Skip to main content

The Ionosphere

What Is the Ionosphere?

The ionosphere is the layer of the earth´s atmosphere that extends throughout the mesosphere, thermosphere, and exosphere. It starts at an altitude of about 60 km and goes all the way up to about 800 km. It is so named because it's a layer in the atmosphere where ions are present. While molecules composing the atmosphere are present in a combined or neutral state in the ionosphere, these molecules are split or ionized by solar radiation (ultraviolet light). Its different regions are categorized as peaks of ionization levels, being denser based on altitude; the higher they are in the atmosphere, the more electrified they become.

To identify these layers, peaks, or regions, they have been designated by distinct letters. E, which stands for electrified was the first historical designation made, as it was the first region discovered. The D region, which is the lowest one, and the F region, the topmost region, were discovered later. There is another region designated with the letter C, but this region is not sufficiently ionized and therefore doesn't have any real effect on radio communications.

Ionization of the Atmosphere

In the ionosphere, extreme ultraviolet and x-ray solar radiation, along with cosmic rays and charged particles, ionize the atoms and molecules present, creating a region of positively charged ions and free electrons. It's the free electrons that cause high-frequency radio waves to be refracted and reflected back to the surface of the earth. The higher frequencies reflected depend on the density of free electrons in the ionosphere.

Cosmic rays originate in the sun but may also come from other bodies outside the solar system and are then known as galactic cosmic rays. They are high-speed particles-atomic nuclei or electrons. These particles interact with the ionosphere at all times but most commonly at night.

The Ionospheric Layers

The ionosphere comprises three distinct regions known as the D, E and F regions. While the F region exists during both day and night, the D and E regions may vary in density. During the day, the D and E regions are more heavily ionized by solar radiation and so does the F layer, which develops an additional weaker region called the F1 region. So, the F region consists of the F1 and F2 regions. The F2 region is present both day and night and is responsible for the refraction and reflection of radio waves.

More on D, E and F Regions

The D layer is the lowest one and it's the one radio waves reach when traveling up the atmosphere. It starts from about 50-80 km (31-50 miles). It´s present during the day when ultraviolet radiation from the sun interacts with the molecules and atoms, stripping one electron off. After sunset, as solar radiation decreases, electrons recombine and this layer disappears. The ionization of the D region is due to a form of radiation known as Lyman-series radiation at a wavelength of 121.5 nanometers and ionizes nitric oxide gas present in the atmosphere.

The D layer attenuates radio signals passing through. The level of attenuation depends on the wavelength of radio signals. Lower frequencies are affected more than higher ones. This varies as the inverse square of the frequency, meaning that lower frequencies are prevented from traveling further, except at night when the D region dissipates.

The E region is the one that follows the D above the atmosphere. It´s found at an altitude of about 90-125 km (56-78 miles). Here, ions and electrons recombine very quickly. The levels of ionization drop fast after sunset, leaving a small amount of ionization present but this also disappears at night. The gas density in the E region is less than it is in the D region; therefore, when radio waves cause electrons to vibrate fewer collisions occur.

As the radio signal travels further up into the region, it encounters more electrons and the signal is refracted away from the higher dense electron region. The amount of refraction diminishes when the signal increases in frequency. The higher frequencies make it through the region and pass on to the next region.

The most important region for long-distance, high-frequency communications is the F region. This region often splits into two distinct regions-the F1 and F2, during the day. Generally, the F1 region is found at about 300 km (190 miles) and the F2 region at about 400 km (250 miles). While the altitude of the regions in the ionosphere varies between regions, the F region varies the most and it´s affected by the variations of the sun, as well as the time of day and season of the year.

The Sun and the Ionosphere

The main cause of ionization of the ionosphere is the sun. The density of the ionosphere varies according to the amount of solar radiation. Solar flares, solar wind variability and geomagnetic storms affect the density of the ionosphere. Since the sun is the main cause of ionization, the night side of the earth and the poles are less ionized than those parts of the planet that point more directly to the sun.

Sunspots—dark areas on the surface of the sun—affect the ionosphere because areas that surround the spots emit larger amounts of ultraviolet radiation, which is the main cause of ionization. The number of spots on the sun varies according to an 11-year cycle. radio communications may be less during a solar minimum than during a solar maximum.

Layers of the Ionosphere

D region 60 km (37 miles)-90 km (56 miles)

E region 95-150 km (59- 93 miles)

F1 region 150-210 km (93-130 miles)

F2 region over 210 km (130 miles)

Ground and Sky Waves

During the day, signals of medium wave frequency travel only as ground waves. As frequency increases, ionospheric attenuation decreases allowing signals to pass through the D region and onto the E region. The E region is where signals are reflected back to earth passing through the D region and landing at a great distance from the transmitter.

As signal frequency increases further, The E region electron density is not sufficient to refract signals and signals reach the F1 region where they are reflected back through the E and D region, eventually landing at an even greater distance from the transmitter.

Higher signal frequencies will make it to the F2 region, the topmost ionospheric region. When those signals reflect off this layer back to earth, the distance traveled will be the greatest. The maximum skip distance that signals can travel when reflected off the E region is 2000 km (1243 miles) and when reflected off the F2 region that increases to about 4000 km (2485 miles).

© 2018 Jose Juan Gutierrez