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Monolayers of transition metal dichalcogenides (TMDCs) are atomically thin direct-gap semiconductors with potential applications in nanoelectronics, optoelectronics, and electrochemical sensing. Recent theoretical and experimental efforts suggest that they are ideal systems for exploiting the valley degrees of freedom of Bloch electrons. For example, Dirac valley polarization has been demonstrated in mechanically exfoliated monolayer MoS2 samples by polarization-resolved photoluminescence, although polarization has rarely been seen at room temperature. Here we report a new method for synthesizing high optical quality monolayer MoS2 single crystals up to 25 microns in size on a variety of standard insulating substrates (SiO2, sapphire and glass) using a catalyst-free vapor-solid growth mechanism. The technique is simple and reliable, and the optical quality of the crystals is extremely high, as demonstrated by the fact that the valley polarization approaches unity at 30 K and persists at 35% even at room temperature, suggesting a virtual absence of defects. This will allow greatly improved optoelectronic TMDC monolayer devices to be fabricated and studied routinely., 14 pages (manuscript) + 4 pages (supplementary materials), including 6 figures in manuscript. To appear in ACS Nano

A topographic parameter inversion method based on laser altimetry is developed in this paper, which can be used to deduce the surface vertical profile and retrieve the topographic parameters within the laser footprints by analyzing and simulating return waveforms. This method comprises three steps. The first step is to build the numerical models for the whole measuring procedure of laser altimetry, construct digital elevation models for surfaces with different topographic parameters, and calculate return waveforms. The second step is to analyze the simulated return waveforms to obtain their characteristics parameters, summarize the effects of the topographic parameter variations on the characteristic parameters of simulated return waveforms, and analyze the observed return waveforms of laser altimeters to acquire their characteristic parameters at the same time. The last step is to match the characteristic parameters of the simulated and observed return waveforms, and deduce the topographic parameters within the laser footprint. This method can be used to retrieve the topographic parameters within the laser footprint from the observed return waveforms of spaceborne laser altimeters and to get knowledge about the surface altitude distribution within the laser footprint other than only getting the height of the surface encountered firstly by the laser beam, which extends laser altimeters’ function and makes them more like radars.

In this study, we develop a bactericidal coating material for micro-implant, tio2 films with ag deposited on were prepared on titanium plates by sol-gel process. their anti-microbial properties were analyzed as a function of the annealed temperature using escherichia coli as a benchmark microorganism. ag nanoparticles deposited on tio2 film were of metallic nature and could grow to larger ones when the annealed temperature increased. the results indicated that the smaller size of ag nanoparticles, the better bactericidal ability. on the other hand, the positive antibacterial effect of tio2 enhanced the bactericidal effect of ag. (c) 2010 elsevier b.v. all rights reserved.

Undoped and C-doped TiO2 thin films have been prepared by sol-gel process. Their structure and optical properties have been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy. It has been observed that C dopants retard the transformation from anatase-to-rutile phase. Namely, C doping effect is attributed to the anatase phase stabilization. The optical analyses show that the optical band gap of anatase C-doped TiO2 decreases with increasing amount of C. Also, it is founded that C dopants have been shown to make TiO2 have a visible light photoresponse. (C) 2009 Elsevier B.V. All rights reserved.

Three-dimensional (3D) composite colloidal photonic crystals with SiO2 core and ZnO shell were fabricated on borosilicate glass (BSG) substrate by a two-stage deposition method. Scanning electron microscopy (SEM) measurements show that both the pre-deposited SiO2 and SiO2/ZnO core–shell structures are oriented with their (1 1 1) axes parallel to the substrates. Optical measurement reveals that the periodic arrays exhibit a photonic band gap in the (1 1 1) direction. The optical properties of SiO2/ZnO core–shell structures strongly depend on the size dispersions of colloidal spheres and the intrinsic defects in the sample.

