The magnetosphere is the region of space where the Earth's magnetic field balances the pressure of the solar wind. The solar magnetic field is carried outward from the Sun and into the solar system by the solar wind. This wind is a plasma of charged particles that travels outward from the Sun at between 300 and 800 kilometers per second. When these charged particles encounter the Earth's magnetic field the vast majority of them are deflected around the Earth. This compresses our planet's magnetic field on the side toward the sun (day side), and stretches it out on the side opposite the sun (night side). This distortion of the Earth's magnetic field results in currents surrounding the Earth that change in response to variations of the solar wind. Some of these currents complete their circuit within the ionosphere and the particles associated with them produce the aurora, or northern lights.
The ionosphere is the region of charged particles surrounding the Earth that is created by the ultraviolet radiation from the Sun. The ionosphere begins at an altitude of about 90km and continues upwards into space. Variations in the composition and density of the ionosphere with altitude produce regions where different physical processes are important. As these regions were discovered they were given letter names F, E, and D.
In addition to ionization from solar radiation, regions of the ionosphere known as the auroral zones experience substantial ionization due to particles that precipitate from space. These rings surround both polar caps at a latitude of about 65 degrees. The precipitation of particles in these regions produces optical emissions by the gases of the atmosphere. We call these emissions the aurora, or northern lights.
It is within the auroral zones that the currents of the Magnetosphere complete their circuit. These currents flow down the Earth's magnetic field lines, through the ionosphere, and then up the field lines out into space. These highly conductive field lines carry electric fields from the magnetosphere into the ionosphere.
In combination with the Earth's magnetic field these electric fields produce several different types of currents in the ionosphere. One type, known as a Hall current, flows perpendicular both to the electric field and the magnetic field. The strong Hall current in the Auroral regions is known as the Auroral Electrojet and it can be extremely intense, especially when lots of bright aurora are being observed.
In the Auroral Electrojet the electrons rapidly move past the ions creating a current. When the electrons are driven to move faster than the sound speed, an unstable situation arises. At this point the rapidly moving electrons can easily cause density pertubations in the surrounding plasma to grow in size. The Ion Acoustic waves that result have frequencies in the range of human hearing. They propagate away from the disturbance at the speed of sound, and perpendicular to the Earth's magnetic field.
These Ion Acoustic waves strongly scatter radio waves of certain wavelengths, and because of this they can be easily observed by ground based radar systems. Coherent scatter radars can measure the locations, velocities, and characteristic shapes of these waves and these observations can be related back to the processes that created them.