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1.
Examples of long period Pc5 magnetic field pulsations near field-aligned current (FAC) regions in the high-latitude magnetosphere, observed by INTERBALL-Auroral satellite during January 11, April 11 and June 28, 1997 are shown. Identification of corresponding magnetosphere regions and subregions is provided by electrons and protons in the energy-range of 0.01–100 keV measured simultaneously onboard the spacecraft. The examined Pc5 pulsations reveal a compressional character. A fairly good correlation is demonstrated between these ULF Pc5 waves and the consecutive injection of magnetosheath low energy protons. The ULF Pc5 wave occurrence is observed in both upward and downward FACs.  相似文献   

2.
The dynamics of the system of field-aligned currents (FACs) and closing ionospheric Pedersen currents is considered with the use of original processing methods and the data from four substorms of different types. The total current system comprises of two parts. One is the well-known substorm current wedge (SCW) system, in which the zonal (westward ) current closes FACs in the R1 zone (region). The component 2 consists of two pairs of meridional currents flowing equatorward and poleward in the R1 region and creating regions R0 and R2 (according to the classification of Iijima and Potemra). It is shown that the total FAC of the disturbed magnetosphere–ionosphere system is dominated by the contribution of component 2, which contradicts the original version of the SCW model but is consistent with new data. The quantitative characteristics of the dawn–dusk asymmetry are determined for the FAC distribution in the ionosphere for each substorm. It is shown that the ratio of the average intensities of FACs in the regions R0 and R2 was IR0/IR2 ≥ 0.4, which contradicts the popular opinion that there are no FACs in the polar cap. In addition, a relatively rare event of the simultaneous start of the substorm explosive phase and the SSC caused by the dynamic impact of the solar wind when the polar cap expands (rather than compresses as usual) is considered.  相似文献   

3.
We present an analysis of two crossings of the plasma sheet boundary layer (PSBL) in Earth’s magnetotail by a quartet of Cluster satellites, accomplished both in the absolutely quiet geomagnetic interval and in the restoration period between two substorms. It is found that in the quiet period, field-aligned electric currents were not observed in the PSBL, in spite of presence in this region of high-velocity ions moving toward Earth along the magnetic field lines at velocities of ∼1400 km/s. This means that in the given event, ions and electrons moved together; i.e., high-velocity ions were a part of the accelerated plasma flow. In the disturbed period, the Cluster satellites detected a system of two oppositely directed field-aligned currents, which can be associated with the presence of the X line rather close to the Earth. Owing to multipoint Cluster observations, we managed to estimate the spatial size of a current structure (along the normal to the PSBL surface), which was equal, for a current flowing toward Earth along the high-latitude PSBL boundary, to about 1600 km, which is comparable with the value of the inertial radius of ions. This agrees with the scenario of spatial separation of charges and formation of the Hall system of currents in the magnetic reconnection region. The duration of observation of the given current structure was ∼12 min., which points to the quasi-steady-state character of reconnection.  相似文献   

4.
地球极区电离层对行星际激波的响应   总被引：1，自引：0，他引：1       下载免费PDF全文
本文就地球电离层对行星际激波的动力学响应进行三维全球数值模拟研究．背景行星际磁场为螺旋场，南北分量为零；初始电离层由Ⅰ区场向电流和相应的晨昏电场所主导；行星际激波沿日地连线方向撞击地球．模拟结果表明，在激波的作用下，电离层Ⅰ区电流系统向子夜方向运动，在向阳侧相继出现与原Ⅰ区电流反向的异常场向电流对和同向的新生Ⅰ区电流对．该异常场向电流对在极盖区形成瞬间昏晨电场，尾随原Ⅰ区电流向夜侧方向漂移直至湮没．与此同时，新生的Ⅰ区电流不断增强并向夜侧和赤道方向延伸，最终取代原Ⅰ区电流，相应极盖区又恢复到原来的晨昏电场状态．这一响应过程和行星际激波强度有关：激波强度越强，新生的Ⅰ区场向电流也越强，它向赤道方向延伸的距离也越大，能到达的纬度也越低．上述结果在趋势上与观测到的输运对流涡旋和亚极光块的运动特征一致．  相似文献   

5.
We have investigated ion outflows observed by the Akebono satellite and the EISCAT radar in the nightside auroral region on February 16, 1993. The Akebono satellite at about 7000 km altitude observed the region of suprathermal ion outflows and inverted-V type electron precipitation alternately with a horizontal separation of 70–150 km at the ionospheric level. These two regions corresponded to the upward and downward field-aligned current region, respectively, and intense ELF waves were observed in the ion outflow region. From the EISCAT VHF radar observation (Common Program 7 mode), it has been suggested that the ion outflow region and the enhanced electron temperature region were aligned along geomagnetic field lines with vertical and horizontal separations of 200–400 and 70–80 km, respectively and these two regions convected equatorward across the EISCAT radar at Tromsø site. Based on these results, we propose a model for this ion outflow as follows. In the nightside auroral region, downward FAC regions exist near the edge of the inverted-V type electron precipitation regions. ELF waves are excited probably by a plasma instability due to the upward thermal electron beam carrying the downward FACs, and these ELF waves cause transverse ion heating at the top of the ionosphere. The produced ion conics contribute significantly to ion outflow.  相似文献   

