The Overturning Structure of Sodium Layer in Lower-thermospheric Region and Its Relevance to Ionospheric Field Align Irregularity (FAI) and Gravity Waves

We report a surge-like overturning structure of the sodium layer in the lower thermospheric region (~110-120km) observed by a sodium lidar located at Haikou (20.0N, 110.3E), China. During 15:00UT ~ 15:30UT (Local time +8h), the sodium density shows an abrupt vertical increasing from ~100km to ~120km, and then falls down with a speed of ~2.5km/hr. The adjacent radio observations from COSMIC satellites and two ionosondes located at Fuke (19.5N, 109.1E) and Sanya (18.4N, 109.6E) indicate complicated ionospheric E region structures, i.e., multiple sporadic-E (Es) layers, covering the Hainan island region (18N-21N, 108-111E). Further more, a VHF radar located at Sanya (18.4, 109.6E) observed the field align irregularities (FAIs), which appear almost simultaneously with the surge-like structure of the sodium. The radar receiving SNR and the corresponding Doppler speed show a similar vertical increasing structure extending from ~100km to ~130km and the charged particles moving away (i.e., upward) from radar at the beginning of the FAIs and follows by descend-layer-like FAIs with a downward speed of 2.9km/hr. From these observational evidence, the surge-like structure of sodium layer is linked to the ionospheric E-region FAIs. We propose the possible process related to the formation of the surge-like sodium layer: the preexisted Es might provide a suitable gradient, and the gravity wave could trigger the K-H instability and push the upward moving of the charged particles, which are the source of Na+ and can be converted to Na through the chemical reactions. The investigations using the lidar temperature (35-55km), meteor radar wind (85-95km) and sodium column density and centroid height indicate that the gravity wave with period of ~4 hours and vertical wavelength of ~10km might be potential driver.