The ionosphere is that part of our atmosphere which is almost entirely ionised by solar radiation. Depending on the purpose of the definition, the ionosphere is sometimes considered the uppermost part of the atmosphere and sometimes the lowest part of the magnetosphere. It extends approximately from 50 kilometres high all the way to over a thousand kilometres high, roughly overlapping the thermosphere and exosphere. Below the thermosphere, air is too dense for the free electrons of ionisation to remain independent of other atoms for any appreciable length of time. Above the exosphere is effective vacuum.
Because the definition of the ionosphere depends on free electrons with an energy high enough to counterbalance the electromagnetic force, the air in this region is actually not gaseous but plasma, the fourth state of matter. The thermosphere gains its name from these high-energy (and thus high-heat) electrons. Temperatures in this region can reach 2500 degrees Celsius - but the air is so thin that energy radiating out from a physical object such as a thermometer far outweighs energy gained from the few electrons and ionised atoms coming into contact with it.
The exact limits of the ionosphere depend on the current level of the sun's activity. The degree of ionisation varies with changes in solar activity over the sunspot cycle and even over the course of each day. Its close association with the earth's magnetic field also means that the ionosphere changes depending upon the earth's geography, with extremes at the magnetic poles and equator. The solar wind distorts the shape of the ionosphere, compressing it to sunward and stretching it out on the night side of the planet.
The ionisphere divides into layers based upon what kind of radiation causes ionisation at that altitude. Layers are not smooth, but extremely irregular. The highest layer is the F layer, where ionisation is due mostly to cosmic rays and the extreme ultraviolet radiation which does not usually reach the earth's surface except under the polar holes in the ozone layer. The lowest layer is the D layer, where the ionisation effect is lowest. It exists only during daylight hours.
The D and E layers of the ionosphere interact with radio transmission. The D layer tends to absorb high frequency radio signals, with the result that the range of AM signals is shorter during the day than during the night. In contrast, during the few times during the summer when it comes into existence, the sporadic E layer (Es) can reflect VHF signals thousands of kilometres beyond their usual range. In addition to solar and stellar influences, the Es layer can also be enhanced by the ionising effect of lightning.
The first mention of the ionosphere was by Robert Watson-Watt in 1926. Its existence was confirmed a year later by Edward Appleton, who would be awarded the 1947 Nobel Prize in Physics for his discovery.