Abstract. Our recent study (Yagova et al., 2017) shows that ultralow frequency (ULF) pulsations are seen in ground-based magnetic and luminosity data in the polar cap for a few hours preceding isolated non-triggered substorms. Such pulsations are characterised by a high level of coherence. There might be two possible origins of these pulsations: ULF disturbances in the solar wind or processes inside the magnetosphere, which lead to a substorm generation. In this case, the first scenario should be considered as a substorm trigger. To understand the role of the mentioned mechanisms in developing of pulsations, a detailed case study has been carried out. It is shown that before a weak substorm, fluctuations with the central frequency 1.5 mHz are observed in the both polar caps in ground-based geomagnetic data. Coherent pulsations with the same main frequency are also seen in the magnetotail magnetic field, in addition to simultaneous electron concentration in the ionosphere, while fluctuations in electron temperature and electron flux in the magnetosphere have a slightly different main frequency. These pulsations appeared after ULF activity in solar wind and interplanetary magnetic field became inappreciable.

中文翻译：

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亚暴前ULF涨落的频谱参数的案例研究，在行星际空间中没有明显的触发

摘要。我们最近的研究（Yagova et al。，2017）表明，在孤立的未触发亚暴之前的几个小时内，极地帽的地面磁和光度数据中出现了超低频（ULF）脉动。这种脉动的特征在于高度的相干性。这些脉动可能有两个起源：太阳风中的ULF扰动或磁层内部的过程，这导致了亚暴的产生。在这种情况下，第一种情况应视为亚暴触发。为了解上述机制在产生搏动中的作用，已进行了详细的案例研究。结果表明，在弱次暴风雨发生之前，地面地磁数据的两个极帽都观测到中心频率为1.5 mHz的波动。除了在电离层中同时集中电子外，在磁尾磁场中还可以看到具有相同主频率的相干脉动，而在磁层中电子温度和电子通量的波动具有略微不同的主频率。这些脉冲在太阳风中的ULF活动和行星际磁场变得不明显之后出现。