The polar wind: Recent observations

The polar wind is an ambipolar outflow of thermal plasma from the high-latitude ionosphere to the magnetosphere, and it primarily consists of H⁺, He⁺ and O⁺ ions and electrons. Statistical and episodic studies based primarily on ion composition observations on the ISIS-2, DE-1, Akebono and Polar satellites over the past four decades have confirmed the existence of the polar wind. These observations spanned the altitude range from 1000 to ∼50,500 km, and revealed several important features in the polar wind that are unexpected from “classical” polar wind theories. These include the day–night asymmetry in polar wind velocity, which is 1.5–2.0 times larger on the dayside; appreciable O⁺ flow at high altitudes, where the velocity at 5000–10,000 km is of 1–4 km/s; and significant electron temperature anisotropy in the sunlit polar wind, in which the upward-to-downward electron temperature ratio is 1.5–2. These features are attributable to a number of “non-classical” polar wind ion acceleration mechanisms resulting from strong ionospheric convection, enhanced electron and ion temperatures, and escaping atmospheric photoelectrons. The observed polar wind has an averaged ion temperature of ∼0.2–0.3 eV, and a rate of ion velocity increase with altitude that correlates strongly with electron temperature and is greatest at low altitudes (<4000 km for H⁺). The rate of velocity increase below 4000 km is larger at solar minimum than at solar maximum. Above 4000 km, the reverse is the case. This suggests that the dominant polar wind ion acceleration process may be different at low and high altitudes, respectively. At a given altitude, the polar wind velocity is highly variable, and is on average largest for H⁺ and smallest for O⁺. Near solar maximum, H⁺, He⁺, and O⁺ ions typically reach a velocity of 1 km/s near 2000, 3000, and 6000 km, respectively, and velocities of 12, 7, and 4 km/s, respectively, at 10,000 km altitude. Near solar minimum, the velocity of all three species is smaller at high altitudes. Observationally it is not always possible to unambiguously separate an energized “non-polar-wind” ion such as a low-energy “cleft ion fountain” ion that has convected into a polar wind flux tube from an energized “polar-wind” ion that is accelerated locally by “non-classical” polar-wind ion acceleration mechanisms. Significant questions remain on the relative contribution between the cleft ion fountain, auroral bulk upflow, and the topside polar-cap ionosphere to the O⁺ polar wind population at high altitudes, the effect of positive spacecraft charging on the lowest-energy component of the H⁺ polar wind population, and the relative importance of the various classical and non-classical ion acceleration mechanisms. These questions pose several challenges in future polar wind observations: These include measurement of the lowest-energy component in the presence of positive spacecraft potential, definitive determination and if possible active control of the spacecraft potential, definitive discrimination between polar wind and other inter-mixed thermal ion populations, measurement of the three-dimensional ion drift velocity vector and the parallel and perpendicular ion temperatures or the detailed three-dimensional velocity distribution function, and resolution of He⁺ and other minor ion species in the polar wind population.     

An experiment to study and control the Langmuir sheath around INTERBALL-2

The satellite INTERBALL-2 has an orbit with high inclination (62.8°), covering the altitude range between a few hundred and about 20000 km. The ambient plasma conditions along this orbit are highly variable, and the interactions of this plasma with the spacecraft body as well as the photo-electron sheath around it are considered to be interesting topics for detailed studies. The electric potential of the spacecraft with respect to the ambient plasma that develops as a result of the current equilibrium reacts sensitively to variations of the boundary conditions. The measurement and eventual control of this potential is a prerequisite for accurate measurements of the thermal plasma. We describe the purpose and technical implementation of an ion emitter instrument on-board INTERBALL-2 utilising ion beams at energies of several thousand electron volts in order to reduce and stabilise the positive spacecraft potential. First results of the active ion beam experiments, and other measures taken on INTERBALL-2 to reduce charging are presented. Furthermore, the approach and initial steps of modelling efforts of the sheath in the vicinity of the INTERBALL-2 spacecraft are described together with some estimates on the resulting spacecraft potential, and effects on thermal ion measurements. It is concluded that even moderate spacecraft potentials as are commonly observed on-board INTERBALL-2 can significantly distort the measurements of ion distribution functions, especially in the presence of strongly aniso-tropic distributions.     

