Specific features of resonance structures in spectra of ULF electromagnetic noise at high latitudes (Barentsburg observatory)

Results of studying resonance spectral structures (RSSs) of the noise background in the frequency range 0.1–5 Hz at Barentsburg high-latitude observatory (Spitsbergen) are presented. It has been indicated that the diurnal variations in the observation probability and the long-term variations in the RSS observation cannot be explained by variations in the illumination of the ionosphere by the Sun. Based on an analysis of the magnetic measurements, the conclusion is drawn that a decrease in the RSS registration probability in daytime hours and in winter months are related to an increase in the variability of ionospheric parameters. It has been indicated that the RSS source is located in the region above an observation point, and the horizontal scale of this source is 100–200 km, which agrees with the concepts of the role of the ionospheric Alfvén resonator in the formation of RSS.     

Specific features of the technique of airborne gravity surveys at high latitudes

Specific features of the technique of airborne gravity surveys in the Arctic are considered. The main requirements for aerogravimetric complexes used for such surveys and for reference GPS stations located at high latitudes are formulated, as well as the conditions necessary for converting an aircraft into an aircraft-laboratory. Specific features of the planning of measurements of aerogravimetric profiles in the Arctic are described. The need in the operations necessary for the rapid estimation of the results is substantiated. The obtained regional gravimetric map on a 1:200000 scale is presented as an example.     

Ozone distribution in winter at high latitudes in the northern hemisphere

The ozone winter maximum at high latitudes in the northern hemisphere is not evenly distributed along the longitudes. This is mainly due to the upper air circulation, both horizontally and vertically. In addition it is also strongly influenced by the largest mountain ranges. During the last two decades the air circulation in the North Atlantic has intensified. This has led to ascending motion in the upper troposphere and the lower stratosphere, which in turn has resulted in a reduced total ozone column in Northwest Europe.The large mounter ranges in Asia are initiating standing waves, with descending motions in the atmosphere behind the mountains. The descending motion leads to adiabatic warming of the lower stratosphere and the upper troposphere. Ozone-rich air is transported downwards to lower levels and stored there, where the ozone is less affected by heterogeneous chemical destruction.     

电离层舒曼共振的空间观测

通过功率谱分析和波阻抗函数计算,本文证实了Aureol-3卫星在电离层高度上(>600km)观测到的极低频(ELF)波场扰动是和舒曼共振相关的电磁振荡.与舒曼共振地面观测相比较,Aureol-3观测到的舒曼共振电场分量具有很好的谐振谱结构,峰值频率和各阶舒曼共振本征频率对应;磁场分量的高阶峰值频率偏离14, 20, 26Hz等舒曼共振本征频率;随着卫星高度的改变,电场与磁场谐振的一阶最大能量峰值并不会发生在同一频率,结合本文分析的数据,分别位于7.8Hz和10Hz;水平方向的磁场分量更接近南北方向的线极化而不是地球－电离层空腔中的椭圆极化;波阻抗随频率表现出不太规则的准正弦振荡,它会随着频率增加和飞行高度上升呈现减小的趋势.虽然舒曼共振信号和电离层密度梯度间的非线性作用可以解释舒曼共振空间观测的部分特征,但需加入其他机制,如电离层不稳定性,传播模式的耦合,进一步了解电离层高度上舒曼共振各种特征产生的原因.     

Estimating external magnetic field differences at high geomagnetic latitudes from a single station

Providing an accurate estimate of the magnetic field on the Earth's surface at a location distant from an observatory has useful scientific and commercial applications, such as in repeat station data reduction, space weather nowcasting or aeromagnetic surveying. While the correlation of measurements between nearby magnetic observatories at low and mid‐latitudes is good, at high geomagnetic latitudes () the external field differences between observatories increase rapidly with distance, even during relatively low magnetic activity. Thus, it is of interest to describe how the differences (or errors) in external magnetic field extrapolation from a single observatory grow with distance from its location. These differences are modulated by local time, seasonal and solar cycle variations, as well as geomagnetic activity, giving a complex temporal and spatial relationship. A straightforward way to describe the differences are via confidence intervals for the extrapolated values with respect to distance. To compute the confidence intervals associated with extrapolation of the external field at varying distances from an observatory, we used 695 station‐years of overlapping minute‐mean data from 37 observatories and variometers at high latitudes from which we removed the main and crustal fields to isolate unmodelled signals. From this data set, the pairwise differences were analysed to quantify the variation during a range of time epochs and separation distances. We estimate the 68.3%, 95.4% and 99.7% confidence levels (equivalent to the 1σ, 2σ and 3σ Gaussian error bounds) from these differences for all components. We find that there is always a small non‐zero bias that we ascribe to instrumentation and local crustal field induction effects. The computed confidence intervals are typically twice as large in the north–south direction compared to the east‐west direction and smaller during the solstice months compared to the equinoxes.     

