this band is caused primarily by hard X-Rays.
60 to 75 miles up the E layer exists. The E layer is roughly 3 to 6 miles thick, and it is primarily ionized by soft X-Rays, and Extreme Ultraviolet, also known as EUV. During the day, the D and E layers are practically the same. The E layer is more highly ionized at local noon than any other time. The E layer ionizes quickly at sunrise, and dissipates just as quickly at night. There were experiments in the 80s, indicating that the E layer does not totally dissipate at the peak of a sunspot cycle.
Starting at around 95 miles up, you encounter the F layers. The E layer is ionoized by EUV, or Extreme Ultraviolet light. During teh daytime, the F layer splits into two layers, the lower, (95 miles high), F1 layer, and the higher, (125 mile high), F2 layer.
The F1 layer is quite weak, and, like the E layer, plays little role in propagation of RF. The F1 layer hits peak ionization at around local noon, when the sun is at it’s highest point in the sky. During winder the F1 layer and the F2 layer merge. This merging is due to lower energy input into the upper atmosphere. The F2 height can vary between 100 and 300 miles, depending on season, and solar conditions. The recombination of electrons and ions in the F2 layer happens slower than in any other ionospheric layer, hence, the F2 layer can, and does last all night a lot of the time. The F2 layer is the most highly ionized layer of all, and as such, plays the largest role in refracting RF signals back to earth.
Each layer can either absorb, refract, or pass radio waves, depending on frequency, and angle of incidence to the specific layer. Each layer is affected by the energy impinging upon it, and each layer is affected by the type of atoms in the upper atmosphere. Each layer is profoundly affected by solar events.
We have learned from Part I what the ionosphere is, how it is created, and destroyed, and how it can affect radio propagation. Part II, has covered how the ionosphere is affected by the sun’s energy, and what changes take place between day and night. Part II has also covered how layers are formed, and what happens between daytime and nighttime to each layer.
Parts I, and II, have been written with the intent of getting you familiar with how the ionosphere operates day to day, Next month, I will cover day to day changes in propagation, based on the changes we learned about in parts I, and II.
Graphic Credits: All graphics were created by either NASA, NASA visualization studio, or Wikipedia. All graphics are used with permission, or via Creative Commons licensing.