Quasi-periodic Fluctuations Research Articles

Quasi-periodic fluctuation in Donchin's speller signals and its potential use for asynchronous control.

When we examine the event-related potential (ERP) responses of Donchin's brain-computer interface (BCI) speller, a type of quasi-periodic fluctuation (FLUC) overlapping with the ERP components can be observed; this fluctuation is traditionally treated as interference. However, if the FLUC is detectable in a working BCI, it can be used for asynchronous control, i.e. to indicate whether the BCI is under the control state (CS) or under the non-control idle state (NC). Asynchronous control is an important issue to address to enable BCI's practical use. In this paper, we examine the characteristics of the FLUC and explore the possibility of using the FLUC for asynchronous control of the BCI. For detecting the FLUC, we propose a method based on the power spectrum and evaluate the detection rates in a simulation. As a result, high true positive rates (TPRs) and low false positive rates (FPRs) are obtained. Our work reveals that the FLUC is of great value for implementing an asynchronous BCI.

Dynamics of populations with delayed density dependent birth rate regulation

This study focuses on the Moran–Ricker model with time lag for the dynamics of limited homogeneous population. The reduction of required resources is caused by their consumption by the previous generation, which corresponds to delayed density dependence. The model shows multimodality. The population can demonstrate different types of dynamics (i.e., stable, periodic, or chaotic) at the same values of demographic parameters. The Moran–Ricker model with time lag can successfully describe the population dynamics of some insect species. The point estimations of population parameters are statistically significant. The obtained point estimates that correspond to real population dynamics are located in a quasi-periodic fluctuation area and adjoin other dynamic modes. A variation of the values of demographic parameters or random fluctuations of the current population size can lead to dynamic mode change. The investigation of the possible model dynamic modes near the found point estimation of parameters is necessary to identify the dynamic modes between which transition may occur.

A phenomenological model for fountain-top entrainment

In theoretical treatments of turbulent fountains, the entrainment of ambient fluid into the top of the fountain, hereinafter fountain-top entrainment $Q_{top}$ ($\text{m}^{3}~\text{s}^{-1}$), has been neglected until now. This neglect, which modifies the energetic balance in a fountain, compromises the predictive ability of existing models. Our aim is to quantify $Q_{top}$ by shedding light on the physical processes that are responsible for fountain-top entrainment. First, estimates for $Q_{top}$ are obtained by applying, in turn, an entrainment closure in the vein of Morton et al. (Proc. R. Soc. Lond., vol. 234, 1956, pp. 1–23) and then of Shrinivas & Hunt (J. Fluid Mech., vol. 757, 2014, pp. 573–598) to the time-averaged fountain top. Unravelling the assumptions that underlie these approaches, we argue that neither capture the dynamical behaviour of the flow observed at the fountain top; the top being characterised by quasi-periodic fluctuations, during which large-scale eddies reverse and engulf parcels of ambient fluid into the fountain. Therefore, shifting our mindset to a periodical framework, we develop a new phenomenological model in which we emphasise the role of the fluctuations in entraining external fluid. Our model suggests that $Q_{top}$ is similar in magnitude to the volume flux supplied to the fountain top by the upflow ($Q_{u}$), i.e. $Q_{top}\sim Q_{u}$, in agreement with experimental evidence. We conclude by providing guidance on how to implement fountain-top entrainment in existing models of turbulent fountains.

Analysis of the Nonlinear Trends and Non-Stationary Oscillations of Regional Precipitation in Xinjiang, Northwestern China, Using Ensemble Empirical Mode Decomposition.

