Lower thermospheric-enhanced sodium layers observed at low latitude and possible formation: Case studies

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1 JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 118, , doi: /jgra.50200, 2013 Lower thermospheric-enhanced sodium layers observed at low latitude and possible formation: Case studies X. H. Xue, 1,2 X. K. Dou, 1,3 J. Lei, 1,2 J. S. Chen, 3,4,5 Z. H. Ding, 5 T. Li, 1,2 Q. Gao, 1,2 W. W. Tang, 1,2 X. W. Cheng, 6 and K. Wei 7 Received 22 November 2012; revised 23 January 2013; accepted 20 February 2013; published 9 May [1] We report two lower thermospheric-enhanced sodium layer (TeSL) cases observed at a low-latitude station, Lijiang, China (26.7 N, E), on 10 March and 10 April 2012, respectively. The TeSLs in the two cases were located at altitudes near 122 and 112 km, respectively. In addition, strong sporadic sodium layers (SSLs) near 100 km accompanied the TeSL observed on 10 March Both the TeSLs and SSLs exhibited tidal-induced downward motion. The adjacent ground-based and space-borne ionospheric radio observations showed strong E s layers before the appearance of the TeSLs, suggesting an E s TeSLs (SSLs) chain formed through the tidal wind shear mechanism. Assuming that the vertical tidal wavelengths remain unchanged, it is found that in different regions caused by the tidal wind shear, different TeSLs evolution processes are expected: (1) in a tidal-convergence region, a TeSL/SSL with a downward propagation phase is enhanced due to a rapid decrease in the Na + lifetime at the lower altitude; (2) in an ion convergencedivergence interface region, a TeSL/SSL will still follow the tidal downward phase progression, but sodium density does not exhibit evident enhancement; and (3) when a TeSL/SSL enters into a tidal wind-divergence zone, the layer density tends to decrease. Citation: Xue, X. H., X. K. Dou, J. Lei, J. S. Chen, Z. H. Ding, T. Li, Q. Gao, W. W. Tang, X. W. Cheng, and K. Wei (2013), Lower thermospheric enhanced sodium layers observed at low latitude and possible formation: Case studies, J. Geophys. Res. Space Physics, 118, , doi: /jgra Introduction [2] The deposition of extraterrestrial material in the Earth s upper atmosphere gives rise to layers of free metal atoms or ions, e.g., Na, Fe, Ca, Ca +, in the mesosphere and lower thermosphere at altitudes ranging from 80 to 105 km. Investigation using the lidars since the 1970s, combined with laboratory experiments, has made it possible to reproduce 1 CAS Key Laboratory of Geospace Environment, Department of Geophysics & Planetary Sciences, University of Science & Technology of China, Hefei, Anhui, China. 2 Mengcheng National Geophysical Observatory, School of Earth & Space Sciences, University of Science & Technology of China, Hefei, China. 3 State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing, China. 4 Graduate University of Chinese Academy of Science, Beijing, China. 5 National Key Laboratory of Electromagnetic Environment, China Research Institute of Radiowave Propagation, Qingdao, China. 6 Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan, China. 7 The Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu, China. Corresponding author: X. K. Dou, CAS Key Laboratory of Geospace Environment, Department of Geophysics & Planetary Sciences, University of Science & Technology of China, Hefei, Anhui , China. (dou@ustc.edu.cn) American Geophysical Union. All Rights Reserved /13/ /jgra the major features of some of the metal layers [e.g., Plane et al. 1999; Plane, 2004; Delgado et al., 2006]. However, some features of the metal layers are still very interesting and remain mysteries to researchers. [3] One of these features is the occurrence of sporadic (or sudden) metal atom layers, which was first reported by Clemesha et al. [1978] using ground-based sodium lidar and is defined as an abrupt increase in the metal density over the level of the background layers. Sporadic metal layers, such as sporadic sodium layers (SSLs), appear widely at low and high latitudes as well as at some middle latitudes [Nagasawa and Abo, 1995; Dou et al., 2009]. The horizontal dimensions of the sporadic metal layers usually extend to a large area, varying from hundreds to thousands of kilometers [Kane et al., 1991; Fan et al., 2007]. Among the many mechanisms for interpreting sporadic metal layer formation, the most promising one is neutralization of ions, and the evidence for this is the high correlation in time and spatial location between metal atoms and sporadic E (E s ) layers [Gardner et al., 1993; Kane et al., 1993; Friedman et al., 2000; Williams et al., 2007; Dou et al., 2010; Delgado et al., 2012], especially for sporadic layers that appear above the main metal layer peak. For example, Friedman et al. [2000] found that above 97 km, the SSLs had associated ion layers, while below that altitude, some SSLs could occur in their absence. The same finding was reported by Hansen and Von Zahn [1990] and Gardner et al. [1993].

