Lidar Observations And Studies Of The Meteoric Potassium Layer Over Beijing

A dual-wavelength resonance fluorescence lidar facility,operating at 770 nm and 589 nm,was set up in Beijing(40.41°N,116.01°E)to detect the meteoric potassium and sodium layers simultaneously.The detailed retrieval procedure of the potassium data is presented,especially in terms of the calculation of the effective backscatter cross section of the potassium D1 line and the fitting method of the raw photon number at the reference altitude.The lidar system has been operational for 220 nights,clocking 1250.8 h in total from November 2010 to October 2011 and May 2013 to April 2014.To observe the potassium layer can contribute to studying the dynamic and chemical processes and the coupling mechanism between the neutral and ionized constituents in the mesopause and lower thermosphere region(80 – 105 km).The observations reveal that the nocturnal annual mean column density of the potassium layer is 8.41×107 cm-2;Peak density is 80 cm-3;Central height is 89.7 km and RMS width is 4.5 km.The potassium layer shows considerable semiannual seasonal variations.Specifically,the column and peak densities vary semiannually with maxima in winter and summer.The centroid height displays semiannual variation,reaching maxima in spring and autumn.The RMS width has a slight semiannual seasonality with maxima in winter and summer.The seasonal behavior of the potassium layer in Beijing is similar to that in other potassium lidar sties.However,the unexpected difference is that the column and peak densities of the potassium layer are extremely large in winter,compared to the maximum values in summer.The special behavior of the potassium layer has been discovered from two yearlong data sets.The peak density of the special potassium layer increases gradually and the height of the peak density evidently descends with time.The concentration increases rapidly at the bottom of the layer,but drops slowly at the top of the layer along with increasing height.We term this peculiar potassium layer as the “lower-triangle potassium layer”.The lower-triangle potassium layer has the highest occurrence ratio in January and makes the column density larger and central height lower.The lower-triangle potassium layer is compared with the sodium layer at the same time and location.The results show that the analogous lower-triangle sodium layer does not occur on 8 of the 11 nights.When the lower-triangle potassium layer forms,the column density of the potassium layer increases obviously and the increasing potassium atoms are located below 90 km.However,the sodium layer changes little.Furthermore,the concentration ratio of potassium to sodium clearly increases from about 0.01 or 0.02 to more than 0.06.The comparisons of the time and altitudes of the largest diurnal tidal amplitude and the lower-triangle potassium layer show that 8 of the 11 lower-triangle events have no significant relation to the atmospheric diurnal tides,which proves that the atmospheric diurnal tides are not the most important determinant of the lower-triangle potassium layer.Assuming that the meteor ablation and atmospheric sedimentation are the major source and sink of the potassium atoms,and taking the continuity equation and eddy diffusion equation as dynamical controlling equations,the total concentration of all potassium species was obtained in the mesopause region.According to the closed chemical reactions between different potassium species,the concentration of each potassium species was solved at each altitude.Then the model of the potassium layer was established preliminarily.Building process,input parameters and simulation results of the potassium model are described in detail.The current potassium model can simulate the vertical distribution of the potassium atoms.Output results of the potassium model are compared with the nocturnal annual mean potassium layer measured by lidar over Beijing.Peak density and centroid height of the potassium layer simulated by the model are in good agreement with observations,while the simulative column density and RMS width is less than the observations.The differences between simulations and observations are mainly from the bottom side of the potassium layer.Establishments and improvements of the potassium model will contribute to the study of characteristics and evolutionary process of the potassium layer.