Changes in precipitation could have crucial influences on the regional water resources in arid regions such as Xinjiang. It is necessary to understand the intrinsic multi-scale variations of precipitation in different parts of Xinjiang in the context of climate change. In this study, based on precipitation data from 53 meteorological stations in Xinjiang during 1960–2012, we investigated the intrinsic multi-scale characteristics of precipitation variability using an adaptive method named ensemble empirical mode decomposition (EEMD). Obvious non-linear upward trends in precipitation were found in the north, south, east and the entire Xinjiang. Changes in precipitation in Xinjiang exhibited significant inter-annual scale (quasi-2 and quasi-6 years) and inter-decadal scale (quasi-12 and quasi-23 years). Moreover, the 2–3-year quasi-periodic fluctuation was dominant in regional precipitation and the inter-annual variation had a considerable effect on the regional-scale precipitation variation in Xinjiang. We also found that there were distinctive spatial differences in variation trends and turning points of precipitation in Xinjiang. The results of this study indicated that compared to traditional decomposition methods, the EEMD method, without using any a priori determined basis functions, could effectively extract the reliable multi-scale fluctuations and reveal the intrinsic oscillation properties of climate elements.

Transmission of the electric fields to the low latitude ionosphere in the magnetosphere-ionosphere current circuit

The solar wind energy is transmitted to low latitude ionosphere in a current circuit from a dynamo in the magnetosphere to the equatorial ionosphere via the polar ionosphere. During the substorm growth phase and storm main phase, the dawn-to-dusk convection electric field is intensified by the southward interplanetary magnetic field (IMF), driving the ionospheric DP2 currents composed of two-cell Hall current vortices in high latitudes and Pedersen currents amplified at the dayside equator (EEJ). The EEJ-Region-1 field-aligned current (R1 FAC) circuit is completed via the Pedersen currents in midlatitude. On the other hand, the shielding electric field and the Region-2 FACs develop in the inner magnetosphere, tending to cancel the convection electric field at the mid-equatorial latitudes. The shielding often causes overshielding when the convection electric field reduces substantially and the EEJ is overcome by the counter electrojet (CEJ), leading to that even the quasi-periodic DP2 fluctuations are contributed by the overshielding as being composed of the EEJ and CEJ. The overshielding develop significantly during substorms and storms, leading to that the mid and low latitude ionosphere is under strong influence of the overshielding as well as the convection electric fields. The electric fields on the day- and night sides are in opposite direction to each other, but the electric fields in the evening are anomalously enhanced in the same direction as in the day. The evening anomaly is a unique feature of the electric potential distribution in the global ionosphere. DP2-type electric field and currents develop during the transient/short-term geomagnetic disturbances like the geomagnetic sudden commencements (SC), which appear simultaneously at high latitude and equator within the temporal resolution of 10 s. Using the SC, we can confirm that the electric potential and currents are transmitted near-instantaneously to low latitude ionosphere on both day- and night sides, which is explained by means of the light speed propagation of the TM0 mode waves in the Earth-ionosphere waveguide.

Temperature fluctuations superimposed on background temperature change

Proxy data allows the temperature of the Earth to be mapped over long periods of time. In this work the temperature fluctuations for over 200 proxy data sets were examined and from this set 50 sets were analyzed to test for periodic and quasi-periodic fluctuations in the data sets. Temperature reconstructions over 4 different time scales were analyzed to see if patterns emerged. Data were put into four time intervals; 4,000 years, 14,000 years, 1,000,000 years, and 3,000,000 years and analyzed with a goal to understanding periodic and quasi-periodic patterns in global temperature change superimposed on a “background” average temperature change. Quasi-periodic signatures were identified that predate the Industrial Revolution, during much of which direct data on temperature are not available. These data indicate that Earth temperatures have undergone a number of periodic and quasi-periodic intervals that contain both global warming and global cooling cycles. The fluctuations are superimposed on a background of temperature change that has a declining slope during the two periods, pre-ice age and post ice age with a transition about 12,000 BCE. The data are divided into “events” that span the time periods 3,000,000 BCE to “0” CE, 1,000,000 BCE to “0” CE, 12,000 BCE to 2,000 CE and 2,000 BCE to 2,000 CE. An equation using a quasi-periodic (frequency modulated sine waves) patterns was developed to analyze the date sets for quasi-periodic patterns. “Periodicities” which show reasonable agreement with the predictions of Milankovitch and other investigators were found in the data sets.

