Compressive fluctuations in the solar wind and their polytropic index

Magnetohydrodynamic compressive fluctuations of the interplanetary plasma in the region from 0.3 to 1 AU have been characterized in terms of their polytropic index. Following Chandrasekhar’s approach to polytropic fluids, this index has been determined through a fit of the observed variations of density and temperature. At least three different classes of fluctuations have been identified: (1) variations at constant thermal pressure, in low-speed solar wind and without a significant dependence on distance, (2) adiabatic variations, mainly close to 1 AU and without a relevant dependence on wind speed, and (3) variations at nearly constant density, in fast wind close to 0.3 AU. Variations at constant thermal pressure are probably a subset of the ensemble of total-pressure balanced structures, corresponding to cases in which the magnetic field magnitude does not vary appreciably throughout the structure. In this case the pressure equilibrium has to be assured by its thermal component only. The variations may be related to small flow-tubes with approximately the same magnetic-field intensity, convected by the wind in conditions of pressure equilibrium. This feature is mainly observed in low-velocity solar wind, in agreement with the magnetic topology (small open flow-tubes emerging through an ensemble of closed structures) expected for the source region of slow wind. Variations of adiabatic type may be related to magnetosonic waves excited by pressure imbalances between contiguous flow-tubes. Such imbalances are probably built up by interactions between wind flows with different speeds in the spiral geometry induced by the solar rotation. This may account for the fact that they are mainly found at a large distance from the sun. Temperature variations at almost constant density are mostly found in fast flows close to the sun. These are the solar wind regions with the best examples of incompressible behaviour. They are characterized by very stable values for particle density and magnetic intensity, and by fluctuations of Alfvénic type. It is likely that temperature fluctuations in these regions are a remnant of thermal features in the low solar atmosphere. In conclusion, the polytropic index appears to be a useful tool to understand the nature of the compressive turbulence in the interplanetary plasma, as far as the frozen-in magnetic field does not play a crucial role.     

Non-Gaussian probability distributions of solar wind fluctuations

The probability distributions of field differences x()=x(t+)-x(t), where the variable x(t) may denote any solar wind scalar field or vector field component at time t, have been calculated from time series of Helios data obtained in 1976 at heliocentric distances near 0.3 AU. It is found that for comparatively long time lag , ranging from a few hours to 1 day, the differences are normally distributed according to a Gaussian. For shorter time lags, of less than ten minutes, significant changes in shape are observed. The distributions are often spikier and narrower than the equivalent Gaussian distribution with the same standard deviation, and they are enhanced for large, reduced for intermediate and enhanced for very small values of x. This result is in accordance with fluid observations and numerical simulations. Hence statistical properties are dominated at small scale by large fluctuation amplitudes that are sparsely distributed, which is direct evidence for spatial intermittency of the fluctuations. This is in agreement with results from earlier analyses of the structure functions of x. The non-Gaussian features are differently developed for the various types of fluctuations. The relevance of these observations to the interpretation and understanding of the nature of solar wind magnetohydrodynamic (MHD) turbulence is pointed out, and contact is made with existing theoretical concepts of intermittency in fluid turbulence.     

Recent observations of MHD fluctuations in the solar wind

A short review of recent observations of solar wind fluctuations in the magnetohydrodynamic (MHD) range of scales is presented. In recent years, the use of high time-resolution data on an extended interval of heliocentric distance has allowed significant advances in our knowledge of MHD fluctuations. We first focus on the origin and evolution of the Alfvénic-type fluctuations. The role of interplanetary sources and the influence of interactions with structures convected by the solar wind are examined. Then compressive fluctuations are investigated, with special attention being given to their nature and origin. Observations are discussed in the light of recent theories and models. Finally, predictions for MHD turbulence in polar regions of the heliosphere are highlighted.     

