Properties of dayside outer zone chorus during HILDCAA events: Loss of energetic electrons
Properties of dayside outer zone chorus during HILDCAA events: Loss of energetic electrons
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DOI：
10.1029/2008ja013353
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发表时间：
2009-03
影响因子：
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通讯作者：
B. Tsurutani;O. Verkhoglyadova;G. Lakhina;S. Yagitani
中科院分区：
文献类型：
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作者：
B. Tsurutani;O. Verkhoglyadova;G. Lakhina;S. Yagitani
[1] 磁层的日侧外区 (DOZ) 部分是统计上发现合唱强度最强烈的区域。在这项研究中，使用 OGO-5 等离子体波和 GEOTAIL 等离子体波、磁场和高能粒子数据对 DOZ 合唱团进行了检查。白天合唱被认为是由~0.1到0.5秒的上升音发射组成，称为“元素”。元素的持续时间及其频率-时间特征在整个合唱活动中（持续数十分钟到几小时）是可重复的，但可能因活动而异。合唱是一种右旋圆偏振电磁平面波。检测到波从沿环境磁场 Bo 传播到简德林角 θGendrin 附近的斜角。发现所有波，无论相对于 Bo 的波传播方向如何，都是一阶圆偏振的。合唱升调元素由持续时间约为 5 至 10 毫秒的连贯“子元素”或“数据包”组成。连续的子元素/数据包随时间按频率步进形成元素。数据包内的峰值幅度可以为 ∼0.2 nT 或更大。子元素或分组幅度比通过功率谱测量获得的先前估计的合唱幅度至少大一个数量级。这种差异是由于合唱元素之间存在间距、元素内子元素/包之间的间距以及升音内子元素/包的不同频率造成的。此处研究的 DOZ 合唱被发现与通过亚暴注入温度各向异性 (/> 1) E = 5 至 40 keV 电子从午夜扇区漂移到 DOZ 区域的损失锥不稳定性产生的一致。使用 ~0.2 nT 的大振幅子单元/包波磁场振幅，表明瞬时 Kennel-Petschek 俯仰角扩散速率 Dαα 为 ~5 × 10−2 s−1。后一个量基于非相干波。如果高能电子在与持续时间为 10 ms 的相干子元件相互作用的整个过程中保持回旋共振，它们可能会“俯仰角传输”约 5°。因此，损失锥 5° 以内的电子可能会在单个波粒相互作用中损失。电子穿过波区时的几种此类相互作用可以导致更大的俯仰角传输角。合唱元素和轫致辐射 X 射线微爆（~0.5 s）的相似时间尺度可以通过上述“俯仰角传输”机制来解释。通过这种机制增加和减少俯仰角传输将导致更高的俯仰角扩散或“超扩散”速率。还检测到峰值幅度约为 20 pT 的各向同性非偏振噪声。噪音远高于乐器噪音水平，推测是合唱或嘶嘶声的残余。
[1] The dayside outer zone (DOZ) portion of the magnetosphere is a region where chorus intensities are statistically found to be the most intense. In this study, DOZ chorus have been examined using OGO-5 plasma wave and GEOTAIL plasma wave, magnetic field and energetic particle data. Dayside chorus is noted to be composed of ∼0.1 to 0.5 s rising-tone emissions called “elements.” The duration of the elements and their frequency-time characteristics are repeatable throughout the chorus event (lasting from tens of minutes to hours), but may differ from event to event. Chorus is a right-hand, circularly polarized electromagnetic plane wave. Waves are detected propagating from along the ambient magnetic field, Bo, to oblique angles near the Gendrin angle, θGendrin. All waves, independent of wave direction of propagation relative to Bo, are found to be circularly polarized, to first order. Chorus rising-tone elements are composed of coherent “subelements” or “packets” with durations of ∼5 to 10 ms. Consecutive subelements/packets step in frequency with time to form the elements. The peak amplitudes within a packet can be ∼0.2 nT or greater. The subelement or packet amplitudes are at least an order of magnitude larger than previously estimated chorus amplitudes obtained by power spectral measurements. This discrepancy is due to the presence of interspacings between chorus elements, the interspacings between subelements/packets within an element, and the different frequencies of subelements/packets within a rising-tone. DOZ chorus studied here were found to be consistent with generation via the loss cone instability of substorm-injected temperature-anisotropic (/ > 1) E = 5 to 40 keV electrons drifting from the midnight sector to the DOZ region. Using a large amplitude subelement/packet wave magnetic field amplitude of ∼0.2 nT, it is shown that the instantaneous Kennel-Petschek pitch angle diffusion rate Dαα is ∼5 × 10−2 s−1. This latter quantity is based on incoherent waves. If energetic electrons stay in cyclotron resonance throughout their interaction with a coherent subelement of duration 10 ms, they may be “pitch angle transported” by ∼5°. Therefore electrons within 5° of the loss cone can be lost in a single wave-particle interaction. Several such interactions as the electrons traverse the wave region can lead to much larger pitch angle transport angles. The similar time-scales of chorus elements and bremsstrahlung X-ray microbursts (∼0.5 s) can be explained by the “pitch angle transport” mechanism described above. Increasing and decreasing pitch angle transport via this mechanism will lead to much higher pitch angle diffusion or “super diffusion” rates. Isotropic unpolarized noise of ∼20 pT peak amplitude has also been detected. The noise is well above instrument noise levels and is speculated to be remnants of chorus or hiss.