Aiming at presenting the impact of nonlinearity on the amplitude of the migrating diurnal tide, a fully nonlinear numerical tidal model is developed, and its reliability and convergence are checked by numerical experiments. Under the same conditions as those applied by the Global Scale Wave Model 2000 (GSWM-00), our model provides the nonlinear quasi-steady solution of the migrating diurnal tide, which is very different from the GSWM-00 result (the linear steady solution). Their large tidal amplitude differences occurring in the MLT region, especially above 90 km, suggest that nonlinearity can intensively impact on the tidal amplitude and cannot be neglected in the study of tidal phenomena in this region. By comparing with the GSWM-00 result, the seasonal characteristics of nonlinear tidal amplitude, such as a semiannual cycle, similar spatial structures for different seasons and hemispherical symmetry are revealed. Furthermore, our simulations also show that the mean flow/tidal interaction will significantly alter the background wind and temperature fields, indicating this interaction should be comprehensively considered in the description of global atmospheric dynamics in the MLT region. The comparisons among our simulations, the GSWMs and some observations of tides suggest that nonlinearity can be responsible for some discrepancies between the GSWMs and observations.

A fully nonlinear numerical tidal model has been developed with the aim of presenting amplitude features of the terdiurnal tide excited by nonlinear interaction between the diurnal and semidiurnal tides. Since the superposition of the migrating diurnal and semidiurnal tidal solutions from the GSWM-00 (Global Scale Wave Model 2000) is taken as the initial disturbance for this nonlinear model, the diurnal and semidiurnal tides are allowed to nonlinearly interact with each other. The analyses on the simulations show that the migrating terdiurnal tide can be significantly excited by the nonlinear interaction between the diurnal and semidiurnal tides in the mesosphere and lower thermosphere (MLT) region, especially above 90 km, and have pronounced amplitudes (wind speeds over 15 m s−1 and temperature over 10 K) in the lower thermosphere (90–110 km). In addition, its amplitudes vary strongly with season and maximize during equinoctial months at low and middle latitudes, and its zonal wind component is larger than the meridional wind component. Simultaneously, the diurnal and semidiurnal tides exhibit evident variations, indicating that the wave-wave and wave-mean flow interactions are the important cause of the tidal variability. Our calculations also illustrate the remarkable alteration of the background fields induced by the mean flow/tidal interaction, implying this interaction should be comprehensively considered in describing global atmospheric dynamic and thermal structures in the MLT region.

Co-doped titanate nanotubes were prepared from hydrothermal treatment on Co-doped anatase TiO2 powders in a concentrated NaOH aqueous solution and characterized by a variety of techniques. We mainly investigated the structural and magnetic properties of these nanotubes. It was found that the obtained nanotubes might possibly be constructed from H2Ti3O7, their undoped counterpart. The result from Raman spectra showed that Ti–OH bonds existed in the tubular structure. The electron paramagnetic resonance and X-ray photoelectron spectroscopy studies clearly showed that the Co was incorporated into the titanate lattice as Co2+ and substituted for Ti cations sites. Also, from the photoluminescence measurement, the presence of oxygen vacancies was detected in the nanotubes. The vibrating sample magnetometer measurement revealed a clear paramagnetic behavior with weakly ferromagnetic ordering at room temperature. The results suggested that ferromagnetism of the nanotubes was related to the oxygen vacancies and could be suppressed by their quantum size.

The microstructure of vanadium oxide nanotubes (VONTs) have been characterized using FTIR spectroscopy and Raman spectroscopy. The temperature effects on the VONTs were studies by changing the laser irradiation power and thermal annealing temperature in air. Raman spectroscopy studies showed that the VONTs could be decomposed even at low laser power irradiation. Also, together with scanning electron microscopy, it was found that thermal annealing in air could lead to the collapse of the tubular structure and convert the nanotubes into V 2 O 5 nanoparticle. It was found that the thermal stability of VONTs was relatively low and the tubular morphology was destroyed at temperatures higher than 300 °C. The spectroscopic analyses showed that the Raman signature of the VONTs could be established for probing tubular structure.

