Ground-based observations show a phase shift in semi-annual variation of excited hydroxyl (OH^∗) emissions at mid-latitudes (43^∘ N) compared to those at low latitudes. This differs from the annual cycle at high latitudes. We examine this behaviour by utilising an OH^∗ airglow model which was incorporated into a 3D chemistry–transport model (CTM). Through this modelling, we study the morphology of the excited hydroxyl emission layer at mid-latitudes (30–50^∘ N), and we assess the impact of the main drivers of its semi-annual variation: temperature, atomic oxygen, and air density. We found that this shift in the semi-annual cycle is determined mainly by the superposition of annual variations of temperature and atomic oxygen concentration. Hence, the winter peak for emission is determined exclusively by atomic oxygen concentration, whereas the summer peak is the superposition of all impacts, with temperature taking a leading role.

中文翻译：

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中纬度激发羟基发射的半年变化

地面观测显示 ，与低纬度地区相比，中纬度地区（^43∘N）的激发羟基（OH ^∗）排放的半年变化具有相移。这与高纬度地区的年度周期不同。我们通过利用OH ^*气辉模型检查这种行为，该 模型已被整合到3D化学-运输模型（CTM）中。通过这种建模，我们研究了中纬度（^30–50∘ N），我们评估了半年变化的主要驱动因素：温度，原子氧和空气密度。我们发现半年周期的这种变化主要是由温度和原子氧浓度的年变化的叠加决定的。因此，冬天的排放峰仅由原子氧浓度决定，而夏天的峰则是所有影响的叠加，温度起主要作用。