The response of saltation to wind speed fluctuations

The response time of saltation to spatial or temporal wind speed fluctuations constitutes an important control parameter for aeolian sediment transport and deposition. In this paper, we present direct measurements of the response time obtained from several field experiments. The sand transport was studied using six small microphones arranged in a vertical profile and collocated with a sonic anemometer, a webcam and a cup anemometer tower. The webcam was coupled with the sonic anemometer via a personal computer and provides information on creeping and saltating grains with a sampling rate of 10 Hz. Sediment transport measurements were obtained over four periods. The Wiener filter, a signal processing technique, is used to obtain a discrete transfer function that relates the horizontal wind speed and the non‐intermittent sand transport. The transfer function can be established using an exponential function with a time constant or characteristic response time τ without time shift. The response time fluctuated between zero and 1·5 seconds depending on the turbulence intensity, the saltation activity, the measuring height and sampling rates. The Wiener filter coefficients suggest that the response of saltation to wind speed alterations is determined by more than one process. Copyright © 2012 John Wiley & Sons, Ltd.     

A statistical study of underestimates of wind speeds by VHF radar

Comparisons are made between horizontal wind measurements carried out using a VHF-radar system at Aberystwyth (52.4°N, 4.1°W) and radiosondes launched from Aberporth, some 50 km to the southwest. The radar wind results are derived from Doppler wind measurements at zenith angles of 6° in two orthogonal planes and in the vertical direction. Measurements on a total of 398 days over a 2-year period are considered, but the major part of the study involves a statistical analysis of data collected during 75 radiosonde flights selected to minimise the spatial separation of the two sets of measurements. Whereas good agreement is found between the two sets of wind direction, radar-derived wind speeds show underestimates of 4–6% compared with radiosonde values over the height range 4–14 km. Studies of the characteristics of this discrepancy in wind speeds have concentrated on its directional dependence, the effects of the spatial separation of the two sets of measurements, and the influence of any uncertainty in the radar measurements of vertical velocities. The aspect sensitivity of radar echoes has previously been suggested as a cause of underestimates of wind speeds by VHF radar. The present statistical treatment and case-studies show that an appropriate correction can be applied using estimates of the effective radar beam angle derived from a comparison of echo powers at zenith angles of 4.2° and 8.5°.     

Short-period fluctuations in the atmospheric electric field over the ocean

Fluctuations of short period in the atmospheric electric field were studied through the measurements of electric field and space charge density on the Mid-Pacific Ocean. The amplitude of fluctuation is about one third of the mean electric field, and the period mainly ranges from 2 to 5 min. The fluctuations are considered to be under the influence of spatial and temporal variation of space charge layer that possibly originates from the electrode effect above the sea surface. The unit of electrical irregularities in the atmosphere above the ocean has horizontal scale of the order of 1.5 km and indicates a tendency to become large as the wind speed increases. The vertical scale of space charge layer is estimated at several tens meters.     

Polarization of compressible turbulent fluctuations in the solar wind plasma

Compressible fluctuations in solar wind plasma are analyzed on the basis of the 1995–2010 WIND and Advanced Composition Explorer (ACE) spacecraft data. In the low-speed solar wind (V ₀ < 430 km/s), correlations between fluctuations in the magnetic field direction and plasma density, as well as between velocity fluctuations and plasma density, are found. The covariance functions of these parameters calculated as functions of the local magnetic field direction are axially symmetric relative to the axis, which is oriented nearly along the regular magnetic field of the heliosphere (the Parker spiral). Fluctuations in the magnetic field and velocity are polarized in the plane that is orthogonal to the axis of symmetry. Plasma oscillations of these properties can be caused by fast magnetosonic waves propagating from the Sun along the Parker spiral.     

Solar wind influence on atmospheric processes in winter Antarctica

Galactic cosmic rays (GCRs) altered by solar wind are traditionally regarded as the most plausible agent of solar activity influence on the Earth's atmosphere. However, it is well known that severe reductions in the GCRs flux, known as Forbush decreases (FDs), are caused by solar wind of high speed and density, which sweeps away the GCRs on its way. Since the FD beginnings are registered at the Earth's orbit simultaneously with dramatic disturbances in the solar wind, the atmospheric effects, assigned to FDs, can be, in reality, the results of the solar wind influence on the atmospheric processes. This paper presents a summary of the experimental results demonstrating the strong influence of the interplanetary electric field on atmospheric processes in central Antarctica, where the large-scale system of vertical circulation is formed during winter seasons. The influence is realized through acceleration of the air masses, descending into the lower atmosphere from the troposphere, and the formation of cloudiness above the Antarctic Ridge, where the descending air masses enter the surface layer. The acceleration is followed by a sharp increase of the atmospheric pressure near-pole region, which gives rise to the katabatic wind strengthening above the entire Antarctica. The cloudiness formation results in the sudden warmings in the surface atmosphere, since the cloud layer efficiently backscatters the long wavelength radiation from the ice sheet, but does not affect the adiabatic warming process of the descending tropospheric air masses. When the drainage flow strengthening the circumpolar vortex around the periphery of the Antarctic continent decays, the surface easterlies typical of the coast stations during the winter season are replaced by southerlies and the cold Antarctic air masses flow out to the Southern ocean.     

