We study quasiperiodic very low frequency (VLF) emissions observed simultaneously by Van Allen Probes spacecraft and Kannuslehto and Lovozero ground‐based stations on 25 December 2015. Both Van Allen Probes A and B detected quasiperiodic emissions, probably originated from a common source, and observed on the ground. In order to locate possible regions of wave generation, we analyze wave‐normal angles with respect to the geomagnetic field, Poynting flux direction, and cyclotron instability growth rate calculated by using the measured phase space density of energetic electrons. We demonstrate that even parallel wave propagation and proper (downward) Poynting flux direction are not sufficient for claiming observations to be in the source region. Agreement between the growth rate and emission bands was obtained for a restricted part of Van Allen Probe A trajectory corresponding to localized enhancement of plasma density with scale of 700 km. We employ spacecraft density data to build a model plasma profile and to calculate ray trajectories from the point of wave detection in space to the ionosphere and examine the possibility of their propagation toward the ground. For the considered event, the wave could propagate toward the ground in the geomagnetic flux tube with enhanced plasma density, which ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point.

中文翻译：

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通过同时进行地面和空间观测，在磁层中准周期性VLF辐射源的定位：一个案例研究

我们研究了Van Allen Probe航天器以及Kannuslehto和Lovozero地面站在2015年12月25日同时观测到的准周期性极低频（VLF）发射。VanAllen Probe A和B都检测到了准周期性发射，这很可能源自共同的来源，并且观察到在地上。为了找到可能的波产生区域，我们通过使用测得的高能电子的相空间密度来计算相对于地磁场，波因廷通量方向和回旋加速器不稳定性增长率的波法线角。我们证明，即使平行波传播和正确的（向下）坡印廷通量方向也不足以声称观测在源区中。对于范艾伦探针A轨迹的受限部分，获得了增长率与发射带之间的一致性，这对应于700 km规模的局部等离子体密度增强。我们利用航天器的密度数据来构建模型等离子体轮廓，并计算从空间中的波检测到电离层的射线轨迹，并检查其向地面传播的可能性。对于所考虑的事件，该波可以在具有增强的等离子体密度的地磁通量管中向地面传播，从而确保了管道传播。通过对地面检测点处的波传播方向的分析来确认波出口的区域。我们使用航天器密度数据来构建模型等离子体轮廓，并计算从空间中的波检测到电离层的射线轨迹，并检查其向地面传播的可能性。对于所考虑的事件，该波可以在具有增强的等离子体密度的地磁通量管中向地面传播，从而确保了管道传播。通过对地面检测点处的波传播方向的分析来确认波出口的区域。我们使用航天器密度数据来构建模型等离子体轮廓，并计算从空间中的波检测到电离层的射线轨迹，并检查其向地面传播的可能性。对于所考虑的事件，该波可以在具有增强的等离子体密度的地磁通量管中向地面传播，从而确保了管道传播。通过对地面检测点处的波传播方向的分析来确认波出口的区域。