Surface microdeformations and regulation of cell movements in Xenopus development

The velocities and directions of movements of individual outer ectodermal cells of Xenopus embryos in the course of normal development from the blastula to the early tail-bud stage, as well as after mechanical relaxation in the early gastrula, were measured. An alternation of the periods of directed movements of large cell masses and local cell wanderings was detected. In both cases, the trajectories of individual cells consisted primarily of orthogonal segments. Cell movements were measured on two scales. At a small-scale consideration (time intervals of the order of several hours and distances of the order of tens of microns), fairly slight linear stretching and compressive deformations were detected, which looked like gentle smooth gradients along which the upward morphogenetic movements of cells were directed. At a large-scale consideration (time intervals of the order of tens of minutes and distances of the order of microns), quasi-periodic fluctuations of velocities of individual cells partly correlated in time were found. The differences between these velocities generated microdeformations, which reached several tens of percent and developed within time intervals not more than 10 min. Measurements of relative magnitudes of mechanical forces influencing the cell walls suggests that microdeformations generate local stretching and compressive deformations modulating smoother tension gradients.

Quasi-Periodic Behavior of the Pacific–Japan Pattern Affecting Propagation Routes of Summertime Wave Patterns and the Associated Tropical Cyclone Tracks over the Western North Pacific

Abstract This study introduces a modified Pacific–Japan (PJ) index that exhibits a substantial periodicity of 5–16 days in the East Asian summer monsoon region. The quasi-periodic fluctuations of the PJ index can indicate changes in the large-scale circulation systems. In the PJ high phase, the wave pattern propagates northwestward from the western North Pacific tropics to an area near northern Luzon and is then forced to move westward because of a stationary, anomalous high pressure system over southern Japan. The tropical cyclones (TCs) associated with the anomalous low pressure systems tend to follow a straight-moving propagation route through the northern South China Sea. The anomalous cyclonic flow causes heavy rainfall in eastern Taiwan. However, in the PJ low phase, the wave pattern and TCs follow a recurving propagation route toward higher latitudes. The circulation pattern typically brings heavy rainfall to northern Taiwan in the PJ low phase. Therefore, wave patterns under the influence of the quasi-periodic fluctuations of the PJ pattern affect rainfall because of the changing propagation routes of the wave patterns, as well as the TC tracks.

NONLINEAR MHD WAVES IN A PROMINENCE FOOT

We study nonlinear waves in a prominence foot using 2.5D MHD model motivated by recent high-resolution observations with Hinode/SOT in Ca~II emission of a prominence on October 10, 2012 showing highly dynamic small-scale motions in the prominence material. Observations of H$\alpha$ intensities and of Doppler shifts show similar propagating fluctuations. However the optically thick nature of the emission lines inhibits unique quantitative interpretation in terms of density. Nevertheless, we find evidence of nonlinear wave activity in the prominence foot by examining the relative magnitude of the fluctuation intensity ($\delta I/I\sim \delta n/n$). The waves are evident as significant density fluctuations that vary with height, and apparently travel upward from the chromosphere into the prominence material with quasi-periodic fluctuations with typical period in the range of 5-11 minutes, and wavelengths $\sim <$2000 km. Recent Doppler shift observations show the transverse displacement of the propagating waves. The magnetic field was measured with THEMIS instrument and was found to be 5-14 G. For the typical prominence density the corresponding fast magnetosonic speed is $\sim$20 km s$^{-1}$, in qualitative agreement with the propagation speed of the detected waves. The 2.5D MHD numerical model is constrained with the typical parameters of the prominence waves seen in observations. Our numerical results reproduce the nonlinear fast magnetosonic waves and provide strong support for the presence of these waves in the prominence foot. We also explore gravitational MHD oscillations of the heavy prominence foot material supported by dipped magnetic field structure.

On observation of so-called “water rhythms” by absorption spectroscopy methods

Experiments on observation of quasiperiodic fluctuations in optical absorption of liquid water (socalled “water rhythms”) were reproduced by IR Fourier spectroscopy methods. Slow in time changes in the optical absorption spectra were observed using different Fourier spectrometers with different metrological characteristics. These changes were within the region of experimental accuracy determined by a slow drift in parameters of the measurement equipment. No “water rhythms” were found in our experiments.