Anisotropy and symmetry of fluctuations in the solar wind magnetic field and velocity

The behavior of correlation tensors of fluctuations in the solar wind magnetic field and velocity is studied during different phases of a solar cycle on the basis of a 45-year measurement series of solar wind parameters. It is found that the orientation of fluctuations in the magnetic field and velocity is approximately axisymmetric relative to the direction of a local magnetic field during high solar activity. This symmetry is violated significantly during periods of low solar activity, and deviations from the symmetry are regular and oppositely directed during minima of even and odd 11-year cycles, which is probably connected with variations in the orientation of the Sun??s magnetic field. The dependence of the power of fluctuations on the local magnetic field direction reveals significant deviations from local symmetry during all phases of a solar cycle, especially for velocity fluctuations.     

太阳活动千年尺度的准周期性波动

2004年,Solanki等人利用树木年轮中δ14C含量变化重建的太阳黑子数序列研究太阳活动的论文被Nature发表,该黑子序列自1895年起向历史时期延伸了11400年.本文采用最大熵谱分析方法和小波变换方法分析了这一重建的太阳黑子序列,重点讨论太阳活动在千年尺度上的周期性波动.结果表明,太阳活动的长期变化中存在接近千年和略大于两千年的准周期信号,以及可能存在约7千年的波动,得到了这些准周期分量的参数.这些准周期分量的周期长度和振幅是随着时间变化的,文中给出了它们的时变图象并讨论了它们的时变特征.     

Tide-induced airflow in a two-layered coastal land with atmospheric pressure fluctuations

Tide-induced airflow is commonly seen in coastal lands and affects ground stability especially with a less permeable pavement on the ground surface. A tide-induced airflow model in a two-layered unsaturated zone consisting of a highly permeable layer underneath a less permeable layer was established by Li and Jiao [Li HL, JJ Jiao. One-dimensional airflow in unsaturated zone induced by periodic water table fluctuation. Water Resour Res 2005;41:W04007. doi:10.1029/2004WR003916] to describe the one-dimensional airflow with constant atmospheric pressure at the ground surface. In this study, we expand the Li and Jiao model by considering the realistic atmospheric pressure fluctuations and the initial condition. A new transient solution to the airflow model is developed for an initial boundary value problem (IBVP). The transient solution can be used not only to calculate the subsurface air pressure at a future time with a known initial condition, but also to evaluate the asymptotic air pressure variations when time becomes long. The amplitude ratio and phase lag of the subsurface air pressure relative to the tide-induced hydraulic head variations inside the unconfined aquifer below the unsaturated zone are investigated. The results reveal that effect on the subsurface pressure due to changes of atmospheric pressure amplitude depends on the configurations of air resistance in the less permeable layer and the air-filled porosity difference in the two layers. The introduction of atmospheric pressure fluctuations into the airflow model leads to insignificant influence on water table level. A field application of the new solution at Hong Kong International Airport in Hong Kong, China is demonstrated. It indicates that the new transient solution can be conveniently used to evaluate the subsurface air pressure with discrete atmospheric pressure data at the ground surface.     

