A spring-fall asymmetry is observed in daytime amplitude values of very low frequency (VLF) radio wave signals propagating over the North Atlantic during 2011–2019. We explore the processes behind this asymmetry by comparing against mesospheric mean temperatures and the semidiurnal solar tide (S2) in mesospheric winds. The solar radiation influence on VLF subionospheric propagation was removed from the daytime VLF amplitude values, isolating the fall-effect. Similarly, the symmetric background level was removed from mesospheric mean temperatures undertaking comparable analysis. During fall, all three analyzed parameters experience significant deviation from their background levels. The VLF amplitude variation during spring is explained by the seasonal variation in solar illumination conditions, while the fall-effect can be interpreted as a mean zonal wind reversal associated with both a S2 enhancement, and temperature reductions. Decreases in temperature can produce decreases in collision frequency, reducing VLF signal absorption, driving the observed VLF asymmetry.

中文翻译：

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在北大西洋地区记录的 VLF 振幅的春秋不对称：坠落效应

在 2011-2019 年期间在北大西洋上空传播的极低频 (VLF) 无线电波信号的白天振幅值中观察到春秋不对称。我们通过与中间层平均温度和中间层风中的半日太阳潮 (S2) 进行比较来探索这种不对称背后的过程。太阳辐射对 VLF 亚电离层传播的影响从白天的 VLF 振幅值中去除，隔离了坠落效应。类似地，对称背景水平从进行可比分析的中间层平均温度中去除。在秋季，所有三个分析参数都与其背景水平发生显着偏差。春季 VLF 振幅变化可以通过太阳光照条件的季节性变化来解释，而坠落效应可以解释为与 S2 增强和温度降低相关的平均纬向风逆转。温度降低会导致碰撞频率降低，从而减少 VLF 信号吸收，从而导致观察到的 VLF 不对称。