Spacecraft potential control aboard Equator-S as a test for Cluster-II

The payload of Equator-S was complemented by the potential control device (PCD) to stabilise the electric potential of the spacecraft with respect to the ambient plasma. Low potentials are essential for accurate measurements of the thermal plasma. The design of PCD is inherited from instruments for Geotail and Cluster and utilises liquid metal ion sources generating a beam of indium ions at several keV. The set-up of the instrument and its interaction with the plasma instruments on board is presented. When the instrument was switched on during commissioning, unexpectedly high ignition and operating voltages of some ion emitters were observed. An extensive investigation was initiated and the results, which lead to an improved design for Cluster-II, are summarised. The cause of the abnormal behaviour could be linked to surface contamination of some emitters, which will be monitored and cured by on-board procedures in future. The mission operations on Equator-S were not at all affected, because of the high redundancy built into the instrument so that a sufficient number of perfectly operating emitters were available and were turned on routinely throughout the mission. Observations of the effect of spacecraft potential control on the plasma remained limited to just one event on January 8, 1998, which is analysed in detail. It is concluded that the ion beam lead to the predicted improvement of the particle measurements even outside the low density regions of the magnetosphere where the effect of spacecraft potential control would have been much more pronounced, and that the similar instruments for the four Cluster-II spacecraft to be launched in 2000 will be very important to ensure accurate plasma data from this mission.     

Electron velocity distribution function in a plasma with temperature gradient and in the presence of suprathermal electrons: application to incoherent-scatter plasma lines

The plasma dispersion function and the reduced velocity distribution function are calculated numerically for any arbitrary velocity distribution function with cylindrical symmetry along the magnetic field. The electron velocity distribution is separated into two distributions representing the distribution of the ambient electrons and the suprathermal electrons. The velocity distribution function of the ambient electrons is modelled by a near-Maxwellian distribution function in presence of a temperature gradient and a potential electric field. The velocity distribution function of the suprathermal electrons is derived from a numerical model of the angular energy flux spectrum obtained by solving the transport equation of electrons. The numerical method used to calculate the plasma dispersion function and the reduced velocity distribution is described. The numerical code is used with simulated data to evaluate the Doppler frequency asymmetry between the up- and downshifted plasma lines of the incoherent-scatter plasma lines at different wave vectors. It is shown that the observed Doppler asymmetry is more dependent on deviation from the Maxwellian through the thermal part for high-frequency radars, while for low-frequency radars the Doppler asymmetry depends more on the presence of a suprathermal population. It is also seen that the full evaluation of the plasma dispersion function gives larger Doppler asymmetry than the heat flow approximation for Langmuir waves with phase velocity about three to six times the mean thermal velocity. For such waves the moment expansion of the dispersion function is not fully valid and the full calculation of the dispersion function is needed.     

On estimating time offsets in the ambient noise correlation function caused by instrument response errors

Broadband seismic networks are becoming more intensive, generating a large amount of data in the long-term collection process. When processing the data, the researchers rely almost on instrument response files to understand the information related to the instrument. Aiming at the process of instrument response recording and instrument response correction, we identify several sources of the instrument response phase error, including pole–zero change, the causality difference in instrument correction method, and the problem of filter coefficient recording. The data time offset range from the instrument response phase error is calculated from one sample point to several seconds using the ambient noise data recorded by multiple seismic stations. With different data delays, the time offset of the noise correlation function is estimated to be 74% to 99% of the data delay time. In addition, the influence of instrument response phase error on the measurement of seismic velocity change is analyzed by using ambient noise data with pole–zero change, and the results show that the abnormal wave velocity with exceeding the standard value is exactly in the time period of the instrument response error, which indicates that the instrument response error affects the study of seismology.     

