Theoretical Study of Auroral Radio Waves

University Of Maryland, College Park, College Park MD

Investigators

Abstract

The Earth's ionosphere is a plasma composed of approximately equal numbers of ions and electrons. In addition to gravity that affects all matter, electric and magnetic forces are important in determining the behavior of the ionospheric plasma. The plasma behaves as a fluid even if there are few collisions between particles because electric and magnetic forces act at a distance. Analogous to waves in the ocean, plasma waves transmit energy through the ionosphere. Electromagnetic, pressure and velocity fields in combination produce a wide variety of plasma waves. This wave environment is extremely complex with a range of generation mechanisms, wave frequencies, and source locations. However, because of this complexity, many of these waves and their generation mechanisms are poorly understood. The auroral region contributes significantly to the natural radio wave environment of the Earth. This project investigates auroral roar and its generation mechanisms, which are complex and nonlinear involving wave-wave and wave-particle interactions. Because the types of waves generated depend on plasma conditions and can be observed far from the source region, they provide a means to infer the characteristics of plasmas remotely. As a result, what is learned about waves in the auroral region can be used to interpret conditions in astrophysical and planetary environments where these same waves are generated. To study the origins of auroral roar, this project uses, and develops further, weak turbulence theory. This, in itself, is an important outcome of the project. A main aim is to contribute to the development of a unifying theory of plasmas and radio emission observed in the Earth's ionosphere and magnetosphere. Non-linear studies of auroral roar are likely to lead to a deeper understanding of the fundamental physics of wave-particle interactions. Four problems are explored in some detail: (1) the formation of the fine frequency structure of auroral roar emissions; (2) the decay of intense upper hybrid waves into multiple harmonics and lower frequency waves; (3) nonlinear coalescence generation of 4fce auroral roar; and (4) decay instability of upper-hybrid, lower-hybrid, and Bernstein waves.