Temporal dynamics of host-landing rate in mosquitoes attacking the imitation of a host

The rates of mosquito landing on a host-imitating device were measured. The device contained an electric heater to imitate the presence of a warm-blooded animal, a gauze screen impregnated with meat juice, a microphone with an amplifier to record the sounds of mosquito landings and take-offs, and a light meter to record the ambient illumination. In addition, the behavior of mosquitoes near the device was studied by video recording. All the experiments were conducted in the first half of June in evening and morning hours in mixed spruce-birch forests of Moscow region. Audio recordings were digitally analyzed to estimate the landing rates. Significant (more than 50% of the average) quasi-periodic fluctuations of the mosquito landing rate were revealed. The peaks of mosquito activity alternated with drops with an average period of 30 min (20–50 min in different recording sessions). These fluctuations were recorded in calm weather during a gradual change in illumination. Experiments with two similar devices positioned 20 m apart showed that fluctuations of mosquito activity developed independently at each site. The fluctuations in the mosquito landing rate seem to be largely determined by endogenous factors: mosquito females, failing to obtain blood from the host-imitating device, lost interest in it and landed to rest on the nearby vegetation. After 20–50 min, the memory trace of the previous experience gradually faded, and mosquitoes returned to the attracting device thus starting the next peak of activity.

Coronal Radio Occultation Experiments with the Helios Solar Probes: Correlation/Spectral Analysis of Faraday Rotation Fluctuations

The coronal Faraday rotation (FR) experiments using the linearly polarized signals of the Helios-1 and Helios-2 interplanetary probes remain a unique investigation of the magnetic field of the solar corona and its aperiodic and quasi-periodic variations. The unexpectedly long lifetime of these spacecraft (1974 – 1986) enabled studies from very deep solar-activity minimum (1975 – 1976) into the strong activity maximum (1979). Important experimental data were also obtained for the rising (1977 – 1978) and declining (1980 – 1984) branches of the solar-activity cycle. Previous publications have presented results of the initial experimental data only for coronal-sounding experiments performed during individual solar-conjunction opportunities. This report is a more detailed analysis of the Helios FR measurements for the entire period 1975 – 1984. Radial profiles of the FR fluctuation (FRF) intensity recorded during the deepest solar-activity minimum in 1975 – 1976 are shown to differ distinctly from those during the strong solar-activity maximum in 1979. In particular, the decrease of the FRF intensity with solar-offset distance is substantially steeper in 1979 than in 1975/1976. In all cases, however, the FR data reveal quasi-periodic wave-like fluctuations in addition to the random background with a power-law spectrum. The dominant period of these fluctuations, recorded during 35 % of the total measurement time, is found to be close to five minutes. Large-scale FR variations at considerably longer periods (1.1 – 2.7 hours) were observed during 20 % of the measurement time. Knowing the intrinsic motion of the radio ray path from spacecraft to Earth and making a reasonable assumption about the solar-wind velocity, FRF observations at widely spaced ground stations have been used to estimate the velocity of coronal Alfven waves. The velocity values range between 290 and 550 km s−1 at heliocentric distances between 3.5 and 4.5 R⊙ and are marginally lower (150 – 450 km s−1) at distances between 5.5 and 6.5 R⊙. Occasional FR variations with a period near 160 minutes and harmonics with periods 60, 30, and 20 minutes were also observed.

Combined experimental and computational investigation of cavitation evolution and excited pressure fluctuation in a convergent–divergent channel

