Statistical Study of the Latitudinal and Seasonal Variations of Intermediate Descending Layers over China

Abstract. This paper presents a statistical investigation of intermediate descending layers (IDLs) in the ionospheric E region using multi-station digital-ionosonde observations at Mohe, Wuhan, and Fuke. The measurements reveal clear descending structures, strong seasonal variability, and pronounced latitude-dependent local-time morphology. Mohe and Wuhan commonly exhibit morning and afternoon occurrence enhancements, with the morning layer generally stronger than the afternoon layer, whereas Fuke shows a broader daytime distribution in which the afternoon enhancement can exceed the morning enhancement and multiple local maxima may occur. Lomb–Scargle analysis of IDL virtual height identifies prominent 12-h and 24-h periodicities, together with a weaker 8-h component in some cases. We interpret the asymmetric morning–afternoon structure as the result of superposed atmospheric tides: the early peak is preferentially associated with diurnal modulation, whereas the later peak is more closely related to semidiurnal modulation. At Mohe and Wuhan, the larger morning occurrence rate is consistent with constructive superposition of the diurnal and semidiurnal components during the morning and weaker reinforcement during the afternoon. At Fuke, the weaker and less stable semidiurnal signature, together with the afternoon-dominant and occasionally three-peak morphology, suggests that a secondary 8-h (terdiurnal) contribution may become relatively more important in shaping the local-time distribution, although the dominant height periodicity remains near 24 h. These results support a latitude-dependent tidal-modulation framework in which several tidal components jointly organize the occurrence and descent of IDLs through wind-shear-driven ion convergence.