The anomaly of plasmapause and ionospheric trough positions from DEMETER data

The paper addresses the study of the specific pattern of the subauroral ionosphere marked with the anomalous positions of the plasmapause, the equatorial boundary of the mid-latitude (main) ionospheric trough, and the light-ion trough under quiet solar and geophysical conditions near the magnetospheric shell with the McIlwain parameter L = 3. The anomaly was identified on the base of data of active experiments with the SURA heating facility on October 2, 2007, which were conducted as part of the SURA-International Space Station (SURA-ISS) program in the framework of the DEMETER satellite mission. Joint analysis of the orbital data from DEMETER and ISS, together with the results of the complex ground-based measurements, shows that the revealed effect, which is characteristic of the premidnight sector north of the Moscow-SURA satellite path, is not local. It is observed in a vast territory, extending from the west to the east along the same L-shell, from at least Sweden to Kamchatka. The conclusions suggested by the DEMETER data are supported by analysis of the meridional distributions of the F2-peak plasma frequencies provided by GPS radio probing of the ionosphere. Comparison of these results with the model latitudinal-longitudinal and meridional distributions of the F2-peak plasma density provided by the IRI 2007 and SMI (Russian standard model of the ionosphere) models shows that the model predictions are at odds with the empirical data.     

格尔木地震台大震速报震级差异分析

选取格尔木地震台2000年7月到2013年5月的680个地震,与中国地震台网中心提供的《国家台网大震速报目录》进行面波震级的对比,得出格尔木地震台的台基校正值、标准误差;通过计算分析,总结出不同方位、震中距、震级大小的地震震级差异性之间的关系。     

Effects of patterned Artemisia capillaris on overland flow velocity under simulated rainfall

Vegetation cover is an important factor for erosion control. Laboratory‐simulated rainfall experiments were conducted to quantify the effectiveness of patchy distributed Artemisia capillaris in retarding overland flow velocity. Simulated storms (60, 90, 120, and 150 mm h^?1) were applied on a bare plot (CK) and four different plant patterns, a banded pattern perpendicular to the slope direction (BP), a single long strip parallel to slope direction (LP), small patches distributed like a checkerboard (SP1), and small patches distributed like a letter “X” (SP2). All treatments had three replicates. Each plot underwent two sets of experiments, intact plant plots and root plots (the above‐ground parts were removed, only roots were reserved), respectively. Results showed that flow velocity increased with rainfall intensity, and the lower slope velocity (V_l) was higher than the upper slope velocity (V_u). The removal of grass shoots increased flow velocity. Compared with bare soil plot, intact plants reduced mean flow velocity by 14%–60%, whereas the reduction declined to <40% for the root plots. BP and both SP treatments performed more effectively than LP in retarding flow velocity, whereas no significant differences were identified between BP and SP. The contributions of A. capillaris shoots and roots to the reductions in flow velocity under different rainfall intensities were different. The shoots made greater contribution of 53%–97% at 60 and 90 mm h^–1, and the roots contributed more (51%–81%) at 120 and 150 mm h^–1. Runoff and sediment rate had significant (p < 0.05) linear correlations with mean flow velocity. Copyright © 2012 John Wiley & Sons, Ltd.     

锡林浩特地震台地磁观测干扰分析

对内蒙古自治区锡林浩特地震台2019年10-11月质子矢量磁力仪FHD-2B的数据进行常规分析,发现D分量测值数据出现噪声大、突跳多的异常现象。对该台质子矢量磁力仪受干扰数据进行傅里叶变换和小波分析,确定干扰源范围,排除外界干扰可能。进一步对仪器进行检测,确定干扰原因为参数漂移,在进行参数校正后,观测数据恢复正常。     

Hafnium and neodymium isotopes in seawater and in ferromanganese crusts: The “element perspective”