2 Table 1. Main Configuration of the USTC Lidar System at Lijiang Transmitter: Nd:YAG Dye Laser Wavelength (nm) Pulse energy (mj) (typ.) Line width(cm 1 ) Pulse width (ns) 6 6 Repetition rate(hz) Beam divergence (mrad) Receiver-Telescope: Type Cassegrain Diameter (mm) 1800 Field of view (mrad) 1.0 Focus type Nasmyth Receiver-Filter: Wavelength (nm) 589 Bandwidth (nm) 1.0 Peak transmission 50% [4] The next one is the topside layer, which is deemed as a high-altitude extension of the neutral metal layer. For instance, Höffner and Friedman [2004] found that the mean K and Ca layers at 54 N exhibited similar summer topside extensions (120 km). Because the metal density enhancement in summer correlated well with the seasonal/annual variation of sporadic micrometeoroid input, independent of meteor showers, the authors suggested a direct link between ablating meteoroids and the topside metal layers. Later, Höffner and Friedman [2005] showed, using case studies near 54 N, that the topside of the mesospheric metal layer (Na, Fe, Ca, and K) exists throughout the year and can be detected up to altitudes of 130 km. Recently, Chu et al. [2011] reported on a wavy neural Fe layer that extended to km, most likely related to internal gravity waves. [5] If the metal atoms are accumulated within the topside layer above 105 km (where the metal atom density is very weak) through a type of mechanism, then a thermosphericenhanced metal layer will appear, and a lidar would detect an additional metal layer located in a lower thermospheric region. Collins et al. [1996] first reported a thermosphericenhanced sodium layer near 109 km at Poker Flat, Alaska, during active aurora. The whole event lasted only 15 min. In another case observed at a high-latitude region, Kühlungsborn (54 N), Germany, reported by Höffner and Friedman [2005], thermospheric Fe, K, and Ca layers were all enhanced near 112 km. In the middle latitude region, Gong et al. [2003] observed a double sodium layer at Wuhan (31 N), China, that lasted about 2 h, with a central peak at approximately 112 km. Ma and Yi [2010] conducted a statistical study of high-altitude SSLs and sporadic Fe layers (Fes) above 100 km and found that 12% of SSLs and 26% of Fes appeared above 105 km with relatively greater widths. Recently, Wang et al. [2012] reported 17 double sodium layer events that were observed on 319 nights in Beijing (40 N), China, between 2009 and 2011, all of which were during the spring and summer. However, due to the lack of conjunction observations, the causes of thermospheric-enhanced metal layers remain unclear. [6] In this paper, we report two lower thermospheric sodium layer (TeSL) events observed during March April 2012 at a low-latitude region, Lijiang (26.7 N), China. Using collocated simultaneous ionosphere and wind observations, we cast some light on the cause-and-effect Table 2. Main Configurations of the Kuming Meteor Radar and MF Radar Systems Factors Meteor Radar MF radar Peak power 20.0 kw 64.0 KW Frequency 37.5 MHz MHz Pulse repetition frequency 430 khz 100 khz Pulse width 24 s 24 s Bandwidth 18.1 khz 60 khz Range resolution 2 km 2 km Sample range km km mechanism of the TeSLs. A description of the observational instruments and analysis methods is given in section 2. The cases observed on 10 March 2012 and 10 April 2012, as well as the possible mechanism of their formation, are described in sections 3 and 4. Section 5 addresses a few features of the observed TeSLs. Conclusions are presented in section Experimental Facilities and Methods [7] A new sodium fluorescence lidar has been developed by the University of Science and Technology of China at Gaomeigu Astronomical Observatory in Lijiang, China (26.7 N, E). This lidar has been used to acquire data during an initial observational campaign from 8 March to 15 April The lidar system is similar to that at Hefei, China [Dou et al., 2009, 2010]. The transmitter of the lidar is a tunable dye laser with 60 mj pulse energy and a 20 Hz repetition rate, pumped by a neodymium:yttrium/aluminum/garnet (Nd:YAG) laser at a wavelength of 532 nm. The dye laser is automatically locked to the sodium D2-resonant absorption line at 589 nm calibrated by a reference sodium atom cell. The backscattered photons collected by the telescope are first filtered by interference filter with center wavelength at 589 nm, before they enter into a photomultiplier tube (PMT). Then, the output signals from PMT are processed by a PC-based photon counting multichannel scalar (MCS) in successive time bins. The main improvement in the lidar performance s is the receiving telescope, which is a 1.8 m Cassegrain optical system used for astronomical observations and operated by the Institute of Optics and Electronics of the Chinese Academy of Sciences. Details of the telescope are given by Weietal.[2010]. The sodium photons are focused on its Nasmyth focus and coupled with the optic receiver through a fiber. The time bin taken here is 640 ns, corresponding to a vertical resolution of 96 m. One thousand (or five hundred) laser shots are accumulated to attain a lidar photo count profile. The corresponding time resolution is 50 s (or 25 s). The main characteristics of the University of Science and Technology of China (USTC) lidar are presented in Table 1. During this campaign period, we obtained 27 clear-night observations of the sodium layer over a total observational time of was more than 130 h. The thermospheric enhanced sodium layer (TeSL) is defined as a secondary sodium layer Table 3. Main Configuration of the Kuming Ionosonde Frequency scan region Frequency resolution Detection range Range resolution 1 32 MHz 0.05 MHz km 5 km 2410

