Impact ionization of neutrals by precipitating electrons is the dominant source of ionization at auroral latitudes during geomagnetically disturbed times [Carlson and Egeland, 1995]. This process gives rise to the enhanced conductivity in these regions, which then permits intense localized currents to flow in the ``auroral electrojets'' along the auroral oval. Of course, these precipitating electrons also yield their energy, via two-step processes, to produce the auroral lights.
The large electric fields, currents, and plasma drifts relative to the neutral plasma at auroral latitudes during disturbed times leads to a variety of instabilities, particularly near the auroral electrojets at altitudes characteristic of the F region [Carlson and Egeland, 1995]. These instabilities cause the initially (relatively) homogeneous plasma to develop density turbulence, typically magnetic field-aligned, with scale sizes of order 100 m to 10 km. Radiation propagating through this density turbulence is refracted, scattered, reflected, as well as suffering phase changes and sometimes Doppler changes. These effects cause scintillation or twinkling of radio sources, analogous to the twinkling of star light by density turbulence in the atmosphere. These scintillations lead to strong amplitude fading and phase fluctuations all the way up to GHz frequencies, thereby disrupting communications up to GHz frequencies, navigation systems etc. In addition, the scattering can blind radar tracking (e.g., over-the-horizon radars), disrupt (and sometimes improve) communications. These scintillation and scattering effects can be used as a diagnostic for space weather. Figure 16.8 illustrates these scintillation effects.
Figure 16.8: Power spectrum of 4 GHz ionospheric scintillations for an event measured in Hong
Kong in 1973 [Lanzerotti, 1979] .
It should be emphasized that the density turbulence generated at auroral latitudes propagates and diffuses through the ionosphere, so that the much of the ionosphere can theoretically become disturbed and inhomogeneous as a result of auroral activity.
Finally, just as a final topic, it is relevant that the ionospheric plasma and atmosphere contain many other waves. Some are associated with tides. Some are global disturbances, called ``travelling ionospheric disturbances'', which can travel several times around the world after a sudden event in the ionosphere and can be seen in VLBI and other astrophysical data.