Beginning roughly 60 kilometers above the surface of the Earth and extending hundreds of kilometers above that, the shell of plasma known as the ionosphere surrounds the planet. This ionized layer of Earth’s atmosphere is a complex structure that conducts electricity, interacts with radio waves, and continuously changes throughout the day. As the Sun shines on it, the onslaught of photons begins to strip away the electrons from molecules in the atmosphere, ionizing them and producing even more plasma. Additional layers begin to form, and layers within those layers.
The reason for the formation of these layers was first proposed by British geophysicist Sydney Chapman in 1931. In its simplest form, his theory states that a layer of ionization appears in a sweet spot between two competing factors. High in the atmosphere floats many photons but not much air. Low to the ground, the atmosphere is thick with particles, but the amount of sunlight reaching it is reduced.
Somewhere in between exists an optimal zone where the availability of sunlight and particles is balanced and can produce the most plasma. The extra layers that arise during the day are due to the heating effect of the Sun, as well as the fact that there are several different kinds of particles in the atmosphere.
But this simple model begins to break down when scientists try to take into account the angle of the Sun shining on the ionosphere. As the Sun rises and sets over the course of the day, striking at different angles, the ability of the ionosphere to conduct electricity—known as its conductance—changes in a fashion that basic Chapman theory can’t predict.
Now, Ieda et al. suggest additional factors to consider. One of these is the counterintuitive observation that at the high altitudes of the ionosphere, as the altitude increases, the atmosphere gets warmer, which causes it to expand slightly, decreasing its density.
Another is the fact that the incoming sunlight does not consist of a single color but the whole spectrum of wavelengths. This means that in the high regions of the ionosphere, sunlight can produce more plasma than Chapman theory predicts.
The final suggestion the team makes is to take into account the fact that the conductance can change depending on the direction that the electricity flows through the electric and magnetic fields of the ionosphere. Specifically, conductance in the direction parallel to the electric field stays stronger in the evening hours than conductance in other directions.
Taking these three factors into consideration, the team finds that their model greatly improves upon the original 1931 theory and all subsequent modifications, making it the most accurate prediction yet of how the angle of the Sun creates such an ionizing atmosphere.