3 Figure 1. (a) A sodium photon count profile at 14:52 UT, corresponding to the maximum of the TeSL, on 10 March The TeSL and SSL peaked at 118 and 101 km, respectively. The spatial and temporal resolution of this event is the 96 m and 50 s. (b) The variations of the column density for the TeSL (dotted line) integrated between 110 to 130 km and main sodium layer (solid line) integrated between 80 to 130 km, together with their ratio (green line) versus observational time on 10 March Note that the column density of the TeSL is multiplied by factor 20. separated from the main sodium layer with a clear peak higher than 105 km. [8] Radar systems, including an all-sky meteor radar, an MF radar, and an ionosonde at the Kunming Radio Observatory (25.6 N, E, approximately 300 km away from Lijiang) operated by the China Research Institute of Radiowave Propagation (CRIRP) were used to detect the wind and ionospheric characteristics during this campaign period. The all-sky meteor radar and the MF radar, as part of the Kuming Atmospheric Radar Facility (KARF) [Zhao et al., 2011], were both manufactured by Atmospheric Radar Systems of Australia and installed at Kuming Radio Observatory in November The ionosonde (type TYC-1) was developed by CRIRP. This type of ionosonde is widely used in other radio observatories of the CRIRP. The basic parameters of the two radar systems and the ionosonde are listed in Tables 2 and 3, respectively. [9] Recently, the Global Positioning System-Low Earth Orbiter (GPS-LEO) radio occultation technique was used to study irregularities in the lower ionospheric E region. LEO satellites, such as the FORMOSAT-3 (Formosa Satellite Mission-3)/COSMIC constellation (Constellation Observing System for Meteorology, Ionosphere, and Climate) and the German TerraSAR X satellite, can receive the two L-band frequencies (L1=1.6 GHz and L2=1.2 GHz) broadcast from GPS satellites. Precise measurements of the time delay between GPS transmitters and LEO receivers can be used to obtain profiles of atmospheric pressure, temperature, and water vapor or ionospheric electron density. In this study, we used the 50 Hz measurement of signal-to-noise ratio (SNR) perturbations at a tangent height of km, which were normalized to their free-space averages (SNR0, a smoothed SNR between 50 and 70 km) according to Wu et al. [2005] s method, during the radio occultation observed by COSMIC or TerraSAR X. If the tangent point of an occultation was located in a region centered at Lijiang (26.7 N, 100. E) with a latitudinal and longitudinal extension of 5 and the occultation time was within 4 h of the lidar observational period, then the SNR perturbation profile was chosen as the effective observation. Figure 2. The development of whole sodium layer during the observational period 13:00 UT 16:30 UT on 10 March 2012, at Lijiang, China. The different color bars for upper and lower layers for better visualization. The blank area is due to the thin cloud covering over the lidar site. The MF received power is contoured as the solid white lines; the contour levels are 10,000 to 30,000 in arbitrary unit, with an interval of

4 XUE ET AL.: TESLS AND THEIR FORMATION Figure 3. (a) The logarithmic contour of sodium layer during the observational period 16:00 UT 23:00 UT on 10 April The MF received power is contoured as the solid white lines, the contour levels are 10,000 to 35,000 in arbitrary unit, with an interval of (b) The peak density (solid line) and peak height (dotted line) of the TeSL on 10 April 2012 versus time. (c) The variations of the column density for the TeSL (dotted line) integrated between 103 to 120 km and main sodium layer (solid line) integrated between 80 to 120 km, together with their ratio (green line) versus observational time on 10 April Note that the column density of the TeSL is multiplied by factor Lidar Observation of the TeSL Events [10] On the night of 10 March 2012, a prominent TeSL was detected by the Lijiang sodium lidar. The overview of backscattered sodium photon counts accumulated for 50 s at 14:52 UT on 10 March 2012 is shown in Figure 1a. The main sodium layer was located at km, with maximum backscattered photons of 3500, and the TeSL between 