地磁场大尺度变化特征分析

利用20世纪50年代至21世纪初中国出版的7次《中国地磁图》资料, 根据构造磁效应和震磁相关性研究了地质构造(构造微动态)、 强震活动与中国地磁图等变线每个不同年代动态演化图像的关系, 从大区域尺度的角度分析了中国大陆及其邻近地区的7级以上强震与地磁分量等变线轴线方向的变化特征。     

Time fluctuations of small ion number density in the atmospheric surface layer

The standard deviations and time-spectra of small ion number density have been measured at ground level under different atmospheric stability conditions. The auto-correlation correlograms and the time-spectra imply a scale of fluctuations corresponding to 1 to 4 min. No clear relationships between standard deviations of small ion number density and micro-meteorological parameters are observed. Thus the results are not a support for Monin-Obukhov similarity theory. For the highest frequencies the slopes of the spectra seem to follow the –5/3-law fairly well and the frequencies of the peak values of individual spectra lnfS(f) increase when the atmosphere stability increases, which is valid for ordinary micrometeorological parameters.     

Catchment scale controls the temporal connection of transpiration and diel fluctuations in streamflow

Diel fluctuations can comprise a significant portion of summer discharge in small to medium catchments. The source of these signals and the manner in which they are propagated to stream gauging sites is poorly understood. In this work, we analysed stream discharge from 15 subcatchments in Dry Creek, Idaho, Reynolds Creek, Idaho, and HJ Andrews, Oregon. We identified diel signals in summer low flow, determined the lag between diel signals and evapotranspiration demand and identified seasonal trends in the evolution of the lag at each site. The lag between vegetation water use and streamflow response increases throughout summer at each subcatchment, with the rate of increase as a function of catchment stream length and other catchment characteristics such as geology, vegetation and stream geomorphology. These findings support the hypothesis that variations in stream velocity are the key control on the seasonal evolution of the observed lags. Copyright © 2012 John Wiley & Sons, Ltd.     

Analysis of large scale disturbances in spiral galaxies

Summary A scale analysis is made of large scale eddying motions superimposed upon the state of mean rotation of spiral galaxies. By this means a balance relationship is derived between the fields of potential on the one hand and the horizontal eddy velocity on the other. This should provide a useful kinematical relationship for observational astronomy. It is obvious to the meteorological reader that the large scale irregular motions of galaxies are analogous to those which occur in our own atmosphere.     

Sidewall effects and sand trap efficiency in a large wind tunnel

Aeolian sand transport is a widespread physical phenomenon on the surface of Earth, as well as on Mars and Titan. Accurate measurements of the components of the transport system are necessary if we are to understand the nature of the physical processes. Sand traps are typically used to measure sediment transport rates, and issues associated with the sampling efficiency of traps and the development of reliable traps have received considerable attention in recent decades. In this study, we measured aeolian transport rate at five distances from a wind tunnel sidewall using a vertically‐segmented sand trap. Total transport rates were determined by weighing the bed sediment before and after each experiment, and with and without a trap installed. The following results were obtained: (1) sand transport increased linearly with the distance away from the sidewall, and the appropriate location to measure maximum transport is within the central 20% of the wind tunnel; (2) current methods overestimate the sampling efficiency of sand traps when comparing trap data to transport rate data obtained by weighing sand moved through the entire tunnel because the effects of the sidewalls in decreasing total transport are neglected; (3) the efficiency of the vertically‐segmented trap that we tested ranged from 11.57% to 31.68% using our revised methods, whereas standard methods caused efficiency to be overestimated by 32–72% of the efficiency; (4) using either method, the efficiency of the trap increased exponentially with shear velocity for the range we used. Copyright © 2017 John Wiley & Sons, Ltd.     

The wind profile from surface through 30 km in atmospheric ballistics problems

Summary The individual wind profile may differ considerably from the mean monthly profile. In cognizance, the correlation method was developed. This method yields some improvement, but requires elaborate mathematical treatment. The computational effort is probably not in accord with the results obtained due to the limitations of the linear correlation concept.The author has developed a new method, called characteristics method. The individual wind profile is represented by mathematically describing the profile with a few characteristic coefficients. This method seems superior to the correlation method.

