A study of the oscillation mode of Pi2 geomagnetic pulsations

利用Active磁层粒子示踪探测器（AMPTE CCE）卫星同时采集的磁层Pi - 2脉动特性的空间变化，研究了磁层Pi - 2脉动特性在径向距离小于6.6地球半径（RE）、地磁纬度为-16 ~ 16以及位于磁壳L=1.23的Kakioka地站的磁场记录。从午夜3小时内采集的Kakioka数据中识别出π 2磁脉动，但对CCE的当地时间没有限制。自动Pi 2选择程序从Kakioka数据中产生249个事件。我们描述了CCE的径向（b_x）、方位角（b_y）和压缩（b_z）分量的磁场变化，根据它们的频谱密度、相干性和相对于Kakioka数据的水平(H)分量中的Pi 2脉动的相位，然后检查了这些参数如何依赖于CCE的位置。More发现，高相干事件（CCE与Kakioka >.6之间的相干）主要发生在CCE位于夜侧且L<4时。对于这些事件，CCE的磁场扰动主要由倍体分量b_x和b_z控制，这些分量表现出-0或-180‰的地星交叉相位，这取决于卫星的位置。空间相位结构与两个反射边界之间激发的压缩腔模型共振的本征模结构一致。我们没有发现证据支持这样的观点，即地面Pi 2是响应于极光带场线上的源波而激发的中纬度环面场线共振。相反，结果表明，在地面上观测到的中纬度(2<L<5) π 2脉动来自于由电离层和Alfven速度梯度组成的内磁层激发的腔型共振。空腔共振可能是由亚暴爆发时由与交叉尾电流破坏相关的大规模磁重构所发射的向地传播的快模波所激发的。少

The spatial variation of the properties of magnetospheric Pi 2 pulsations is studied using magnetic field records acquired simultaneously by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite at radial distances less than 6.6 Earth radii (RE) and at geomagnetic latitudes from -16ﾟ to 16ﾟ and at the Kakioka ground station located at magnetic shell of L=1.23. Pi 2 magnetic pulsations are identified from the Kakioka data acquired within 3 hours of midnight, but no restriction is imposed on the local time of CCE.An automated Pi 2 selection procedure resulted in 249 events from the Kakioka data. We have characterized magnetic field variations in the radial (b_x), azimuthal (b_y), and compressional (b_z) components at CCE in terms of their spectral density, coherence, and phase relative to those of the Pi 2 pulsation in the horizontal (H) component of the Kakioka data and then examined how these parameters depend on the location of CCE.It is … More found that high-coherence events (coherence between CCE and Kakioka>0.6) are observed primarily when CCE is on the nightside and at L<4. For these events the magnetic field perturbations at CCE are dominated by the ploidal components b_x and b_z and these components exhibit a ground to satellite cross phase of either -0 or -180ﾟ, depending on the location of the satellite. The spatial phase structure is consistent with the eigenmode structure of a compressional cavity-mode type resonance excited between two reflecting boundaries. We find no evidence supporting the view that ground Pi 2 are mid-latitude toroidal field line resonances excited in response to source waves on auroral zone field lines. Rather, the results imply that mid-latitude (2<L<5) Pi 2 pulsations observed on the ground originate from a cavity-mode type resonance excited in the inner magnetosphere bounded below by the ionosphere and at high altitudes by an Alfven velocity gradient. The cavity resonance is probably excited by earthward propagating fast mode waves launched at substorm onset by the large scale magnetic reconfiguration associated with cross-tail current disruption. Less