110 and 130 km was separated from the main layer by an order of magnitude less in photon count number. Meanwhile, an SSL superimposed on the main layer appeared at 102 km. [11] Figure 2 shows the development of the whole sodium layer during the period 13:00 UT to 16:30 UT. The TeSL was not formed when data acquisition began at approximately 13:00 UT, and a data gap exists in the data due to a thin cloud that appeared between 13:15 UT to 14:18 UT, but after that, the TeSL was present for the rest of the observing period. The peak sodium density of TeSL increased from 60 cm 3 to a maximum of 120 cm 3 (approximately 6.7% of the peak density of the main sodium layer) between 14:20 and 15:00 UT and then decayed gradually to 45 cm 3. The full width at half maximum (FWHM) of the TeSL extended from 4.5 to 8.0 km and recovered to 4 km at the end of the observation period. The peak altitude descended from approximately to km, with an average downward speed of 1.5 km/hr. [12] The main sodium layer had a peak value 1800 cm 3 near 91.5 km before 15:20 UT and was lifted approximately 1.5 km between 15:30 and 16:30 UT. It should be noted that before the TeSL formation, an SSL with a maximum density of 1000 cm 3 had already appeared at 105 km (at 13:10 UT) and continued during the whole observation period. At approximately 15:20 UT, the SSL density began to increase rapidly and reached 5500 cm 3 at 16:25 UT, three times greater than the peak density in the main sodium layer. At almost the same time that the SSL became strong, a second SSL, with a peak density of 2000 cm 3, appeared at 104 km, and was followed the leading SSL. The downward speeds of the first and second SSLs were approximately 1.9 and 1.3 km/hr, respectively. [13] The column density of the main layer is indicated by the solid black line in Figure 1b. During the observation period, the column density of the main layer increased from cm 2 to cm 2. The column density of TeSL (dotted line in Figure 1b) increased to a maximum value of cm 2, approximately 3.8% of that of the main layer (solid green line in Figure 1b) at 15:00 UT. The two SSLs (especially the lower SSL) increased the column density of the main layer gradually after 15:00 UT, while the TeSL decreased the ratio of column density to that of the main layer to less than 1.5% after 16:00 UT. [14] Another case of a TeSL, which occurred on the night of 10 April 2012, is shown in Figure 3a. The TeSL developed at approximately 16:15 UT with a central height at 112 km and propagated with downward speed of 1.2 km/hr. Figure 3b shows that the peak density in the TeSL increased slowly and was less than 100 cm 3 before 20:30 UT. After that time, the TeSL seemed to merge into the main layer and trigger a weak SSL at 105 km at 21:15 UT. The SSL s peak density of 330 cm 3 occurred at 22:40 UT, and the central height was 104 km. The column density of the TeSL (dotted line in Figure 3c) increased gradually from cm 2 to cm 2. The column density of the main layer varied between cm 2 and cm 2, as indicated by the solid line in Figure 3c. The larger ratio of the TeSL column density to that of the main layer was approximately 3.6%. 4. Possible Formation Mechanism of the TeSL Events [15] To explore the possible formation mechanism of the TeSL events, data from the radio observational systems mentioned in section 2 were used. For simplicity, we focused on the 10 March 2012 event. [16] For the 10 March 2012 event, the hourly critical frequency fo Es and the virtual height h0 Es observed by the CRIRP ionosonde at Kunming for the 3 days from 9 to 11 March 2012 are indicated by the stars and dotted line, respectively, in Figure 4a. An fo Es greater than 4 MHz began at 8:00 UT and lasted until 16:00 UT on 10 March It can also be seen that an extremely large fo Es, approximately 20 MHz, emerged between 12:00 and 13:00 UT, just before the formation of the TeSL. From 5:00 UT to 11:00 UT on 10 March 2012, the virtual height h0 Es was first elevated from 110 to 155 km and then fell to 110 km. When fo Es 2412

5 Figure 4. (a) The critical frequency f o E s (stars) and virtual height hes (dots) observed by ionosonde located at Kunming Radio Observatory, China, about 300 km away from Lijiang, on 9 11 March The lidar observational period is marked by dash lines. (b) The normalized SNR (signal-to-noise ratio) perturbation of the 50 Hz TerraSAR-X sampled data at 10:32 UT on 10 March 2012, over a tangent point (30 N, 100 E), the free-space SNR 0 is obtained from the SNR profile between 50 and 70 km for normal quiet ionospheric conditions. (c) The same as Figure 4b but observed by COSMIC at 20:37 UT, with a tangent point (31 N, 98 E). Figure 5. (a) The ratio r i (solid line) of ion-neutral collision frequency ( i ) to ion gyrofrequency ( i ) and the diffusion time scale (dashed line) for a typical diurnal tidal (DT) wind with a vertical wave number k z and diffusion coefficient D a = k B (T i + T e )/m i i, assuming T e = T i = T n and k z =2/(25 km). (b) Estimates of vertical ions convergence times at Lijiang, China, for z =1 km for strong (100 ms 1 )and weak (10 ms 1 ) zonal (black) and meridional (red) wind driving. The solid lines and dash lines represent the strong and weak wind, respectively. 2413

