The ionosphere is the complex outer region of Earth’s atmosphere. This layer is made up of plasma, which forms by the Sun’s radiation ionizing neutral particles. The ions and electrons within this plasma interact with electric and magnetic fields. New research shows that features in the ionosphere can trap density-driven waves, causing regions to heat.
Plasma waves, which are prolific in the ionosphere, are changes in electric and magnetic fields that propagate through the plasma. These waves can create density variations in the plasma or add energy to the plasma in the form of heat.
Plasma waves fall into two broad classifications: electrostatic and electromagnetic. Electrostatic waves can move along or across magnetic field lines without disturbing the magnetic field—only the electric field and the plasma are affected. Electromagnetic waves, on the other hand, produce a disturbance in the magnetic field as well.
Typically, electron density is constant along a magnetic field line, or at least changes smoothly, because electrons can move quickly and freely along the magnetic field, but sometimes, electrostatic waves in the ionosphere can heat the plasma in such a way that they produce density depletions, where the electron density drops suddenly. These unique regions are called magnetic field–aligned density striations and are generally only a few meters wide but can extend along field lines in the ionosphere for tens of kilometers.
Using numerical simulations, Najmi et al. introduced an electromagnetic wave into an already formed field-aligned density striation, then studied the turbulence it created. In their simulations, when an electromagnetic wave encountered the sparse region, it was converted into an electrostatic wave. This wave became trapped within the region and decayed into three additional types of plasma waves: upper hybrid oscillations, lower hybrid oscillations, and Bernstein waves.
The first two types of waves interacted with each other to create a great deal of turbulence within the density-depleted region. The appearance of the third type of wave, however, caused the electrons to be heated by several thousand kelvins. This is because the Bernstein waves travel at just the right frequency to pump energy into the electrons as they oscillate around the magnetic field.
These results provide insight into the interplay between density variations and heating in the ionosphere and are important for further studies of ionospheric turbulence. (Radio Science, doi:10.1002/2015RS005866, 2016).