The objective of this paper is to investigate the cavitation evolution and excited pressure fluctuation in unsteady sheet/cloud cavitating flows in a convergent–divergent channel using a combination of experimental and computational studies. In the experiments, a simultaneous sampling technique is used to synchronize the cavitation images with wall-pressure signals. The computations are based on the Navier–Stokes equations solved with a mass transfer cavitation model and a modified RNG k–ε turbulence model. The experimental results indicate that, for non-cavitating flow, neither significant fluctuation nor periodic fluctuation can be observed. However, in the case of sheet/cloud cavitating conditions, significant quasi-periodic pressure fluctuations are captured in the entire cavitation area. The numerically predicted unsteady sheet/cloud cavitation behaviors agree well with the experimental results; both the experimental and computational results indicate that a typical quasi-periodic sheet/cloud cavitation development process can be divided into three main stages: (1) the growth of the attached cavity, (2) shedding of the attached cavity, and (3) growth and collapse of the detached cavities. The quasi-periodic pressure fluctuations in sheet/cloud cavitation are found to be highly related with the time evolution of the cavitation behaviors. The acceleration due to the changes in the cavity volume is found to be the main source of the excited pressure fluctuations. The pressure signal decreases in the middle of the growing process of the attached cavity and the detached cavity. Next, the pressure signal increases and reaches a peak with the collapse of large detached cavity near transducer 4#.

盐池县2000—2012年植被变化及其驱动力分析

PDF HTML阅读 XML下载 导出引用 引用提醒 盐池县2000-2012年植被变化及其驱动力 DOI: 10.5846/stxb201403050374 作者: 作者单位: 1. 西北土地退化与生态恢复国家重点实验室培育基地宁夏大学,1. 西北土地退化与生态恢复国家重点实验室培育基地宁夏大学 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点基础研究计划(973)前期专项(2012CB723206);国家自然科学基金项目(41201438);宁夏大学211建设项目 Vegetation dynamics over 2000-2012 and its driving factors in Yanchi County, Ningxia Province Author: Affiliation: 1.Cultivation Base for State Key Lab. of Restoration and Reconstruction of Degraded Ecosystem in Northwestern of China,Yinchuan,1.Cultivation Base for State Key Lab. of Restoration and Reconstruction of Degraded Ecosystem in Northwestern of China,Yinchuan Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:荒漠草原区的植被对防治荒漠化、维护生态屏障具有决定性作用,宁夏盐池县作为其典型代表,近13年的植被变化深受气候变化和人类活动的综合影响.基于MODIS NDVI等数据,运用趋势分析、经验模态分解和空间叠置分析等方法,对盐池县2000-2012年的植被动态变化进行研究,结果表明:(1)2000-2012年盐池县NDVI在0.2-0.4之间呈波动上升趋势,上升幅度为0.078/10 a,上升趋势显著;总体来说,植被稳定性低,年际间波动或转换频繁、幅度大;(2)NDVI的波动分量与残余分量方差贡献率各占50%,且NDVI波动呈减弱趋势.促使NDVI波动的主控因子是年降水量,但其影响在减弱;(3)推动NDVI趋势性上升的主要因素是土地利用方式改善和类型变化,但土地利用方式改善对NDVI的贡献远远大于土地利用类型变化对NDVI的贡献.因此,荒漠草原区的生态改善应以保护为主,辅之以必要的生态重建,走以适度开发带动整体保护的道路. Abstract:The vegetation of the desert steppe plays an important role in preventing desertification, maintaining ecosystem stability, and constructing "Ecological Barrier" in Northwest China. However, most desert steppe ecosystems are very fragile and constantly face the risk of degradation. Yanchi County, located in eastern Ningxia province, is a typical desert steppe, and its vegetation dynamics in the past 13 years have been affected by both climate change and human activities. In order to explore the process and driving factors based on the Normalized Difference Vegetation Index (NDVI) derived from the Moderate-resolution Imaging Spectroradiometer (MODIS), we studied the vegetation dynamics in Yanchi County from 2000 to 2012. We used 296 scenes of MODIS NDVI data, all of these were converted to an Albers conical projection system and GeoTIF format by using the MODIS Reprojection Tool, and smoothed using the Savitzky-Golay filter to reconstruct a high-quality NDVI time-series data set. Annual and quarterly NDVI were synthesized using the Maximum Value Composite (MVC) method. In addition to MODIS NDVI data, the land use data of Yanchi in 2000 and 2011, meteorological data, and social statistical data were also used in this study. Multiple methods were used to analyze the vegetation dynamics in Yanchi County. A linear regression with an F test was used to analyze the trend of NDVI and its significance. The non-parametric Mann-Kendall test was used to detect the abrupt change in the long-term NDVI from 2000 to 2012. A non-linear and non-stationary signal analysis method, Empirical Mode Decomposition (EMD), was used to isolate the amplitude-frequency determining the temporally varying trend of NDVI, and spatial overlay analysis was used to analyze the influence of land use and land cover change on vegetation dynamics. The results showed: (1) the average NDVI values of all pixels in Yanchi County, which were composited annually by the MVC method, ranged from 0.2 to 0.4 in the period 2000-2012. The vegetation index in this area was very low because there is a typical desert steppe, but significantly (P < 0.05) increased by 0.078 per 10 a, which was faster than that in the Three-North Shelter Forest Program region. Overall, the vegetation in Yanchi County lacks stability and has frequent, large-amplitude inter-annual fluctuations. (2) The Empirical Mode Decomposition found that the NDVI time-series data included two Intrinsic Mode Function (IMF) components with 4 and 9 year quasi-periodic fluctuations. The variance contribution of the first IMF component was about 50%, almost the same as that of the NDVI residue component, which increased significantly over the 13 years. However, the intensity of NDVI fluctuation decreased because the fluctuation in precipitation, one of its main driving factors, declined. (3) Improvements in land use and land cover change were the main drivers for NDVI increase. The former made a larger contribution than the latter. Therefore, protection is the first option for improving the ecological environment, and proper reconstruction can be used as a supplement in desert steppe. A proper strategy for vegetation restoration and maintenance should be adopted, and overall protection can be implemented through scientific and harmonized development. 参考文献 相似文献 引证文献

