The Observations And Studies Of Thermospheric Na Layers From Midlatitude Lidar Stations

The first detection of the neutral atmospheric sodium layer by resonant fluorescence lidar(light detection and ranging)was reported by Bowman et al.(1969).From then on,large studies indicated that the neutral metal atomic layers are mainly distributed from 80 to 105 km including Na,K,Li,Fe,Ga,Ga+,Mg,Ni and so on.Also,lidar is one of the remote sensing techniques that offer the greatest promise for high-resolution observations of the temperature and wind of the upper atmosphere.The rare layers above the main layer extending to bout 120 km,were reported(mostly in Na)over the past twenty years(Collins et al.,1996)have received renewed interest by Gong et al.(2003)and others with different names.The newest discovery of thermospheric neutral metal layers extending to 130-170 km have stimulated considerable community interest in part due to their use as tracers for resonance lidar temperature and wind measurements,thus potentially tripling the altitude range for atmospheric dynamics studies into the thermosphere.Fe layers between 110 and 155 km over McMurdo,Antarctica(77.8°S,166.7°E)by Chu et al.(2011)was the first report of this type.Here,we report our search from Na lidar stations of the Meridian Project.The key results are as follows:1 Low thermospheric sodium layer:(1)We found 38 1ow thermospheric sodium layers using 11607 h of observational lidar data from 2010 to 2016.The statistical analyses of the peak altitudes,peak local times,FWHMs,and peak densities of the low thermospheric sodium layers are presented.And we focus on the analysis of the climatic characteristics of the low thermospheric sodium layers;(2)We analyzed the climatic characteristics of Es for six years and the statistical correlations of the individual low thermospheric sodium layers and Es layers.We found more than 90% of the low thermospheric sodium layers were accompanied by Es;(3)By comparing the low thermospheric sodium layers observed from Yanqing station and Pingquan station simultaneously,we found all of low thermospheric sodium layers have large horizontal spatial scale.(4)About two thirds low thermospheric sodium layers exhibited apparent downward phase progressions,while the descending rates of the low thermospheric sodium layers are consistent with the phase speeds of the tide.(5)The low thermospheric sodium layers and low thermospheric potassium layers always observed at same night and most of the low thermospheric sodium layers and low thermospheric potassium layers are distributed at the same height and have the same evolution trend.Only different between them is the variation of density.(6)We described physical and chemical processes of wind-Es-low thermospheric sodium layers mechanism using a case analysis of 30 May 2011 and the vertical transport mechanism using a case analysis of 7 June 2017.2 Thermospheric sodium Layers:(1)We report our search from sodium lidar stations of the Meridian Project,and found 4 nights with thermospheric layers,including one that yielded highest Na density of ~35 cm-3 and reached highest altitude of 200 km;(2)The first concurrent observations by two lidar stations 237 km apart have provided clear evidence that more thremospheric layers are localized,likely with horizontal dimension no more than 250 km.(3)In addition to small horizontal dimension,comparing descending rate with CTMT tidal phase fronts,allowing the association of descending rates to the gravity wave phase fronts.On the other hand,the weakness in geomagnetic variations and local TEC during the layer occurrence raises a dark cloud on whether the ionosphere electric field or the fountain effect is strong enough to provide the uplift of ions necessary for the formation of thermospheric layers at central midlatitude.