Earth's atmosphere contains a series of regions that have a relatively large number of electrically charged atoms and molecules. As a group, these regions are collectively called the ionosphere.
High-energy X-rays and ultraviolet (UV) "light" from the Sun are constantly colliding with gas molecules and atoms in Earth's upper atmosphere. Some of these collisions knock electrons free from the atoms and molecules, creating electrically charged ions (atoms or molecules with missing electrons) and free electrons. These electrically charged ions and electrons move and behave differently than normal, electrically neutral atoms and molecules. Regions with higher concentrations of ions and free electrons occur at several different altitudes and are known, as a group, as the ionosphere.
There are three main regions of the ionosphere, called the D layer, the E layer, and the F layer. These regions do not have sharp boundaries, and the altitudes at which they occur vary during the course of a day and from season to season. The D region is the lowest, starting about 60 or 70 km (37 or 43 miles) above the ground and extending upward to about 90 km (56 miles). Next higher is the E region, starting at about 90 or 100 km (56 or 62 miles) up and extending to 120 or 150 km (75 or 93 miles). The uppermost part of the ionosphere, the F region, starts about 150 km (93 miles) and extends far upward, sometimes as high as 500 km (311 miles) above the surface of our home planet.
The regions of the ionosphere are not considered separate layers, such as the more familiar troposphere and stratosphere. Instead, they are ionized regions embedded within the standard atmospheric layers. The D region usually forms in the upper part of the mesosphere, while the E region typically appears in the lower thermosphere and the F region is found in the upper reaches of the thermosphere.
The height, fraction of ionized particles, and even the existence of the different regions of the ionosphere varies over time. The ionosphere is very different in the daytime versus night. During the day, X-rays an UV light from the Sun continuously provides the energy that knocks electrons free from atoms and molecules, producing a continuous supply of ions and free electrons. At the same time, some of the ions and electrons collide and re-combine to form normal, electrically neutral atoms and molecules. During the day, more ions are created than are destroyed, so the number of ions in the three regions increases. At night, the recombination process takes over in the absence of sunlight, and the number of ions drops. Over the course of most nights, the D region disappears entirely and the E region weakens as the number of ions in that layer plummets. Each morning, as solar X-rays and UV light return, the D and E regions are repopulated with ions. The highest altitude F region sticks around throughout the night, but generally splits into an upper F2 layer and a lower F1 layer during the day.
Before communication via satellites became common, the operators of radio communication systems often used the ionosphere to extend the range of their transmissions. Radio waves generally travel in straight lines, so unless a tall transmission tower can "see" the top of a receiver tower, the curvature of the Earth limits the range of radio transmissions to stations that are not over the horizon. However, some frequencies of radio waves bounce or reflect off of the electrically charged particles in certain ionosphere layers. Pre-satellite radio communications often took advantage of this phenomenon, bouncing radio waves off of the "sky" to extend the range of the signals. Radio operators had to account for the constant changes in the ionosphere, particularly the shifts or disappearance of the layers between day and night, to effectively take advantage of these mirror-like reflections of radio waves.
The ionosphere regions can absorb or dampen radio signals, or they can bend radio waves, as well as reflecting the signals as described above. The specific behavior depends on both the frequency of the radio signal as well as the characteristics of the ionosphere region involved. Since Global Positioning System (GPS) satellites use radio signals to determine locations, the accuracy of GPS can be severely reduced when those signals bend as they pass through ionosphere regions. Similarly, some radio communications can be disrupted if the frequency used is one that an ionosphere layer dampens or absorbs entirely, resulting in a weakened signal or even total loss of communications. Scientists constantly measure and produce computer models of the ever-changing ionosphere so that people in charge of radio communications can anticipate disruptions.
Scientists use radio waves in various ways to probe and monitor the otherwise invisible ionosphere. Various radio antennas and radar systems, on the ground and on satellites, are used to monitor the constantly evolving ionosphere. Radio antennas "listen" for radio signals generated by the ionosphere itself, radar systems bounce signals of the different layers, and pairs of transmitters and receivers shoot signals through the ionosphere to determine how much those signals are dampened or redirected.
Along with the daily fluctuations in the ionosphere, there are also seasonal and longer-term variations in this complex set of regions. Different latitudes warm and cool with the seasons as the intensity of sunlight varies from place to place due to the tilt of Earth's axis. Similarly, the ionosphere varies seasonally as the location of the peak intensity of solar X-rays and UV light, which drive the rate of formation of ions, moves around on the globe. Seasonal changes in the chemistry of the atmosphere also play a role, influencing the rate of recombination events which remove ions from the atmosphere. Longer term, the 11-year sunspot cycle has a strong influence on the upper reaches of the atmosphere, including the ionosphere. The brightness of the Sun, in visible light wavelengths that we can see, varies by less than 1/10th of one percent between the high point and the low point of the sunspot cycle. However, the X-ray and UV output of the Sun varies much more throughout the solar cycle, fluctuating by a factor of 10 or more. Since these X-rays and UV radiation control the rate of ion formation that produces the ionosphere, large variations in these types of radiation lead to big changes in the ion densities in the ionosphere regions. Also, large geomagnetic storms triggered by solar flares and coronal mass ejections from the Sun can create severe temporary disruptions in the ionosphere.