The Co-doped TiO 2 nanotube arrays have been synthesized by a sol–gel template method and were characterized by a variety of techniques. The obtained nanotube arrays were identified to be of an anatase structure without any impurity phase. Also, the Co was incorporated in the anatase TiO 2 lattice as Co 2+ and substituted for the Ti site with no evidence of metallic Co. The ferromagnetic behavior was clearly observed at room temperature in the as-synthesized nanotube arrays, which is related to the oxygen vacancies created by the Co 2+ substitution of Ti 4+ site in TiO 2 , not being caused by Co metal.

Vanadium oxide nanotubes were synthesized by hydrothermal treatment from V2O5·nH2O sols as precursor and dodecylamine as structure-directing template. The morphology and structure of the nanotubes were characterized by SEM, TEM, XRD, TG-DTA and FTIR. The experimental results reveal that the duration of the hydrothermal treatment is of importance for obtaining VOx-NTs which have a layered structure. TG-DTA study indicates that V5+ cations in nanotubes are partially reduced to V4+ cations. The results from FTIR spectra indicate the difference in V-O vibrations between before and after hydrothermal treatment. From the results, it suggests that during hydrothermal treatment, the rearrangement of the vanadium oxide structure leads to the formation of VOx nanotubes from lamellar structure because of the presence of V4+ species.

Ti 1− x Co x O 2 ( x = 0–0.07) films were prepared by sol–gel spin coating and characterized by a variety of techniques. We mainly investigated the effect of Co dopant on the structural and optical properties of Ti 1− x Co x O 2 films. XRD studies showed that the magnetic Co impurity atoms were substituted for the Ti sites in rutile TiO 2 . Also, the FTIR spectra clearly showed the substitution of Co atom for Ti in TiO 2 , resulting in a disorder of the crystal structure and the presence of oxygen vacancies in Ti 1− x Co x O 2 lattice. By vibrating sample magnetometer (VSM) measurement, the films with x = 0.03, 0.05 and 0.07 were found to be ferromagnetic at room temperature. For the case of ferromagnetic Ti 1− x Co x O 2 films, an emission band in the range of 450–500 nm from oxygen vacancies at various energies was detected from the photoluminescence (PL) measurement. VSM and PL analyses showed that oxygen vacancies probably played a major role in ferromagnetism of the Ti 1− x Co x O 2 system with Co substituting for Ti.

In this paper, a new ionospheric index pair—the single-station index (Js) and the planetary index (Jp), which describe ionospheric disturbances—is constructed using the spectral-whitening method. Observations covering more than 30 years and obtained from 21 ionosonde stations widely distributed across middle- and low-latitude areas are used in the study. Variations in the Js index at all stations generally follow the variation of the geomagnetic Dst index during periods of strong to moderate geomagnetic activity. Nevertheless, the inertial response of Js to weak geomagnetic activity results in a weak correlation between Js and Dst. The extrema of Js and Dst during the same geomagnetic activity period have a much higher correlation coefficient, since they avoid the delaying effect of the ionospheric response to geomagnetic activity. To smooth out local differences among ionospheric disturbances, the Jp index was defined as the means of all Js indices and so describes ionospheric disturbances at the planetary scale. The Jp index is significantly correlated to the Dst index, and the correlation coefficient between the extrema of Jp and Dst is up to −0.8. This correlation remains robust when the data set length varies, and data from approximately 10 stations are sufficient to derive a reasonable estimate of Jp. The quantitative relationship between Jp and Dst can be used to reproduce Jp by adopting Dst as the precursor, and this raises the possibility of predicting planetary ionospheric disturbances based on geomagnetic activity.