Hafnium and Nd isotopes are increasingly used as paleoceanographic proxies. Comparing the “mantle–crust array” and the “seawater array” in plots of ε_Hf vs. ε_Nd, it has been observed that for a given ε_Nd value the corresponding ε_Hf value is higher for seawater than it is for terrestrial rocks. While this difference had initially been explained by significant hydrothermal input of mantle Hf into seawater, the currently favoured explanation is incongruent weathering of continental rocks producing radiogenic riverine Hf input.We here address this topic from the perspective of the behaviour of these two elements in seawater and in ferromanganese (Fe–Mn) crusts. We distinguish between a “truly dissolved” and a “dissolved” Hf and Nd pool, the latter being comprised of truly dissolved and colloid-bound (“colloidal”) Hf and Nd. While there exists a hydrothermal pathway for colloid-bound dissolved mantle Hf into the oceans, there is, in marked contrast to Nd, no important riverine pathway for colloidal or truly dissolved continental Hf. Owing to their respective chemical speciation in seawater, there exists truly dissolved Nd in the ocean, while the amount of truly dissolved Hf is insignificant.Neodymium is in exchange equilibrium between local seawater and both, the hydrous Fe and Mn oxides hydrogenetic Fe–Mn crusts are composed of. Due to continuous ad- and desorption there is continuous isotopic re-equilibration and the isotopic composition of Nd in a Fe–Mn crust reflects that of truly dissolved Nd in local ambient seawater. In contrast, Hf is only associated with the hydrous Fe oxides on which it forms surface precipitates that do not exchange with seawater. Due to this lack of isotopic re-equilibration, the isotopic composition of Hf in a Fe–Mn crust is the average of that of all the Hf scavenged during the lifetime of the hydrous Fe oxide particles. Since the Hf-bearing hydrous Fe oxides in a Fe–Mn crust do not form from local ambient seawater at the crust's growth site but are advected as colloids or fine particles, their Hf isotopic composition depends on the origin and migration pathway of these colloids. Hence, while Nd isotopes in Fe–Mn crusts provide reliable information on truly dissolved Nd in local ambient seawater, Hf isotopes rather indicate the origin and pathway of hydrous Fe oxide colloids, and might differ from truly dissolved Hf in local ambient seawater. This may explain the occasional decoupling of Nd and Hf isotopes in Fe–Mn crusts and supports the notion of a significant hydrothermal mantle signal of Hf in seawater.     

Monitoring ionospheric scintillations and TEC at the Polish Polar Station on Spitsbergen: instrumentation and preliminary results

As a partial contribution to the International Polar Year, the Space Research Centre of the Polish Academy of Sciences has installed on the Polish Polar Station, located in the region of the Hornsund Bay in southern Spitsbergen, the equipment intended to monitor ionospheric scintillation and total electron content. We describe the equipment setup and the aims of the experiment. As an illustration of data analysis, we present the reconstruction of irregular structure of the ionosphere from the complex amplitude data, and the estimate of the form and drift of diffraction pattern based on the spaced receiver measurements.     

Monitoring temporal variations in instrument responses in regional broadband seismic network using ambient seismic noise

High‐quality broadband data are required to promote the development of seismology research. Instrument response errors that affect data quality are often difficult to detect from visual waveform inspection alone. Here, we propose a method that uses ambient noise data in the period range of 5?25 s to monitor instrument performance and check data quality in situ. Amplitude information of coda waves and travel time of surface waves extracted from cross‐correlations of ambient noise are used to assess temporal variations in the sensitivity and poles–zeros of instrument responses. The method is based on an analysis of amplitude and phase index parameters calculated from pairwise cross‐correlations of three stations, which provides multiple references for reliable error estimates. Index parameters calculated daily during a two‐year observation period are evaluated to identify stations with instrument response errors in real time. During data processing, initial instrument responses are used in place of available instrument responses to simulate instrument response errors, which are then used to verify our results. The coda waves of noise cross‐correlations help mitigate the effects of a non‐isotropic field and make the amplitude measurements quite stable. Additionally, effects of instrument response errors that experience pole–zero variations on monitoring temporal variations in crustal properties appear statistically significant of velocity perturbation and larger than the standard deviation. Monitoring seismic instrument performance helps eliminate data pollution before analysis begins.     