Experimental investigation of flow development and gap vortex street in an eccentric annular channel. Part 1. Overview of the flow structure

AbstractFlow visualization, laser Doppler velocimetry and planar and stereoscopic particle image velocimetry were used to investigate the isothermal velocity field along an eccentric annular channel with a diameter ratio of 0.5 and an eccentricity of 0.8 for a Reynolds number of 7300. Observation of the flow development has identified three distinct regions: the entrance region, the fluctuation-growth (FG) region and the rapid-mixing (RM) region. Weak quasi-periodic velocity fluctuations were first detected in the downstream part of the entrance region, and grew into very strong ones, reaching peak-to-peak amplitudes in the narrow gap that were nearly 60 % of the bulk velocity. Two mixing layers were identified on either side of the gap, which generated a street of counter rotating vortices and thorough large-scale mixing of the fluid in the channel.

500-year climate cycles stacking of recent centennial warming documented in an East Asian pollen record

Here we presented a high-resolution 5350-year pollen record from a maar annually laminated lake in East Asia (EA). Pollen record reflected the dynamics of vertical vegetation zones and temperature change. Spectral analysis on pollen percentages/concentrations of Pinus and Quercus, and a temperature proxy, revealed ~500-year quasi-periodic cold-warm fluctuations during the past 5350 years. This ~500-year cyclic climate change occurred in EA during the mid-late Holocene and even the last 150 years dominated by anthropogenic forcing. It was almost in phase with a ~500-year periodic change in solar activity and Greenland temperature change, suggesting that ~500-year small variations in solar output played a prominent role in the mid-late Holocene climate dynamics in EA, linked to high latitude climate system. Its last warm phase might terminate in the next several decades to enter another ~250-year cool phase, and thus this future centennial cyclic temperature minimum could partially slow down man-made global warming.

The role of advection and of work done by forces in the gustiness of the Bora flow