We adopted 14 year (from 1998 to 2011) radiosonde data from 16 stations at latitudes between 14.33°S to 29.37°N to investigate the low-frequency oscillations of inertial gravity wave energy densities (including kinetic energy density, potential energy density, and total energy density) in the lower atmosphere from 2 km to 30 km. Apparent signatures of 11 year solar cycle and quasi-biennial oscillation (QBO) were found in the gravity wave energy densities in the troposphere and lower stratosphere, respectively. Detailed analyses suggested that the 11 year oscillation and QBO of gravity wave activity might be due to the 11 year oscillation of convection activity and zonal wind QBO modulation, respectively. Besides, the gravity wave energy also showed a 35.2 month oscillation in the lower stratosphere, which might be generated by the interaction between the QBO (with the period of 26.7 months), and the 11 year solar cycle, while more possible causes are the direct modulation from the background zonal wind. Our results showed that the QBO signature in gravity wave total energy density could extend northward to over 20°N in the Northern Hemisphere, while the 11 year solar cycle in gravity wave activity could only extend to lower than 7.5°N. The oscillation with the period of 35.2 months may be a prolonged QBO in both gravity wave activity and background zonal wind.

The correlation between liver stiffness (LS), measured by ultrasonic transient elastometry (FibroScan), and the presence and severity of esophageal varices (EV) in patients with viral cirrhosis of the liver has not been well documented to date. The study described here investigated the value of using FibroScan to predict EV. Patients with cirrhosis (200 patients: 167 cases caused by hepatitis B virus and 33 cases caused by hepatitis C virus) underwent both upper gastrointestinal endoscopy and FibroScan. Demographic, clinical, biochemical and endoscopic data and FibroScan-obtained LS parameters were collected. The mean LS value in patients with EV (33.2 kPa) was significantly higher than the mean LS value in patients without EV (18.6 kPa) (p < 0.05). The mean LS value in patients with grade 2 and 3 EV (38.3 kPa) was significantly higher than that in patients with grade 1 EV (24.8 kPa) (p < 0.05). Overall, FibroScan was 86.4% sensitive and 72.2% specific in predicting the presence of EV, with an area under the receiver operating characteristic curve (AUROC) of 0.84. The sensitivity and specificity for the patients with grade 2 or 3 EV were 84% and 73% (AUROC = 0.86). When FibroScan was combined with platelet count, the overall sensitivity and specificity of prediction increased to 84% and 80% (AUROC = 0.88), respectively, and 84% and 75% (AUROC = 0.89), respectively, in patients with grade 2 and 3 EV. FibroScan alone or combined with platelet count might predict the presence and severity of EV in patients with hepatitis B or C-related viral cirrhosis.

The understanding of the metal and transition metal dichalcogenide (TMD) interface is critical for future electronic device technologies based on this new class of two-dimensional semiconductors. Here, we investigate the initial growth of nanometer-thick Pd, Au, and Ag films on monolayer MoS2. Distinct growth morphologies are identified by atomic force microscopy: Pd forms a uniform contact, Au clusters into nanostructures, and Ag forms randomly distributed islands on MoS2. The formation of these different interfaces is elucidated by large-scale spin-polarized density functional theory calculations. Using Raman spectroscopy, we find that the interface homogeneity shows characteristic Raman shifts in E2g(1) and A1g modes. Interestingly, we show that insertion of graphene between metal and MoS2 can effectively decouple MoS2 from the perturbations imparted by metal contacts (e.g., strain), while maintaining an effective electronic coupling between metal contact and MoS2, suggesting that graphene can act as a conductive buffer layer in TMD electronics.

Electronic valleys are energy extrema of Bloch bands in momentum space. In analogy to electrons with spin degrees of freedom, valley indices can be considered as pseudo-spins for new modes of electronic and photonic device operation. In this paper, we will review our experimental progress on the investigation of these pseudo-spins using atomically thin semiconductors (MoS2, MoSe2 etc.), which are either monolayer or bilayer group VI transition metal dichalcogenides. We will show that these new 2D semiconductors not only behave as remarkable excitonic systems, but also provide an ideal platform for realizing the optical manipulation and electrical control of valley degrees of freedom.