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Zusammenfassung Das individuelle Windprofil kann im Einzelfalle erheblich vom Monatsmittelprofil abweichen. Um dieser Tatsache Rechnung zu tragen, wurden bis jetzt die Korrelationskoeffizienten mit herangezogen. Die dadurch erzielten Verbesserungen sind jedoch mit einem Rechenaufwand verbunden, der in keinem Verhältnis zu dem Gewinn steht, da der Verwendung des linearen Korrelations Koeffizienten natürliche Grenzen gesetzt sind.Es wird daher eine neue Methode diskutiert, die vom Autor entwickelte Charakteristik Methode. Dabei wird das individuelle Windprofil mathematisch durch wenige charakteristiche Terme beschrieben. Aus Vergleichsbeispielen geht hervor, dass die neue Methode der (linearen) Korrelationsmethode überlegen ist. Auch der Rechenaufwand ist gering, wenn einmal die Grundkoeffizienten gewonnen sind.

     

Long-term large wood load fluctuations in two low-order streams in Southern Chile

The eco-hydrogeomorphic significance of large wood (LW) and its potential for increasing downstream hazards during extreme floods have been widely recognized. We used LW data collected for a 10-year period from the two low-order streams of Pichún (Pi) and Vuelta de Zorra (VZ) in Southern Chile to (a) determine if the abundance and dimensions of individual LW pieces change with time, (b) quantify wood load fluctuations during the 10-year period, and (c) assess the role of LW recruitment from the riparian forests to explain wood load fluctuations during the study period. Nine years after the first survey, the number of LW pieces in Pi and VZ diminished by 60 and 40%, respectively. Despite the reduction in these numbers, in Pi, the LW dimensions did not change significantly during the study. In VZ, the dimensions exhibited statistically significant differences, despite being within the same class. In both catchments, the LW load fluctuated during a 10-year period, but the drivers of change differed. Although tree toppling was the recruitment mechanism responsible for LW in both stream cases, the high wood load measured in Pi at the beginning of the study suggested massive tree recruitments before the first survey, followed by wood exports which were higher than inputs in the subsequent 10-year period. In VZ, LW load decreased during the first 9 years (mean annual rate of ~9.2 m³ year⁻¹) and then increased by ~12.1 m³ year⁻¹ in year 10. At VZ, the inputs consisted of single trees that were recruited from the riparian area and by upstream flotation, while exports occurred by downstream fluvial transport. Wood inputs and exports occurred asynchronously and led to LW load fluctuations at decadal and annual intervals. Land management and tree species thus exert a major influence on wood inventory and budget in streams. © 2020 John Wiley & Sons, Ltd.     

Active regions and the large scale flow in the solar photosphere

Summary Statistics on sunspot leaders and followers in active regions are derived from the 50 years of Greenwich records 1905–1954. The results are combined with the probable structure of large scale solar eddies as synthesized from various relevant pieces of information. It is found that active regions tend to be located in anticyclonic ridges of the large disturbances. The statistical leader and follower populations are further subdivided, each into new and old spot classifications. The new leaders, especially, show motions different from those of other spots, suggesting the presence of a life cycle for the large disturbances. It is found that the divergence and vorticity are positive and anticyclonic, respectively, for the new active regions, the former quantity being numerically larger than the latter in this case. Various possible implications of the material are discussed.     

城市布局规模与大气环境影响的数值研究

为探索城市建设对局地及周边大气环境的影响,本文采用典型代表性天气条件,以北京主城区及其东部发展带小城镇群的发展变化为例,设计算例进行数值模拟.分析结果表明：城镇群建设发展通过地气的相互作用对局地环境产生显著影响,在本文选择的夏日晴好天气条件下,就1980～2004年城市区域布局状况,模拟域内北京城市用地增加19%,城市区域平均气温增加1.91℃,植被覆盖率减少20%,城市区域平均比湿减少3.3 g·kg-1,并且城市发展的格局规模不同,对城市气象环境的影响程度也不同. 此外,由于地气多因子的相互影响和反馈作用,城建规模的变化对周边的环境也存在显著的影响,城建规模越大,对周边的影响越大．例如, (1) 北京主城区的存在对周边小城镇午间14:00近地面温度影响最大可达到1.2℃,混合层高度可增高150 m左右; (2) 城市建设在影响周边气象环境的同时,也改变了城市污染物的输送扩散能力,北京主城区的存在使周边小城镇PM10的允许排放总量减小18.02 t·d-1,同时,随着周边小城镇城市规模的扩大,影响主城区PM10逐渐由净的输出转变为净的收入,小城镇群的存在对主城区PM10净收支的贡献率达到0.192 t·d-1.