Measurement of Solar and Cosmic Radiation during Spaceflight

The external platform of the International Space Station (ISS) will provide aunique opportunity for exobiological studies under space conditions, e.g., spacevacuum, solar UV radiation, cosmic radiation, and temperature extremes. In orderto facilitate this research, ESA is developing the EXPOSE facility to be attachedto the External Pallet of the truss structure of the ISS. The experiment is plannedfor a duration of 18 months during the ISS early utilization period. Experimentson the ``Responses of Organisms to the Space Environment (ROSE)' will studythe survival of spores from bacteria, fungi, ferns, etc., under space conditions andthus investigate whether life could potentially survive extended travel through space.In parallel to the exposure of the biological material, the charged particle and solarextraterrestrial radiation will be measured with a multichannel dosimeter. Thisinstrument is based on two separate developments. The dosimeter which determinessolar radiation in four channels is based on the ground-based ELDONET (Europeanlight dosimeter network) instrument which has been developed to measure solar UVand visible radiation in three channels. The other part of the instrument is based on a256 channel dosimeter of cosmic radiation which was developed and used on theRussian space station Mir. A final miniature prototype has been developed to fit thespace available. The instrument incorporates the latest technologies such as SurfaceMounted Devices (SMDs), switchable amplifiers, as well as on-board microprocessorcontrol. Several software packages have been developed to record the solar visible andUV, and charged particle, radiations, to display them in graphical form and to store them for future analysis. All recorded data will be made available to the public via the Internet.     

Auroral activity caused by high-power radioemission from the SURA facility

A series of experimental modifications of the ionosphere in the HF range, performed at the SURA facility base, together with optical measurements onboard the International Space Station (ISS), indicated that such impacts on the ionosphere are effective when the facility operational frequency is higher than the critical plasma frequency (for the main ionospheric F2 layer). The experimental measurements were supported by measurements at ground-based observatories, ISS, and the Demeter and GPS satellites. The analysis results of the entire data set are presented. The ray HF radio tracing for the experiment of October 2, 2007, has been calculated, and it has been indicated that the ionosphere to the north of the facility up to 60°–62° N latitudes was irradiated by the facility beam (the effects of ray redistribution and refocusing) due to refraction on the gradient of the F2 layer critical frequencies. An analysis of the ground-based and satellite measurements (both in the vicinity of a heater and in the magnetically conjugate region) indicates that it is possible to trigger a substorm in experiments with the Sura heating facility.     

Frequency–Wavenumber (FK)-Based Data Selection in High-Frequency Passive Surface Wave Survey

Passive surface wave methods have gained much attention from geophysical and civil engineering communities because of the limited application of traditional seismic surveys in highly populated urban areas. Considering that they can provide high-frequency phase velocity information up to several tens of Hz, the active surface wave survey would be omitted and the amount of field work could be dramatically reduced. However, the measured dispersion energy image in the passive surface wave survey would usually be polluted by a type of “crossed” artifacts at high frequencies. It is common in the bidirectional noise distribution case with a linear receiver array deployed along roads or railways. We review several frequently used passive surface wave methods and derive the underlying physics for the existence of the “crossed” artifacts. We prove that the “crossed” artifacts would cross the true surface wave energy at fixed points in the f–v domain and propose a FK-based data selection technique to attenuate the artifacts in order to retrieve the high-frequency information. Numerical tests further demonstrate the existence of the “crossed” artifacts and indicate that the well-known wave field separation method, FK filter, does not work for the selection of directional noise data. Real-world applications manifest the feasibility of the proposed FK-based technique to improve passive surface wave methods by a priori data selection. Finally, we discuss the applicability of our approach.     

Middle Mantle Seismic Structure of the African Superplume

I present the results of statistical hypothesis testing of Grand’s (2002) global tomography model of three-dimensional shear velocity variations for the middle mantle underneath eastern and southern Africa. I apply an F test to evaluate the validity of a model where a tilted, slow-velocity anomaly in the deepest mantle under southern Africa, known as the African superplume, is continuous with a slow-velocity anomaly in the upper mantle under eastern Africa. This null hypothesis is tested against alternative hypotheses, in which various “obstruction volumes” in the middle mantle are constrained to zero perturbation from the one-dimensional reference velocity during the tomographic inversion. I find that there is an equal probability of accepting an alternative hypothesis with a thin “obstruction volume” at 850–1,000 km depth, whereas volumes at other depths are rejected. But the alternative hypothesis, where a connection is forced at 850–1,000 km depth, is rejected. I conclude that the African superplume rises to at least 1,150 km depth, and that the upper mantle slow-velocity anomaly continues from the surface to below the mantle transition zone. I interpret the “obstruction volume” as a weakening of the superplume in the middle mantle.     