6 Figure 6. (a) The zonal wind and (b) meridional wind observed by the meteor radar at km on 9 to 11 March 2012 at Kunming Radio Observatory, China. The lidar observational period is marked by dash lines. The fitted tidal wind amplitudes and phases from the meteor radar observations versus the altitude are represented as pluses in the following figures, i.e., (c and d) the amplitudes and phases for zonal DT, (e and f) for zonal SDT, (g and h) for meridional DT, and (i and j) for meridional SDT, respectively. The stars in the figures represent the corresponding global scale wave model (GSWM) tidal (DT or SDT) amplitude or phase on March at 27 N for zonal or meridional wind, while the triangles are the extending amplitude and phase from the meteor radar wind above 100 km for the corresponding tidal (DT or SDT) component. increased abruptly, h 0 E s rose to 125 km at 12:00 UT and then returned to 100 km during the TeSL period. The critical frequency of f o E s 20 MHz indicated the electron density of cm 3, assuming that the relationship between the electron density (in cm 3 ) and the ionosonde frequency (in MHz) satisfies N = f 2. Thus, the sodium ion density can be estimated as 10 5 cm 3, according to Hansen and Von Zahn [1990]. It suggested an extremely strong E s leading the TeSL. The white contour line in Figure 2 shows the power amplitude received by the KARF MF radar, with a resolution of 3 min and 2 km at km. The strong power echoes, which are deemed as the bottom of the E s [Tsuda et al. 2011], appeared at 100 km near 15:00 UT and well followed by the descending of the SSL. This confirms again the role of E s in forming the SSL. [17] Additional evidence of the existence of the E s is the radio occultation observation. According to the data selection criterion in section 2, a pre-tesl SNR (10:32 UT) and a post-tesl SNR (20:37 UT) were identified from TerraSAR X and COSMIC observations, respectively, as shown in Figures 4b and 4c. For the pre-tesl SNR perturbation (Figure 4b), four thin irregular electron density layers (or E s ) with the SNR perturbations as high as 50% appeared at heights of approximately 126 km, 122 km, 105 km, and 93 km. Considering the peak altitudes of the TeSL and SSL, the upper two E s layers might be related to the TeSL, and the E s layer located at 105 km might be responsible for the SSL. The post-tesl SNR observation by COSMIC occurred about 4 h after the end of the lidar observation, but a weak E s layer was still approximately 101 km with SNR perturbations of approximately 15%. [18] According to wind shear theory, vertical shears in the horizontal wind play a key role in forming E s layers [Mathews, 1998], and atmospheric tidal winds are the main drivers in the formation of E s layers. Our analysis showed that both the TeSL and the SSLs on 10 March 2012 tended to descend at a speed of approximately km/hr, indicating an effect of the downward progression of the tide. 2414

7 Figure 7. The calculated divergence dw iz /dz of the ion vertical motion on 10 March The negative divergence is shown as blue while the positive as red. The peak height of TeSL and SSLs versus time are shown as the black dots. Therefore, we investigate the possible dynamical process associated with enhanced metal and ion layers. [19] From the ion momentum equation, ignoring the pressure gradient force and the electric field force, we can obtain the vertical velocity (w iz ) for the ions at Lijiang (where the magnetic declination approximately zero and the magnetic strength is 45,000 nt) as follows [Kirkwood and Nilsson, 2000]: w iz = r i cos I 1+r 2 i U + cos I 1+r 2 i ( V sin I )+ XUE ET AL.: TESLS AND THEIR FORMATION 1 cos2 I 1+r 2 i W, (1) where the magnetic dip angle I is equal to 37 ı at the Lijiang station. Neutral wind velocity U n =(U, V, W ) components are positive upward, eastward, and northward directions. Because the vertical wind velocity W is usually an order of magnitude less than the zonal and meridional wind velocities (U, V ), the third term in the above equation can be omitted. r i is the ratio of the ion-neutral collision frequency ( i ) to the ion gyrofrequency ( i ). For simplicity, the ion gyrofrequency ( i ) in equation (1) was taken as 180 Hz, and the ion-neutral collision frequency was estimated based on the literature of Kelley [2009] as i = N n / p m n, where N n and m n are the neutral atmospheric density (from Mass Spectrometer - Incoherent Scatter Empirical (MSISE)-00 model) and molecular mass, respectively. The solid line in Figure 5a shows a typical value of r i versus the altitude. In the region of r i << 1, the vertical motion of the