The amplitude of Very Low Frequency (VLF) transmissions propagating from transmitter to receiver between the Earth's surface and the ionospheric D-region is a useful measurement to detect changes in the ionization within the D-region ranging from 60 to 90 km. The VLF signal amplitude is disturbed by geomagnetic, solar, and atmospheric phenomena. To be able to identify perturbations in the VLF signal amplitude, we determine its averaged seasonal variation under quiet solar and geomagnetic conditions. Here it is challenging, that long time series of the VLF signal amplitude show significant jumps and outliers, which are caused artificially by technical adjustments/maintenance work. This paper presents a new approach for processing long VLF data time series over multiple years resulting in level 2 data. The new level 2 data enables the consideration of time series with artificial jumps since the jumps are leveled. Moreover, the outliers are removed by a robust and systematic 2-step outlier filtering. The average seasonal and diurnal variation for different transmitter-receiver combinations can be computed with the new level 2 data by applying a composite analysis. A subsequently applied polynomial fit obtains the quiet time lines for daytime and nighttime, representing the typical seasonal variation under undisturbed conditions of the VLF signal amplitude for each considered link. The developed quiet time lines may serve as a tool to determine perturbations of the VLF signal amplitude with solar and geomagnetic as well as atmospheric origin. Also, they allow comparison of the VLF signal amplitude variation for different transmitter-receiver links.

中文翻译：

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VLF 幅度测量的处理：安静时间季节变化的推导

在地球表面和电离层 D 区域之间从发射器传播到接收器的甚低频 (VLF) 传输的幅度是检测 60 至 90 公里 D 区域内电离变化的有用测量值。 VLF 信号幅度会受到地磁、太阳和大气现象的干扰。为了能够识别甚低频信号幅度的扰动，我们确定了其在安静的太阳和地磁条件下的平均季节性变化。这里具有挑战性的是，VLF 信号幅度的长时间序列显示出明显的跳跃和异常值，这是由技术调整/维护工作人为造成的。本文提出了一种处理多年长 VLF 数据时间序列并产生 2 级数据的新方法。新的 2 级数据可以考虑具有人工跳跃的时间序列，因为跳跃是水平的。此外，通过稳健且系统的两步异常值过滤来去除异常值。不同发射器-接收器组合的平均季节和昼夜变化可以通过应用复合分析，使用新的 2 级数据来计算。随后应用的多项式拟合获得白天和夜间的安静时间线，代表每个考虑链路的 VLF 信号幅度在未受干扰条件下的典型季节变化。开发的安静时间线可以作为确定太阳、地磁以及大气起源的甚低频信号幅度扰动的工具。此外，它们还可以比较不同发射器-接收器链路的 VLF 信号幅度变化。