Theoretical Study of Auroral Radio Waves

地球的电离层是由大约相同数量的离子和电子组成的等离子体。除了影响所有物质的重力外，电磁场对决定电离层等离子体的行为也很重要。即使粒子之间很少发生碰撞，等离子体也表现为流体，因为电磁作用力在一定距离内起作用。与海洋中的波类似，等离子体波通过电离层传输能量。电磁场、压力场和速度场结合在一起会产生各种各样的等离子体波。这种波动环境极其复杂，有一系列的产生机制、波动频率和震源位置。然而，由于这种复杂性，人们对许多这样的波及其产生机制知之甚少。极光区域对地球的自然无线电波环境有很大贡献。该项目研究极光咆哮及其产生机制，它是复杂的、非线性的，涉及波-波和波-粒子相互作用。由于产生的波的类型取决于等离子体条件，并且可以在远离源区的地方观察到，因此它们提供了一种远程推断等离子体特性的方法。因此，有关极光区域波的知识可以用来解释在天体物理和行星环境中产生这些相同波的条件。为了研究极光咆哮的起源，该项目使用并进一步发展了弱湍流理论。这本身就是该项目的一个重要成果。其主要目的是促进发展在地球电离层和磁层观测到的等离子体和无线电发射的统一理论。对极光咆哮的非线性研究可能会加深对波粒相互作用的基本物理的理解。较详细地讨论了四个问题：(1)极光咆哮发射的精细频率结构的形成；(2)强烈的高层混合波衰变为多个谐波和低频波；(3)4fce极光咆哮的非线性合并产生；(4)上混合波、低混合波和Bernstein波的衰变不稳定性。

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.