ions (equation (1)) is dominated by the meridional wind. While in the region of r i >> 1, zonal wind controls the vertical motion of the ions. [20] Figure 6 shows the zonal and meridional winds observed by the KARF meteor radar for the 3 days centered around 10 March The sodium lidar observational period is indicated by dashed lines. The tidal structures in (U, V ) with downward phase progression are clear. The diurnal tidal (DT) and semidiurnal tidal (SDT) amplitudes and phases (when the amplitudes reach the maximum) from the least-square-fitting for both the zonal and meridional winds 2415 are illustrated by plus symbols in Figures 6c 6j, respectively. The zonal DT/SDT amplitude and phase are shown in Figures 6c/6e and 6d/6f, and the meridional DT/SDT amplitude and phase are shown in Figures 6g/6i and 6h/6j. [21] Unfortunately, above 100 km, there is no wind observation data available, and the tidal amplitude/phase has to be inferred to identify the possible process associated with the formation of the TeSL. The stars in Figures 6c 6j indicate the corresponding amplitudes and phases of the zonal and meridional DT (SDT) of the global scale wave model (GSWM-00) in March at 27 N. The following should be noted: (1) the phase variation of the zonal/meridional DT (SDT) versus the altitude in GSWM is quasi-linear in the range of km; (2) the least-squares fitted phase of the observed wind component has a similar altitudinal dependence with the GSWM phase in the range of km, in spite of some phase difference; and (3) the fitted amplitude of the observed tidal wind component is different from that in GSWM in the range of km. Thus, we assume that the vertical wavelength of a tidal component (DT or SDT) remains unchanged in the range of km. The inferred tidal phase above 100 km is obtained using a linear extension of the observed tidal phase between 80 and 100 km. In addition, the inferred tidal amplitude above 100 km is obtained by multiplying the GSWM tidal amplitude above 100 km by the ratio of an averaged observed tidal amplitude to the GSWM tidal amplitude between 86 and 92 km, due to the large difference between the observed tidal amplitude and the GSWM tidal amplitude. The inferred tidal amplitude and phase are also indicated by the triangles in Figures 6c 6j. [22] The zonal (meridional) wind U(V ) above 100 km in equation (1) is constructed from the zonal (meridional) DT, SDT wind, and mean flow. The DT and SDT above 100 km can be obtained if the inferred amplitude and phase of DT and SDT are given, and the mean flow is assumed no change with altitude in the region of TeSLs or SSLs (>100 km). Thus, the divergence of the ions vertical motion dw iz dz can be calculated using equation (1). The divergence of ions vertical motion at six altitude levels above 100 km on 10 March 2012 is shown in Figure 7, in which the blue stars Figure 8. The same as Figure 7, but for 10 April 2012 event.

8 indicate the ion-convergence region dw iz <0 and the red dz stars indicate the ion-divergence region dw iz >0. The black dz dots in Figure 7 represent the trend of the peak height for the TeSL ( km) and SSLs ( km). [23] As shown in Figure 7, before the appearance of the TeSL (at approximately 13:00 UT) on 10 March 2012, strong ion convergence appeared at km, with dwiz dz 4 ms 1 km 1, which might trigger the formation of the TeSL observed by lidar. As the TeSL reached its peak value near 14:55 UT, the ions vertical motion changed from convergence to divergence at 120 km. After that, the ions diverged gradually. This might be one reason why the TeSL only ascended for a short period of time and then became weak and disappeared from this region. [24] Note that the inferred large meridional SDT amplitude of approximately 160 m/s at 120 km (due to the ratio of GSWM SDT amplitude at 120 km to that at 90 km being approximately 7; Figure 6i) causes a strong gradient of ion vertical motion between 115 and 125 km. However, an SDT amplitude of 160 ms 1 might be not reasonable. We tried using an inferred meridional SDT amplitude of 40 ms 1 at 120 km (corresponding to the ratio of GSWM SDT amplitude at 120 km to that at 90 km being 2), the ions vertical motion divergence calculated with DT, and this new SDT winds was nearly the same as that shown in Figure 7, but the value of dwiz was limited to within 2 ms 1 km 1. dz [25] In the SSLs region, on the other hand, the ions convergence continued and tended to be enhanced by the wind shear at 103 km, which most likely contributed to maintaining the E s and enhancing the SSLs at the end of the lidar observation. [26] On 10 April 2012, the CRIRP ionosonde and COSMIC also detected an E s before the formation of the TeSL. For example, an f o E s of 10.5 MHz, indicating a sodium ion density of 10 4 cm 3, appeared an hour before the beginning of the TeSL. By comparing the observed tidal wind with GSWM (figure not shown) as mentioned above, the ions vertical motion divergence dw iz dz can be derived as shown in Figure 8. Between 16:00 UT and 17:00 UT, the ions convergence zone was consistent with the peak height of the TeSL (dots in Figure 8) at approximately km, indicating the wind shear mechanism plays an important role in forming the E s and TeSL at the beginning of the lidar observation. Later, the peak height of the TeSL followed the interface of the ions convergence and divergence zones. The ions near the interface did not totally diverge and might be enough to maintain a weak TeSL. On the other hand, these ions were not able to form a strong E s to support the remarkable sodium density increase. This might explain why the SSL did not enhance greatly when propagating downward. 