Variations in the near-surface atmospheric electric field at high latitudes and ionospheric potential during geomagnetic perturbations

We have analyzed variations in the near-surface atmospheric electric field (Ez) normalized to their daily averages that were simultaneously observed in different high-latitude regions at moderate geomagnetic activity (Kp ∼ 3). The Ez data were measured under fair weather conditions at the Vostok Antarctic research station (Φ′ = −83.5°) in the southern polar cap and at the Hornsund Arctic observatory (Φ′ = 74.0°) on Svalbard close to the polar boundary of the auroral oval in the Northern Hemisphere. It is established that variations in the atmospheric electric field in the polar cap region at the Vostok station are controlled (the correlation coefficient R ∼ 0.7–0.9) by variations in the overhead ionospheric potential. The situation at the Hornsund observatory is more complicated. During intervals when Hornsund occurred below the westward electrojet, the correlation was typically positive with R ∼ 0.60–0.85; however, while this observatory was in the region of the eastern electrojet, the correlation could be negative with R ∼ 0.7–0.8. Normally, during such periods, the westward electrojet was detected polarwards of Hornsund while, according to the SuperDARN radar data, the observatory was located below the negative vortex of the polar ionospheric convection.     

Studies on the shift in the frequency of the first Schumann resonance mode during a solar proton event

The variation of the first Schumann resonance (SR) frequency spectra observed from the recorded data over Kolkata (22.56°N, 88.5°E) during a solar proton event (SPE) on July 14, 2000 has been presented. It shows increase in frequency during X-ray bursts and decrease during the period of occurrence of an SPE. The results from some other locations for the same event are also reported. The severe X-ray bursts recorded just before the proton event exhibit enhancement in frequency of the first mode due to enhancement of ionization in the D-region of the ionosphere. Some attempts are made to explain the observed variation during solar proton events in terms of the perturbations within the Earth–ionosphere waveguide on the basis of two-layer-model.     

Changes of the first Schumann resonance frequency during relativistic solar proton precipitation in the 6 November 1997 event

The variations of the first mode of Schumann resonance are analyzed using data from Kola peninsula stations during the solar proton event of 6 November 1997. On this day the intensive flux of energetic protons on GOES-8 and the 10% increase of the count rate of the neutron monitor in Apatity between 1220 and 2000 UT were preceded by a solar X-ray burst at 1155 UT. This burst was accompanied by a simultaneous increase of the Schumann frequency by 3.5%, and the relativistic proton flux increase was accompanied by 1% frequency decrease. These effects are explained by changes of the height and dielectric permeability of the Earth-ionosphere cavity.     

Wave structure of magnetic substorms at high latitudes

A new type of high-latitude magnetic bays is revealed at geomagnetic latitudes higher than 71°, called ??polar substorms.?? It is shown that polar substorms differ from both classical substorms and high-latitude geomagnetic disturbances of the type of polar boundary intensifications (PBIs). While classical substorms start at latitudes below 67° and then expand poleward, polar substorms start almost simultaneously in the evening-night polar region of the oval. In contrast to PBIs, accompanied by auroral streamers expanding southward, polar substorms are accompanied by auroral arcs quickly traveling northward. It is shown that polar substorms are observed before midnight (20?C22 MLT) under weak geomagnetic activity (Kp ?? 2) during the late recovery phase of a magnetic storm. It is shown that a typical feature of polar substorms is the simultaneous excitation of highly intensive Pi2 and Pi3 geomagnetic pulsations at high latitudes, which exceed the typical amplitude of these pulsations at auroral latitudes by more than an order of magnitude. The duration of pulsations is determined by the substorm duration, and their amplitude decreases sharply at geomagnetic latitudes below ??71°. It is suggested that pulsations reflect fluctuations in ionospheric currents connected with polar substorms.     

