An overturning-like thermospheric Na layer and its relevance to Ionospheric field aligned irregularity and sporadic E

We report an overturning-like structure of the thermospheric sodium layer (TSL) in the altitude region of ∼100-120 km observed by a sodium lidar at Haikou (20.0°N), China, on July 29, 2012. The overturning-like sodium layer was first seen as upwelling from the top of the sodium layer (∼102 km) to an altitude of ∼118 km from 14:55 to 15:50 UT and then descending gradually from its apex with a speed of 3.5 km/h. The ionospheric observations from the COSMIC radio occultation and three ionosondes exhibited abrupt perturbations in the radio occultation (RO) SNR profiles and spread Es in the ionograms, respectively, indicating the existence of complex Es around Haikou. On the other hand, VHF radars located at Sanya (18.4°N, 220 km away from Haikou) and Fuke (19.5°N, 130 km away from Haikou) both recorded strong E region field-aligned irregularity (FAI) echoes altitude-extended structure covering altitudes of 100-140 km, which are well correlated with the overturning-like structure of the thermospheric sodium layer. The good agreement between occurrence time of sodium layer (and FAI) structure and of complex Es could indicate a close correlation between them. One possibility is that the chemical reaction in the course of the complex Es (with altitude-deformed structure) could produce sufficient sodium atoms and thus lead to the formation of sodium layer upwelling structure. Correspondingly, FAI altitude-extended structure could be generated through the gradient drift instability in the altitude-extend (deformed) Es which provide plasma density gradients to support the instability development.