L Versus Time Structures of Dayside Magnetic Pulsations Detected by the European Quasi‐Meridional Magnetometer Array
L Versus Time Structures of Dayside Magnetic Pulsations Detected by the European Quasi‐Meridional Magnetometer Array
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DOI：
10.1029/2019ja026796
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发表时间：
2019-08
期刊：
影响因子：
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通讯作者：
Kazue Takahashi;B. Heilig
中科院分区：
文献类型：
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作者：
Kazue Takahashi;B. Heilig
通过将欧洲准子午磁强计阵列（EMMA）探测到的磁场扰动显示为磁场L值随时间空间变化的二维图像（称为EMMAgrams），研究了日侧地面磁脉动的时空结构。我们根据2015年8月15日的观测结果生成了电磁图，其中包括先前研究过的与行星际冲击相关的脉动事件。除了由腔模振荡驱动的场线共振特征外，我们还在Pc2波段发现了具有L无关周期的极向传播结构。Pc2结构归因于源自离子前震的周期性磁流体动力脉冲（上游波），并沿着Tamao提出的路径在磁层中传播。在L依赖周期（瞬态脉动）的局部场线的振铃也被清楚地检测为色散的向极传播结构，不仅在冲击冲击后立即出现，而且在不太明显的外部干扰期间也会出现。瞬态脉动在几个波周期后衰减，在两个纬度间隔较小的台站检测到的瞬态脉动的交叉谱分析表明，在由台站的场线共振频率所划分的频带内，交叉相位升高。由太阳风动压变化引起的瞬态脉动的连续激发，似乎对类似的交叉相位峰的形成有重要贡献，这种峰被广泛用于磁震研究。
We studied the spatiotemporal structure of ground magnetic pulsations on the dayside by displaying magnetic field perturbations detected by the European quasi‐Meridional Magnetometer Array (EMMA) as 2‐D images in the magnetic L value versus time space, called EMMAgrams. We generated EMMAgrams from observations made on 15 August 2015, including a previously studied pulsation event associated with an interplanetary shock. In addition to signatures of field line resonance driven by a cavity mode oscillation, we found poleward propagating structures with L‐independent periods in the Pc2 band. The Pc2 structures are attributed to periodic magnetohydrodynamic pulses (upstream waves) originating from the ion foreshock and propagating in the magnetosphere along the path proposed by Tamao. Ringing of local field lines at L‐dependent periods (transient pulsations) is also clearly detected as dispersive poleward propagating structures not only immediately after the shock impact but also during time periods of less obvious external disturbances. A transient pulsation decays after a few wave periods, and cross‐spectral analysis of transient pulsations detected at two stations with a small latitudinal separation indicates elevation of the cross phase in a band delimited by the field line resonance frequencies at the stations. Successive excitation of transient pulsations by variations of the solar wind dynamic pressure appears to contribute significantly to formation of similar cross‐phase peaks that are widely used in magnetoseismic studies.