Local wind changes within the flow of the Adriatic Bora are investigated in the case of 8 December 2001 that was simulated with the WRF/ARW model in a 333 m horizontal resolution and with all relevant model variables archived during a 3 h time interval with 1 s temporal resolution. Along a selected cross section, two locations of strong gusts in the lee of a Dinaric mountain ridge are found – the upper one on the slope and the other over the sea close to the coastline; in between the wind is considerably weaker. About 16 to 18 gusts developed in the 3 h interval at both locations with an average period of approximately 8.5 min. The advective transport of kinetic energy (KE), the work of the unbalanced part of the pressure gradient force and the diffusion and dissipation of KE cause the local change D KE, and their quasi-periodic fluctuations lead to gusts and lulls. The results of the numerical simulation enable an estimation of the contributions of advection, the work of forces and of turbulence and dissipation effects. Advective contributions to local changes are more or less in the phase with DKE and ahead of the phase of KE, while the contributions of work are generally smaller and in the opposite phase: so in general the advection and the work effects oppose each other. The advection and the work effects are not spatially homogeneous, i.e. they cease and even change their sign between the two locations of the strongest gusts and so there is no continuous displacement of KE from the upper to the lower location by either of the two effects: their contributions interchange with each other down the slope.

Comparison of sporadic sodium layer characteristics observed at different time resolutions

Abstract. Sporadic sodium (Nas) layers, occurring in roughly the same height range as ionospheric sporadic-E layers, were first detected by lidar some 30 yr ago. Nas layers have a typical thickness of a few hundred meters to a few km, with peak atom concentrations several times that of the background layer. Despite a great deal of excellent work over the past decades, the source of Nas layers is still not altogether clear, partly as a result of our incomplete knowledge of Nas layer characteristics. In this paper we concentrate on some typical case studies chosen from the ~127 h of sporadic sodium layer observations made at a time resolution of 1.5 s at Yanqing (115.97° E, 40.47° N), Beijing, China. This is a much better time resolution than what has been employed in most earlier measurements. The results show that the Nas layer peak heights are dispersed at slightly different although adjacent heights. When averaged over several minutes, as has been the case with most earlier measurements, the height scatter results in an apparent layer thickness of a few km. We conclude, therefore, that these dispersed peaks at different but adjacent heights constitute the 5 min Nas layer. Similar to the observations of sporadic-E-ion (Es) layers and meteor rate, we observe quasi-periodic fluctuations on a timescale on the order of several minutes in the peak height and the peak density of sporadic layers, which is a universal feature but concealed by the lower temporal resolution previously adopted. Spatially localized multiple scatterers and multiple thin layers with similar apparent movement in Nas layers are also found. We discuss the possible formation mechanism by the direct deposition of large swarms of micrometeoroids and demonstrate a typical example of meteor trails evolving into a Nas layer, which suggests that this mechanism might indeed occur.

Planetary wave oscillations observed in ozone and PMSE data from Antarctica

The effect of temperature variations caused by planetary waves on the occurrence of Polar Mesospheric Summer Echoes (PMSE) has been a subject of recent research. These same temperature fluctuations have also been shown to modulate the ozone volume mixing ratio above 30km. In this study, ground-based radiometer measurements of ozone mixing ratio profiles at Troll station (72°S, 2°E), Antarctica are compared with PMSE extracted from the near-range measurements of the Sanae (72°S, 2°W) Super Dual Auroral Radar (SuperDARN) over the radiometer field of view. We show here that the resulting quasi-periodic fluctuations in PMSE correlate with the variations seen in the ozone. The ozone mixing-ratio variations may then be used to trace the phase variation of planetary waves with height to demonstrate that they extend from the stratosphere up to the mesopause. The results indicate that the modulation of PMSE occurrence frequency during the summer of 2009/10 is the result of two planetary waves with similar zonal structure and period, but with different vertical phase structures.

Nonlocal resistance and its fluctuations in microstructures of band-inverted HgTe/(Hg,Cd)Te quantum wells

We investigate experimentally transport in gated microsctructures containing a band-inverted HgTe/Hg_{0.3}Cd_{0.7}Te quantum well. Measurements of nonlocal resistances using many contacts prove that in the depletion regime the current is carried by the edge channels, as expected for a two-dimensional topological insulator. However, high and non-quantized values of channel resistances show that the topological protection length (i.e. the distance on which the carriers in helical edge channels propagate without backscattering) is much shorter than the channel length, which is ~100 micrometers. The weak temperature dependence of the resistance and the presence of temperature dependent reproducible quasi-periodic resistance fluctuations can be qualitatively explained by the presence of charge puddles in the well, to which the electrons from the edge channels are tunnel-coupled.