First-order phase transitions in solids are notoriously challenging to study. The combination of change in unit cell shape, long range of elastic distortion, and flow of latent heat leads to large energy barriers resulting in domain structure, hysteresis, and cracking. The situation is still worse near a triple point where more than two phases are involved. The famous metal-insulator transition (MIT) in vanadium dioxide, a popular candidate for ultrafast optical and electrical switching applications, is a case in point. Even though VO2 is one of the simplest strongly correlated materials, experimental difficulties posed by the first-order nature of the MIT as well as the involvement of at least two competing insulating phases have led to persistent controversy about its nature. Here, we show that studying single-crystal VO2 nanobeams in a purpose-built nanomechanical strain apparatus allows investigation of this prototypical phase transition with unprecedented control and precision. Our results include the striking finding that the triple point of the metallic and two insulating phases is at the transition temperature, T_tr = T_c, which we determine to be 65.0 +- 0.1 C. The findings have profound implications for the mechanism of the MIT in VO2, but in addition they demonstrate the importance of such an approach for mastering phase transitions in many other strongly correlated materials, such as manganites and iron-based superconductors., 15 pages including figures and supplementary information

How to parallelize the great amount of legacy sequential programs is the most difficult challenge faced by multi-core designers. The existing parallelization methods at the compile time due to the obscured data dependences in C are not suitable for exploring the parallelism of streaming applications. In this paper, a software pipeline for multi-layer loop method is proposed for streaming applications to exploit the coarse-grained pipeline parallelism hidden in multi-layer loops. The proposed method consists of three major steps: 1 transform the task dependence graph of a streaming application to resolve intricate dependence, 2 schedule tasks to multiprocessor system-on-chip with the objective of minimizing the maximal execution time of all pipeline stages, and 3 adjust the granularity of pipeline stages to balance the workload among all stages. The efficiency of the method is validated by case studies of typical streaming applications on multi-core embedded system.

[1] Atmospheric waves and their interactions in the thermospheric neutral wind are studied based on Arecibo incoherent scatter radar observations. Our analysis suggests that the thermospheric atmosphere is usually disturbed by certain types of waves, including tides, gravity waves, and planetary waves, of which the diurnal tide is almost always the dominant disturbance. Strong interactions (defined as the coexistence of strong positive and negative correlations among the interacting waves) between the diurnal tide and gravity waves are frequently observed during the entire observation period. These strong interactions can persist for several days, although they are highly intermittent. Moreover, the sum and difference interactions between the diurnal tide and gravity waves always occur simultaneously and the energy exchange between the interacting waves is sometimes reversible. In addition to tide–gravity wave interactions, tide–planetary wave and tide–tide interactions are also found in our data. In tide–planetary wave interactions, the tidal oscillations are modulated at the interacting planetary wave periods. A combination of bispectral and correlation analyses verifies the occurrence of nonlinear interactions among different tidal components in the middle thermosphere. Moreover, during tide–tide interactions, the energy transfer trend changes very frequently, indicating that tidal energy is frequently redistributed among different tidal components. Generally, our study provides proof of strong tide–gravity wave, tide–planetary wave, and tide–tide interactions in the middle thermosphere, which has rarely been reported to date.

[1] We proposed a broad spectral data-analyzing method to study the altitude and seasonal variability of gravity wave (GW)-associated dynamics in the lower atmosphere at a midlatitude by using the radiosonde data from Miramar Nas (32.87°N, 117.15°W), California, during 1998–2008. Generally, the presented primary statistical features of GW parameters and their seasonal variation are consistent with previous radiosonde observations by using the conventional hodograph analysis method based on monochromatic GW theory. These consistencies suggest our proposed analyzing method is feasible in extracting GW parameters from the radiosonde data. More interesting, our analyses can reveal the altitude variations of GW parameters, which have been seldom reported in previous radiosonde observations. Similar to previous observations, most seasonal and height variability of GW parameters is closely connected with tropospheric jet, suggesting the important role of the jet in determining GW parameters as well as the lower atmospheric dynamics and thermal structure. Mainly due to the broad spectral nature of the observed GWs, there are also some differences between our results and previous studies based on monochromatic GW extraction. By using the perturbation of vertical ascent rate, we directly calculated GW momentum and heat fluxes, which can only be indirectly derived from the hodograph analysis. Furthermore, the directly derived heat flux can explain the frequently observed tropospheric inversion in winter. Besides GW parameters, turbulent energy dissipation rate and diffusion coefficient were also derived. The derived turbulence parameters and their altitude variations are in good agreement with radar observations reported elsewhere.

