The October effect is known as a rapid and strong decrease in the signal amplitude of radio waves with very low frequency (VLF), reflected at the lowest edge of the ionosphere. This strong decrease can be observed only during the daytime. Although the October effect is long known, it is hardly investigated and its mechanism is still unknown. To get closer to a mechanism, we answer why the October effect does not occur during nighttime. Therefore, average characteristics of the October effect are obtained from different VLF transmitter-receiver combinations. The occurrence of the October effect is then compared with characteristics of the neutral atmosphere temperature at VLF reflection heights as it seems to act as a proxy for the unknown mechanism. The temperature shows an asymmetric seasonal behavior at daytime VLF reflection heights poleward of 50°N but not during the nighttime, resulting in the October effect.

中文翻译：

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为什么十月效应不会在夜间发生？

十月效应被称为极低频无线电波 (VLF) 信号幅度的快速强烈下降，反映在电离层的最低边缘。这种强烈的下降只能在白天观察到。尽管“十月效应”早已为人所知，但对其研究却很少，其机制仍不清楚。为了更接近一个机制，我们回答为什么十月效应不会在夜间发生。因此，十月效应的平均特征是从不同的甚低频发射机-接收机组合中获得的。然后将十月效应的发生与甚低频反射高度处的中性大气温度特征进行比较，因为它似乎可以作为未知机制的代理。白天 VLF 反射高度为 50°N 极地时，温度显示出不对称的季节性行为，但夜间则不然，从而导致了十月效应。