5. Discussion [27] The observation of sodium layers in the thermosphere, at much greater altitudes than the meteoric metal layer, is interesting. Observations of these have only begun to appear widely since the first such report by Collins et al. [1996]. If the topside metal layer is common phenomenon [Höffner and Friedman, 2005], then a TeSL in lower thermospheric region is somewhat similar to an SSL in the main XUE ET AL.: TESLS AND THEIR FORMATION layer but has a broad width. From previous analysis, the TeSLs and SSLs both begin within tidal induced ions convergence zones, which can accumulate the ions (including Na + ) to form strong E s layers. This confirms a wind shear- E s -TeSL (SSL) chain. However, the following issues needed to be addressed: 5.1. (1) The Convergence Time of Ions Associated With TeSLs [28] The wind shear necessary to produce strong E s is likely not produced by the tides alone. The gravity waves, which was not investigated here, and their interaction with the tides are also an important mechanism to produce strong wind shear [Li et al., 2012]. Moreover, the GSWM tends to overestimate the tidal amplitudes, partly due to the lack of the accurate gravity wave parameterization [Lu et al., 2011]. Thus, the tidal amplitudes derived above might have a large uncertainty. From equation (1), one can obtain a simple estimation of the convergence time (t z, t m ), which is needed for ions to move vertically a distance z, as a function of altitude and neutral wind velocity for the zonal and meridional wind shear mechanisms, respectively (this is the same as equation (4) in [Haldoupis, 2012]) as follows: 1+r 2 i t z = V r i cos I z; t 1+r 2 i m = z (2) U cos I sin I Figure 4b shows estimates of vertical ions convergence times at Lijiang, China, for z =1 km for strong (100 ms 1, solid line) and weak (10 ms 1, dash line) zonal (black) and meridional (red) wind driving. Near 120 km, both zonal and meridional wind are efficiency for converging ions due to the ratio of ion-neutral collision frequency to ion gyrofrequency, i.e., r i 1. In this altitude, a typical zonal and meridional wind can force the ions to drift 1 km vertically in a few to tens of minutes. Below 110 km, the convergence caused by zonal wind becomes important due to the large ratio r i. Therefore, the wind velocity needed for a short ion convergence time can be allowed to vary in a wide range above 110 km. However, equation (2) is also true for vertical ion divergence time estimation. Then, in the region of TeSLs (typically km), the ion layers are not stable since the ions convergence and divergence are much faster. As a result, the condition needed for forming a TeSL is more severe compared with that for an SSL in a lower altitude region. This may be a reason of rare observations of the TeSLs The Small Sodium Densities in TeSLs [29] Both cases showed that the peak densities of the TeSLs were approximately times smaller than that of the main sodium layer. One possible reason may be the fast ions vertical convergence or divergence times as mentioned above. Another clue to explain this further can be found in Xu and Smith [2005], who noted that the lifetime of Na + differs from 100 h at 120 km to about 10 min at 95 km. Thus, in the TeSL region, the recombination process between Na +! Na is very slow, and only a small part of Na + has a chance to be neutralized through the chemical process, even though the ion density in the upper 2416 E s layer increases substantially (e.g., f o E s > 20 MHz on 10 March 2012). This could be a possible explanation for why the larger ratio of peak density (or column density) of