How to manage the message passing among inter processor cores with lower overhead is a great challenge when the multi-core system is the contemporary solution to satisfy high performance and low energy demands in general and embedded computing domains. Generally speaking, the networks-on-chip connects the distributed multi-core system. It takes charge of message passing which including data and synchronization message among cores. The size of most data transmission is typically large enough that it remains strongly bandwidth-bound. The synchronization message is very small which is primarily latency bound. Thus the separated networks-on-chip are needed to transmit the above two types of message. In this paper we focus on the network for the transmission of synchronization messages. A hardware module - message passing unit (MPU) is proposed to manage the synchronization message passing for the heterogeneous multi-core system. Compared with the original single network approach, this solution reduces the run-time object scheduling and synchronization overhead effectively, thereby, improving the whole system performance.

In order to ensure the optimum allocation of resources, the collecting and aggregating information among management computers, power apparatus, communication devices, transmission lines and so on in electrical network will grow more and more important. In this paper, the management system enable exchange and readily use information securely and effectively between two or more networks, systems, devices, applications, or components, and with little or no inconvenience to the user. This management system is based on the Simple Network Management Protocol (SNMP) and implements the classical management functions of configuration, fault, account performance and security management. We expand MIB resources by defining MIB objects to monitor the resources of sever, and use multi-threading technology to collect data and process them, which can improve the collection efficiency.

[1] The complete seasonal variation patterns of the nocturnal mesospheric Na and Fe layers over Wuhan, China (30°N), have been established on the basis of several years of Na and Fe lidar measurements. Both the Na and Fe layer column abundances show strong annual variations as well as moderate semiannual variations with maxima in winter and double minima from late spring to midautumn (note that only one night of Fe data is presently available between mid-May and mid-July). The seasonal variation in the Fe abundance is evidently stronger than that of Na. The Na layer abundance has an annual mean of ∼2.5 × 109 cm−2, while this value for Fe is ∼7.5 × 109 cm−2. The Na and Fe centroid heights are dominated by semiannual oscillations with similar phases. The mean centroid heights are ∼91.4 km for Na and ∼88.7 km for Fe. The Na RMS width exhibits a strong semiannual oscillation with the layer slightly broader in winter, whereas the Fe width varies principally annually with a maximum in winter. The mean RMS widths of the Na and Fe layers are 4.5 and 4.1 km, respectively. The seasonal characteristics of the Na and Fe layers observed at 30°N have been compared with those currently available at other latitudes. The seasonal ratios of their abundances are smaller compared with 40°N and the South Pole. Their centroid heights and RMS widths also show less seasonal variations than the counterparts at all other latitudes. The annual mean Na and Fe abundances are about 60–77% of the counterparts at 40°N, 18°N, and the South Pole. This suggests that both the nocturnal Na and Fe layers have a low-abundance region around 30°N. On the basis of the results observed at the three latitudes in the Northern Hemisphere, the annual mean Fe layer width decreases with increasing latitude.

[1] High-accuracy atom density profiles, obtained by the simultaneous and common-volume Fe and Na lidar measurements at Wuhan, China (30.5°N, 114.4°E), reveal some ubiquitous features of the Fe and Na layers on their borders. The Fe and Na lower boundaries show consistently a delicate stratification in which the lower boundary of the Fe layer is in general slightly higher than or coincident with that of the Na layer, with an overall mean altitude difference being about 0.2 km. Despite the existence of considerable vertical displacements, the two lower boundaries vary always following almost the same track. The overall correlation coefficient between them is as high as 0.96. This ubiquitous delicate stratification of the measured lower boundaries (nearly coincident density cutoff) suggests strongly that the undersides of Fe and Na layers are controlled by the same or very similar processes. The upper boundary of the Na layer is always several kilometers higher than that of the Fe layer. A relatively weak positive correlation is also persistently observed between the two upper boundaries. Weak sporadic layering events frequently occur on the upper extent of the metal layers. They may impair the correct determination of the upper boundaries of the normal metal layers and consequently weaken the correlation. Both the Fe and Na layers often show an evidently steeper density gradient on the underside than on the upper extent, and the borders of the Fe layer are clearly steeper than those of the Na layer. The explanation to these ubiquitous features needs further experimental and modeling efforts.

