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Earth's middle and upper atmosphere exhibits several dominant large-scale\udoscillations in many measured parameters. One of these oscillations is the\udsemi-annual oscillation (SAO). The SAO can be detected in the ionospheric\udtotal electron content (TEC), the ionospheric transition height, the wind\udregime in the mesosphere–lower thermosphere (MLT), and in the MLT\udtemperatures. In addition, as we report for the first time in this study,\udthe SAO is among the most dominant oscillations in nighttime very low\udfrequency (VLF) narrowband (NB) subionospheric measurements. As VLF\udsignals are reflected off the ionospheric D region (at altitudes of\ud ∼  65 and  ∼  85 km, during the day and night, respectively), this implies that the upper\udpart of the D region is experiencing this oscillation as well, through\udchanges in the dominating electron or ion densities, or by changes in the\udelectron collision frequency, recombination rates, and attachment rates, all\udof which could be driven by oscillatory MLT temperature changes. We conclude\udthat the main source of the SAO in the nighttime D region is\udNO<sub><i>x</i></sub> molecule transport from the lower levels of the thermosphere,\udresulting in enhanced ionization and the creation of free electrons in the\udnighttime D region, thus modulating the SAO signature in VLF NB\udmeasurements. While the cause for the observed SAO is still a subject of\uddebate, this oscillation should be taken into account when modeling the\udD region in general and VLF wave propagation in particular

Semi-annual oscillation (SAO) of the nighttime ionospheric D region as detected through ground-based VLF receivers