9 the TeSL to that of the main layer always appeared at a relatively low altitude (usually <110 km, as reported by Wang et al. [2012]). [30] From another point of view, the direct recombination Na +! Na is a very inefficient process, i.e., the rate coefficient is cm 3 s 1 [Delgado et al., 2012]. However, the production of Na is also affected by the electron density and Na + density within the E s layer. The sodium density in a TeSL would be greatly dependent on the strength of the E s. A strong E s is able to produce an obvious Na enhancement in short time. For example, the electron density and Na + density were inferred as 10 6 cm 3 and 10 5 cm 3, respectively, on 10 March Thus, the time needed for producing a sodium density of 100 cm 3 can be estimated as 100 s, much shorter than the Na lifetime above 100 km [Xu and Smith, 2005] The Broad Widths of the TeSLs [31] Another interesting feature of the TeSLs observed is that they had broader widths (e.g., FWHM 8 kmfor TeSL on 10 March 2012) than the SSLs. This feature has also been confirmed by recent lidar observations in Beijing, China [Wang et al., 2012]. [32] At high altitudes, where the ion-neutral collisions decrease, the effect of ambipolar plasma diffusion is increasing, i.e., diffusion coefficient D a = k B (T i + T e )/m i i [Haldoupis, 2012]. The diffusion time scale for a typical tidal wind with a vertical wave number k z is then D = 1/ k 2 z D a. Figure 5a (dotted line) shows altitude variation of the Na + diffusion time, assuming T e = T i = T n and k z =2/25 km. Rapid diffusion of Na + (<100 min) appears at 125 km and slow diffusion (>1000 min) appears at 105 km. Thus, in the TeSL region, the ambipolar diffusion acts against the convergence caused by the tidal wind. These may be the reason why the TeSLs are wider in extent but shorter in durations than the lower SSLs. [33] Please also note that we ignored the vertical wind (W ) in equation (1). This is suitable for the TeSL altitude region (i.e., km), where the ratio r i varies from 5 to 0.5. In the SSLs region (i.e., km), r i becomes larger than 10, which makes the vertical wind (W )termin equation (1) comparable to the horizontal wind terms. Thus, the vertical wind can also contribute to the vertical motion of the ions and the formation of the SSLs. However, there was no vertical wind observation available, and the effect of the vertical wind was not clear here. 6. Conclusion [34] In this paper, we report two TeSL events observed at a low latitude location in China. To the best of our knowledge, the 10 March 2012 TeSL event, with peak height of 122 km, is the highest sodium layer ever reported. The TeSL was independent of the main sodium layer, but it was accompanied by a lower SSL at approximately 105 km at the beginning of the observation period. At the end of the observation, the sodium density in the lower SSL increased greatly. The 10 April 2012 event began at approximately 112 km and merged into the main sodium layer while propagating downward. There was no evident sodium density enhancement observed in this TeSL event, although it occurred in a lower altitudinal region [35] Strong E s layers observed by an ionospheric ionosonde appeared approximately 1 or 2 h before the two events, and the space-based GPS radio occultation also detected the existence of the E s layer at the same altitude range. These results suggest that high-altitude TeSLs and the lower altitude SSLs might be formed by the same mechanism associated with E s layer formation. After extending the wind observed by meteor radar to altitudes above 100 km, it was found that the different ions vertical motion dynamo regions indicates different evolution processes for TeSLs (SSLs). Namely, (1) in a tidal ion convergence region, a TeSL or SSL can be enhanced gradually when propagating downward due to a rapid decrease in the lifetime of Na + at lower altitude (such as at the beginning of the TeSLs on 10 March/April 2012, and the SSLs on 10 March 2012); (2) at an ion convergence-divergence interface, a TeSL or SSL can still follow downward tidal progression, but the density does not have a notable enhancement (such as for the case of the TeSL on 10 April 2012 after 17:00 UT); and (3) when a TeSL or SSL enters into a tidal wind-divergence zone, the layer tends to decrease (such as for the case of the TeSL on 10 March 2012 after 15:00 UT). Thus, in a suitable dynamo region, where wind shear is enough to maintain the E s layer against divergences caused by other processes (e.g., ambipolar diffusion or de-layering waves), a TeSL or SSL could develop and become strong as propagating downward. [36] However, questions still remain, such as: what is the source of the strong high-altitude E s that later forms a TeSL above 110 km? Could the micrometeoroids with fast injection speeds or the temperature enhancement contribute to the high-altitude E s and TeSL? It should also be pointed out that lacking horizontal information makes us lose some of the TeSL evolution segments. However, the distance between the lidar site (Ljiang) and the Kunming Radio Observatory is approximately 300 km, this may suggest that these events extend to large horizontal scales. Further observation and modeling efforts are required to answer these questions and address the issue of missing information. [37] Acknowledgment. We thank the COSMIC and TerraSAR-X radio occultation data as well as the GSWM-00 and NRL-MSISE00 model data used in this paper. 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