[1] The focus of this paper is to study the relationship between sporadic Fe (Fes) and Na (Nas) layers through simultaneous and common volume Fe and Na lidar observations. A total of 37 sporadic layering events were identified from one year (195 hours) of observations at Wuhan (30.5°N, 114.4°E), China. Out of the 37 events, 23 (62%) are characterized by the simultaneous formation of Fes and Nas layers. The most prominent feature for each of the 23 events is that the Fes and Nas layers occurred in overlapping altitude ranges and moved following almost the same track. On occasion the Fes and Nas layers exactly simultaneously reached their maximum peak densities at nearly the same altitude. These observational results strongly suggest that Fes and Nas layers are formed via the same or very similar mechanisms. This conclusion contradicts the previous suggestion based on those independent observations of Fes and Nas layers, that the Fes and Nas layers may be formed via different mechanisms. Out of the 37 events, 14 (38%) belong to single-species sporadic layering events. It is noticed that the formation of each single-species sporadic atom layer was usually accompanied by a weak density enhancement in the other metal atom. This supports the suggestion that Fes and Nas layers are formed via the same or very similar mechanisms. Both the Fes and Nas layers over Wuhan showed a tendency to strengthen with decreasing occurrence altitude. This tendency is consistent with the earlier Nas layer observations at high and low latitudes. Moreover, it is noticed by comparing the currently available Fes and Nas layer characteristics, which came from the observations at five different locations (including Wuhan) during different periods, that a lower average altitude could link to a higher average peak density and vice versa. The statistics-based link might perhaps represent a universal feature of sporadic layers. From our simultaneous Fe and Na density data we have found that the undersides of the normal Fe and Na layers follow nearly the exact movements and occur at nearly the same altitude. The normal Fe layer tended to be narrower than the corresponding Na layer nearly at all times, and this difference was generally reflected in the extent of the upper edge of the layer. The nearly persistent underside overlap strongly suggests that on the undersides of these meteoric metal layers there exist some sink mechanisms leading to the concurrent removal of different sorts of free neutral metal atoms.

[1] An observational study of nonlinear interaction between the quasi 2 day wave (QTDW) and the diurnal and semidiurnal tides from meteor radar measurements at Maui is reported. The diurnal and semidiurnal tides show a short-term variation with the QTDW activity. The variation of amplitude of the semidiurnal tide is opposite to that of the QTDW. The minimum amplitudes of the diurnal tide appear several days later than the maximum amplitudes of the QTDW, and the diurnal tide obviously strengthens when the QTDW drops to small amplitudes. The bispectrum analysis shows significant nonlinear interactions among the QDTW and the tidal components. The two quasi 16 h modes with periods of 16.2 h and 15.8 h generated in the interactions of the QTDW with the diurnal and semidiurnal tides can clearly be distinguished because of the slight deviation of the QTDW period from 48 h. The bicoherence spectrum demonstrates that the QTDW and the semidiurnal tide have quite strong levels of coherence, indicating that the nonlinear interaction is a mechanism responsible for the variability of the semidiurnal tide. Although there is also some interaction between the QTDW and the diurnal tide, their coherence level is low. When the QTDW drops to very weak amplitudes, the background wind decreases and reverses. During this time, the diurnal tide holds large amplitudes. These results support the notion that the variability of the diurnal tide is mainly attributable to the strong QTDW-induced changes in the background atmosphere, which was shown in the modeling study by Chang et al. (2011). Hence, both the nonlinear interaction and the background flow changes are responsible for the observed variation of the diurnal tide.