基于双通道参考技术的磁共振地下水探测电磁噪声分布规律研究

磁共振地下水探测（MRS）技术能直接测量水中氢质子的拉莫尔进动,可表征地下水含水量和介质孔隙度等信息,因而获得了广泛应用.然而,MRS信号微弱,实际探测中电磁噪声强度大,若噪声未被有效去除,会导致后续水文参数解释不准确,采用"8"字形线圈或运用带参考线圈的噪声抵消等方法可抑制噪声干扰,但这些方法消噪效果的好坏取决于电磁噪声的空间分布.另外,在强噪声或多噪声源环境下应用MRS方法,选择恰当的探测位置对提高探测效率以及增加探测结果的有效性具有重要意义.因此,本文使用双通道噪声采集装置在实验前记录环境中的电磁噪声,通过双通道参考技术推导同一时刻不同测点的噪声强度,再得到测区电磁噪声分布结果,该噪声分布情况对MRS方法探测方式的选取具有指导性的作用.同时,可根据噪声分布情况为探测线圈选择最佳测点以避开噪声强度大的区域,进而提高采集信号的信噪比.最后,通过采集室外环境下的电磁噪声,验证本文提出的基于双通道参考技术的电磁噪声分布规律研究方法的有效性,为可靠高效地开展磁共振地下水探测提供了新的技术手段.     

Regional deep sounding at high latitudes of the Northern Hemisphere

Methods were developed for carrying out spherical harmonical analysis of disturbed diurnal variations over a limited territory at high latitudes of the Northern Hemisphere. The ratios ?_n^{|staggered||m} of the external and internal parts of the magnetic potential were evaluated for spherical harmonics 4 ? n ? 12. Different from generally accepted ?_n^{|staggered||m} values for S_q variations (2 ÷ 3), they proved to be considerably higher and attained 10 for n ? 8. This result testifies to a small contribution of induced fields into disturbed diurnal variations observed at high latitudes.A deep sounding was carried out from spherical analysis data. It is shown that the mean distribution of the electrical conductivity at high latitudes σ(r) is the same as the global mean distribution for the whole Earth and that, consequently, it may serve as a reference for local soundings.     

低纬地区地磁场谐波特征

利用我国低纬地区琼中、广州和泉州３个地磁台１９８２－１９８９年的地磁静日实测值，分析了Ｈ、Ｚ和Ｙ三个地磁分量的谐波特征。结果表明，①Ｈ、Ｚ两分量的谐波振幅一般在春秋季最大，冬季最小，Ｙ分量的谐波振幅在夏季最大，冬季最小，它们的变化形态基本上与太阳黑子数的变化形态同步；②三分量的相位都受到太阳活动和季节的影响。     

Statistical characteristics of the spatial distribution of Pc3-4 geomagnetic pulsations at high latitudes in the antarctic regions

The diumal variations in the parameters of Pc3 (20–60 mHz) and Pc4 (10–19 mHz) pulsations at latitudes of the dayside cusp and polar cap have been studied using data of the magnetic stations of the trans-Antarctic meridional profile for the time interval from January to March 1997 (local summer) under weakly disturbed geomagnetic conditions (AE ≤ 250 nT). The technique for estimating pulsation parameters is based on the separation of the wave packets and noise. The diumal variations in the hourly average parameters of the wave packets in the Pc3 and Pc4 bands and noise in the Pc3-4 band (10–60 mHz)—the average number of wave packets, energy of wave packets and noise, and energy of a single wave packet—turned out to be different for the stations located deep in the polar cap (Φ ～ 87°) and at the latitudes of the dayside polar cusp (Φ ～ 70°) and auroral oval (Φ ～ 66°). Several sources of pulsations caused by different channels of wave energy penetration into the magnetosphere through the dayside cusp, dayside magnetopause, and dawn flank of the magnetotail apparently exist at high latitudes.     

地电阻率场地噪声水平探讨

地电阻率场地的噪声水平将直接决定地电阻率的观测精度,决定观测结果是否适合地震预报的需要。本文主要分析了早年规范和现行标准对场地噪声定义的一些问题,试图给出一个比较适合观测的场地噪声水平。     

Relation between the near-Earth electric field at high latitudes and intense cloud-to-ground lightning strokes initiating VLF emission bursts

Since 1998, the gradient of the near-Earth atmospheric electric field potential (|Ez|) has been continuously registered at Vostok Antarctic station within the scope of the joint Russian-Australian project. The data of the continuous 10-day period of fine weather in April 1998 have been selected for the following analysis. The field |Ez| behavior at Vostok station was compared with a number of lightning strokes obtained from data of the ground-based network of electromagnetic measurements. It has been found out that the average hourly values of |Ez| evidently negatively correlate with the number of intense lightning strokes. The causes of these relations are discussed. The obtained results are interpreted based on a theory of global electric circuit.     

