Pitch-angle diffusion and the origin of temporal and spatial structures in morningside aurorae
Pitch-angle diffusion and the origin of temporal and spatial structures in morningside aurorae
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晨间极光的俯仰角扩散与时空结构起源
DOI：
10.1007/bf00217428
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发表时间：
1990
影响因子：
10.3
通讯作者：
G. Davidson
中科院分区：
文献类型：
--
作者：
G. Davidson
在低于70°的纬度上，晨间极光显示出复杂的空间和时间结构，这与在夜晚或午夜地区看到的任何东西都不同。晨边结构被认为是由极光亚暴中注入的捕获电子沉淀形成的;在晨边极光区，主导傍晚侧的大规模平行电场尚未确定任何重要作用。这些空间和时间结构是如何产生的一直是许多猜测的主题;大多数理论机制都集中在驱动俯仰角扩散的波粒相互作用上。在极光区的俯仰角扩散的作用有关的主要证据进行审查。观测证据主要涉及大气中的极光发射、从火箭和卫星观测到的高能粒子、高空甚低频波、磁层冷等离子体和地面探测到的磁脉动。借助于这些证据，加上与外部“永久捕获”辐射带有关的观测和理论，几个调制赤道地区VLF波增长的理论模型被拼凑在一起。这些模型，以及支持它们的观测数据，被检查，看看它们是否符合观测图片，看看它们在未来的研究中可能会导致什么。这些模型分为两类：调制是外部施加的和调制是自激的。对于时间变化，自激机制现在是有利的。领先的候选者涉及非线性弛豫振荡器;非线性可能会产生重要的后果。对于空间结构的起源，这两个类别都有几个竞争者，但没有一个完全符合从观察中得出的推论。所有的理论都涉及尚未精确确定的关键参数。关键的研究需要列出和讨论。
Morningside aurorae at latitudes below about 70° display complex spatial and temporal structures unlike anything seen in the evening or midnight sectors. The morningside structures are believed to be formed by the precipitation of trapped electrons injected in auroral substorms; no significant role has yet been identified in the morningside auroral regions for the large-scale parallel electric fields that dominate the evening side. How those spatial and temporal structures originate has been the subject of much speculation; most theoretical mechanisms focus on the wave-particle interactions that drive pitch-angle diffusion. The principal evidence pertaining to the role of pitch-angle diffusion in the auroral regions is reviewed here. The observational evidence concerns mainly auroral emissions in the atmosphere, energetic particles observed from rockets and satellites, VLF waves at high altitudes, magnetospheric cold plasma, and magnetic pulsations detected on the ground. With the aid of such evidence, plus observations and theories related to the outer ‘permanently trapped’ radiation belts, several theoretical models for the modulation of VLF wave growth in the equatorial regions have been pieced together. Those models, and the observational data supporting them, are examined to see how well they fit the observational picture and to see where they might lead in future research. The models fall into two categories: those in which the modulations are externally imposed and those in which the modulations are self-excited. For the temporal variations the self-excited mechanisms are now favored. The leading candidate involves a nonlinear relaxation oscillator; the nonlinearity may have important consequences. There are several contenders in both categories for the origin of the spatial structures, none of which agrees fully with inferences from the observations. All the theories involve critical parameters that have not yet been precisely fixed. The critical research needs are listed and discussed.