Observations of discrete harmonics emerging from equatorial noise.
Observations of discrete harmonics emerging from equatorial noise.
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DOI：
10.1038/ncomms8703
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发表时间：
2015-07-14
影响因子：
16.6
通讯作者：
Weiss B
中科院分区：
文献类型：
--
作者：
Balikhin MA;Shprits YY;Walker SN;Chen L;Cornilleau-Wehrlin N;Dandouras I;Santolik O;Carr C;Yearby KH;Weiss B
空间等离子体中可能存在多种粒子和场的振荡模式。自20世纪70年代初以来，太空任务在地磁赤道附近观测到了类似噪音的等离子体波，即所谓的赤道噪音。有几种理论被提出，但没有提供由现实模型支持的明确的观测证据。在这里，我们报告了集群飞行任务的观测结果，这些观测清楚地显示了这些波的高度结构化和周期性的模式。在与质子回转频率(绕场线旋转的频率)的精确倍数相对应的频率(围绕场线旋转的频率)从第17次谐波到第30次谐波的频率处观察到了非常窄带的发射，表明这些波是由质子分布产生的。与波中的这些相干周期结构同时，星团航天器观察到为波提供自由能量的质子在速度空间中的“环形”分布。根据离子分布计算的波增长与观测结果非常相似。自1970年代以来，空间飞行任务观测到“赤道噪音”--类似噪音的等离子体波紧密地局限在地球S磁层的磁赤道区。在这里，作者揭示了它们的结构和周期频率模式，揭示了它们是由质子分布产生的。
A number of modes of oscillations of particles and fields can exist in space plasmas. Since the early 1970s, space missions have observed noise-like plasma waves near the geomagnetic equator known as ‘equatorial noise'. Several theories were suggested, but clear observational evidence supported by realistic modelling has not been provided. Here we report on observations by the Cluster mission that clearly show the highly structured and periodic pattern of these waves. Very narrow-banded emissions at frequencies corresponding to exact multiples of the proton gyrofrequency (frequency of gyration around the field line) from the 17th up to the 30th harmonic are observed, indicating that these waves are generated by the proton distributions. Simultaneously with these coherent periodic structures in waves, the Cluster spacecraft observes ‘ring' distributions of protons in velocity space that provide the free energy for the waves. Calculated wave growth based on ion distributions shows a very similar pattern to the observations. Since the 1970s space missions have observed `equatorial noise' — noise-like plasma waves closely confined to the magnetic equatorial region of Earth s magnetosphere. Here, the authors uncover their structured and periodic frequency pattern, revealing that they are generated by proton distributions.