Effects of geomagnetic disturbances in daytime variations of the atmospheric electric field in polar regions

We have studied the influence of daytime polar substorms (geomagnetic bays under the IMF Bz > 0) on variations of the vertical gradient of the atmospheric electric field potential (Ez) observed at the Polish Hornsund Station (Svalbard, Norway). Only the observations of Ez under “fair weather” conditions were used, i.e. in the absence of strong wind, precipitations, low cloud cover, etc. We studied more than 20 events of daytime polar substorms registered by the Scandinavian chain of IMAGE magnetometers in 2010–2014 during the “fair weather” periods at the Hornsund Station. Analysis of the observations showed that Ez significantly deviates from the its background variations during daytime, as a rule, when the Hornsund Station is in the region of projection of the daytime auroral oval, the position of which was determined from OVATION data. It was shown that the development of a daytime polar substorm leads to fluctuating enhance of Ez values. It was found that Ez surges are accompanied by intensification of field-aligned electric currents outflowing from the ionosphere, which were calculated from the data of low-orbit communication satellites of the AMPERE project.     

Cluster探测到磁尾等离子体注入的特征

利用Cluster卫星2001～2004年磁尾运行期间RAPID仪器的数据,确定了115例磁尾等离子体注入事件,借助时序叠加法统计研究磁尾等离子体注入现象的特征.注入事件主要分布于磁地方时夜晚20时至凌晨04时.与同步轨道区观测到的粒子注入事件类似,可以将磁尾粒子注入事件分成五类：（1）只有离子注入;（2）离子先于电子注入;（3）离子和电子同时注入;（4）电子先于离子注入;（5）只有电子注入.磁尾粒子注入时,质子（能量范围0～40 keV）的温度和数密度同时显著增加,沿地球径向的传播速度也明显增大.统计分析磁尾注入期间同时观测到的晨昏对流电场,发现电场可分为两类：（A）注入后电场突然增大,电场强度为正;（B）注入后电场突然增大,电场强度为负.利用磁层磁场（T89c）和电场（Volland-Stern）模型模拟粒子注入后赤道面的电漂移速度矢量,模拟结果与统计结果基本一致,表明晨昏对流电场引起的电漂移是驱动磁尾（-18R_EE）等离子体沿地球径向注入的机制之一.     

大别-苏鲁及其邻近地区基于背景噪声的瑞雷波和勒夫波相速度层析成像

采用与作者2014年发表的“大别-苏鲁及其邻近地区基于背景噪声的勒夫波群速度成像”文章相同的资料,用频时分析提取5 000余条瑞雷波和4 000余条勒夫波相速度频散曲线,反演得到了8—32 s的瑞雷波和勒夫波相速度分布图像．结果显示,瑞雷波与勒夫波相速度分布具有很好的一致性．8 s的相速度分布与地表构造特征相吻合,造山带与隆起区均表现为高速,盆地因其规模不同而显示不同程度的低速．随着周期的增大,大别 苏鲁的高速带由强变弱,但始终存在．16—24 s的高速可能主要受到中地壳高速的控制,而32 s的高速则可能与上地幔顶部的高速有关．比较大别造山带与苏鲁造山带的平均频散曲线,发现大别造山带和苏鲁造山带的勒夫波频散曲线均高于AK135模型计算的理论频散曲线,而瑞雷波则没有这一现象. 这可能意味着两个地区有比较强烈的径向各向异性．      

On the dispersion of two coexisting nongyrotropic ion species

Space observations in the solar wind and simulations of high Mach number bow-shocks have detected particle populations with two coexisting nongyrotropic ion species. We investigate the influence of these two sources of free energy on the stability of parallel (with respect to the ambient magnetic field) and perpendicular propagation. For parallel modes, we derive their dispersion equation in a magnetoplasma with protons and alpha particles that may exhibit stationary nongyrotropy (SNG) and discuss the characteristics of its solutions. Kinetic simulations study the behaviour of perpendicular electrostatic (Bernstein-like) waves in a plasma whose ion populations (positrons and fictitious singly-charged particles with twice the electron mass, for the sake of simulation feasability) can be time-varying nongyrotropic (TNG). The results show that the coexistence of two gyrophase bunched species does not significantly enhance the parallel SNG instability already found for media with only one nongyrotropic species, whereas it strongly intensifies the growth of Bernstein-like modes in TNG plasmas.