Particle-precipitation fine structure during the development of substorms at high latitudes

The energy of precipitating particles that cause auroras can be characterized by the ratio of different atom and molecule emissions in the upper atmospheric layers. It is known that the spectrum of precipitating electrons becomes harder when substorms develop. The ratio of the I ₆₃₀₀ red line to the I ₅₅₇₇ green line was used to determine the precipitating-electron spectrum hardness. The I ₆₃₀₀/I ₅₅₇₇ parameter was used to roughly estimate the electron energy in auroral arcs observed in different zones of the auroral bulge at the bulge poleward edge and within this bulge. The variations in the emission red and green lines in auroral arcs during substorms that occurred in the winter season 2007–2008 and in January 2006 were analyzed based on the zenith photometer and all-sky camera data at the Barentsburg and Longyearbyen (LYR) high-latitude observatories. It has been indicated that the average value of the I ₆₃₀₀/I ₅₅₇₇ emission ratio for arcs within the auroral bulge is larger than this value at the bulge poleward edge. This means that the highest-energy electron precipitation is observed in arcs at the poleward edge of the substorm auroral bulge.     

Variations in the near-ground electric field at high latitudes and the potential drop across the polar cap during morning polar substorms

This paper studies time variations in the near-ground atmospheric electric field (Ez) at the geomagnetic latitude of 74° (Hornsund observatory) during polar substorms. Ez variations are compared with those in the potential drop across the polar cap (Up), according to SuperDARN radar observations. It is found that in the morning sector, time variations in Ez are strongly driven by time variations in the electrojet and almost do not depend on time variations in Up, which is presumably due to the penetration of the electric field of the electrojet into tropospheric altitudes.     

Ground images at high latitudes of ULF wave processes in the outer magnetosphere

Magnetohydrodynamic (MHD) wave processes in space and laboratory plasmas make possible the resonant conversion of global compressional modes into localized Alfven oscillations. In this way MHD signals excited by local non-steady processes in space can convey information to the ground. This review focuses on long-period ultra-low-frequency (ULF) phenomena specific to high latitudes, including those in the cusp region, nightside oval, and polar cap, and highlights several unresolved problems related to the physics of ULF waves at these latitudes. Several new results are highlighted: (1) In the high-altitude cusp an MHD waveguide/resonator without conductive boundaries could occur, where hydromagnetic turbulence penetrating from the magnetosheath can accumulate and transform into Alfven waves escaping along the field lines. (2) Broadband long-period wave activity near the cusp exhibits temporal structure that is as yet unexplained, as well as frequent long intervals of non-conjugate levels of wave power. (3) Occurrence of field-aligned potential drops in the auroral acceleration region may cause severe damping of Pc5/Pi2 waves as compared with ionospheric Joule dissipation or dispersive leakage. (4) Specific long-period irregular variations in the nightside polar cap of still unknown origin exist, which might be related to the flapping of the magnetotail or turbulent trans-polar convection. Finally, we point out what remains unknown, to what extent our current knowledge is incomplete, what observations have been rare and what future research could focus on.     

Laboratory analogue modelling of the Schumann Resonance source field

An oscillating vertical line current source was used in the laboratory to simulate the naturally occurring 8 Hz Schumann Resonance field due to lightning strokes. Vertical to horizontal magnetic field ratios (coast effect), and magnetotelluric ratios, for traverses over a laboratory model of the Assistance Bay region of Cornwallis Island (N.W.T. Canada) for the vertical line current source field are compared with those for an overhead uniform source field. The E- and H-polarizations of the source fields used are defined as the cases of the horizontal magnetic field of the source roughly perpendicular and parallel to the general coastline, respectively.The magnetic field ratios are essentially identical for the two field sources for E-polarization, but differ by a factor of approximately two for H-polarization, with the ratios greater for the overhead source than for the vertical line current source. The magnetotelluric ratios are found to be identical for the two field sources of each of the E- and H-polarizations.     

Irregular structures of the F layer at high latitudes during ionospheric heating

Results on heating the ionospheric F region above Tromsø, Norway are presented. The ionosphere was monitored by satellite tomography and amplitude scintillation methods as well as the EISCAT incoherent scatter radar. No effect of heating was observed in the daytime. In the evening and in the pre-midnight sector, noticeable tilts of the F region were observed during heating periods. The tilts overlapped the heating cone, where the electron density decreased and irregularities exceeding 10 km in size appeared. Between the heating periods the F layer was restored to its horizontal shape. The anisotropic parameters of small-scale irregularities with scale lengths of hundreds of metres were also determined. It was found that the perpendicular anisotropy points in the direction of F region plasma flow. In some cases the results can be explained by assuming that the small-scale irregularities were generated within the heating cone and drifted out of the heating region where they were subsequently observed.