地球等离子体层嘶声的内部源区

Internal source of plasmaspheric hiss on Earth

地球等离子体层是向阳侧中低纬度电离层沿着闭合磁力线向上的自然延伸,其中所出现的一类频率覆盖数十赫兹至数千赫兹的哨声模波动称为等离子体层嘶声.自发现以来,等离子体层嘶声就被广泛认为是沉降损失辐射带高能电子的主要等离子体波动之一.但是,等离子体层嘶声的起源一直没有定论.潜在的来源分成两类:一类为等离子体层内部背景等离子体噪声,另一类为等离子体层外部波动.2012年发射升空的Van Allen Probes搭载了完备的磁层粒子、场和波动探测仪器,为这一问题的解决带来了新的机遇.本文综述了近5年来利用Van Allen Probes探索等离子体层嘶声内部源区的研究工作,强调背景等离子体噪声可以通过高能电子线性和非线性不稳定性叠加放大成可观测的嘶声,突出内源的广泛分布特性,即在结构上涵盖等离子体层核心和羽流,在地方时上涵盖向阳和背阳侧等离子体层,在径向距离上涵盖外层和内层等离子体层.

Plasmasphere is an upward extension of the ionosphere at low and middle latitudes along the closed magnetic field lines rooted in the Earth,where the trapped whistler-mode waves in the frequency range from tens to thousands of Hertz are named as plasmaspheric hiss.Since its discovery,plasmaspheric hiss has been considered an important plasma wave to drive the precipitation loss of the Van Allen radiation belt electrons.However,the origin of plasmaspheric hiss has long been under debate.There have been two classes of sources proposed for the plasmaspheric hiss:(1)background plasma noises inside the plasmasphere and(2)other plasma waves outside the plasmasphere.In 2012,the Van Allen Probes mission capable of measuring an enormous range of particles,fields and waves was launched,providing an unprecedented opportunity to investigate the origin of plasmaspheric hiss.Here we briefly review the progress on the internal sources of plasmaspheric hiss during the recent five years.It has been suggested the background plasma noises can be amplified to the plasmaspheric hiss by a combination of linear and nonlinear instabilities of energetic electrons.During geomagnetically active times,energetic electrons are transported from the magnetotail into the plasmaspheric body and plume.The linear growth rate of whistler-mode waves destabilized by these energetic electrons exhibits an analogous frequency-dependence to the observed hiss power spectral density.When the seed wave amplitude exceeds a threshold,the nonlinear instability can act and then amplify the seed wave to the observable level.Such internal sources could exist over a broad range of magnetic local times,radial distances,and other structures of the plasmasphere.