6.
Observations made with the co-located Rayleigh lidar and MST radar systems at Aberystwyth (52.4°N, 4.1°W) in Wales and radiosondes from Valentia (51.9°N, 10.2°W) in Eire are used to investigate the changes in the vertical propagation of gravity waves during periods of 4 days in June 1995 and February 1993. In each month, the lidar observations show that the wave activity in the upper stratosphere and lower mesosphere changes between two pairs of days. The radar and radiosonde measurements indicate that mountain waves make no contribution to the changes in intensity. Instead, the changes seem to arise largely from the presence or absence of long-period waves with vertical wavelengths near 8 and 10 km in June and February, respectively. The influence of such waves on the vertical wavenumber spectra is examined and related to the evidence for convective instabilities provided by the temperature profiles.  相似文献   

7.
We investigate the inclinations of heliospheric current sheet at two sites in interplanetary space, which are generated from the same solar source. From the data of solar wind magnetic fields observed at Venus (0.72 AU) and Earth (1 AU) during December 1978–May 1982 including the solar maximum of 1981, 54 pairs of candidate sector boundary crossings are picked out, of which 16 pairs are identified as sector boundaries. Of the remainder, 12 pairs are transient structures both at Venus and Earth, and 14 pairs are sector boundaries at one site and have transient structures at the other site. It implies that transient structures were often ejected from the coronal streamer belt around the solar maximum. For the 16 pairs of selected sector boundaries, we determine their normals by using minimum variance analysis. It is found that most of the normal azimuthal angles are distributed between the radial direction and the direction perpendicular to the spiral direction both at Venus and Earth. The normal elevations tend to be smaller than ≈45° with respect to the solar equatorial plane, indicating high inclinations of the heliospheric current sheet, in particular at Earth. The larger scatter in the azimuth and elevation of normals at Venus than at Earth suggests stronger effects of the small-scale structures on the current sheet at 0.72 AU than at 1 AU. When the longitude difference between Venus and Earth is small (<40° longitudinally), similar or the same inclinations are generally observed, especially for the sector boundaries without small-scale structures. This implies that the heliospheric current sheet inclination tends to be maintained during propagation of the solar wind from 0.72 AU to 1 AU. Detailed case studies reveal that the dynamic nature of helmet streamers causes variations of the sector boundary structure.  相似文献   

8.
Layered Velocity Models of the Western Bohemia Region   总被引：1，自引：0，他引：1  
A new robust and effective optimization algorithm – isometric algorithm – was used for the inversion of layered velocity models, with constant gradient in each layer, to find suitable 1-D models for the location of microearthquakes in the individual four subregions of the West Bohemian earthquake swarm region. Models which are considered as optimal yield the minimum sum of the absolute values of the travel-time residua in locating the whole group of earthquakes in the given subregion. The results obtained from the inversion of P and S waves and from P waves only are shown. For comparison, optimum homogeneous models derived by the grid search method, again using both P and S waves and P waves only, are given. The computations indicate that the models for the individual subregions differ from each other. For layered models the differences are more pronounced, as expected, in the upper parts, down to depths of about 5 km. In comparison with the subregions Nový Kostel and Plesná, the P and S wave velocities for subregion Lazy are relatively higher and the P and S velocities for subregion Klingenthal relatively lower. In the lower parts the differences are smaller and the velocities have practically identical gradients. The highest velocities were obtained for subregion Lazy and the lowest velocities for subregion Klingenthal, as well for the homogeneous models. The model that represents the whole swarm region was determined in a similar way. This model is compared with the previously published velocity-depth distribution, obtained from DSS profile VI/70 in the vicinity of the area under study.  相似文献   

9.
The up-down asymmetry of the type I plasma waves in the equatorial electrojet region during daytime is investigated. It is shown that the asymmetry is exhibited not only in the vertical direction but also is manifested in the oblique direction (24.6° off zenith). The results further indicate the effect of neutral winds on the phase velocities of type I plasma waves.  相似文献   

10.
The maps of the field-aligned current (FAC) density distribution in the ionosphere obtained by the TIM-2 magnetogram inversion technique are used to investigate the August 27, 2001 substorm. The open magnetic flux Ψ and intensity J of the substorm current wedge (SCW) have been determined with a step of 1–5 min. The substorm onsets are divided into two types, PSR (plasma sheet reconnection) and TLR (tail lobe reconnection). The fast expansion tailward of the PSR region is described as the transition from PSR to PSR+TLR. Assuming that the SCW FACs flow down into the ionosphere from the edges of the disruption region of the cross-tail dawn-dusk current, several parameters of the disruption region have been estimated. The disrupted magnetic field has been found to be ∼5% of the undisrupted one for PSR and ∼95% for PSR+TLR. The disturbance power Q for PSR is an order of magnitude lower than that for PSR+TLR. The abrupt growth of Q during the transition from PSR to PSR+TLR is observed over the entire SCW area from its near-Earth part to the midtail and distant tail.  相似文献   

11.
Numerical calculations of the thermospheric and ionospheric parameters above EISCAT are presented for quiet geomagnetic conditions in summer. The Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) was used. The numerical results were obtained both with a self-consistent calculation of the electric fields of magnetospheric and dynamo-action origin and with the magnetospheric electric fields only. It was found that the dynamo-electric field has some effect on the ionospheric convection pattern during quiet geomagnetic conditions. It has a marked effect mainly on the zonal neutral wind component above EISCAT (±20m/s at 140 km altitude). We have studied the effects of various field-aligned current (FAC) distributions on thermosphere/ionosphere parameters and we show that a qualitative agreement can be obtained with region-I and -II FAC zones at 75° and 65° geomagnetic latitude, respectively. The maximum FAC intensities have been assumed at 03–21 MLT for both regions with peak values of 2.5 × 10–7 Am–2 (region I) and 1.25 × 10–7 A m–2 (region II). These results are in agreement with statistical potential distribution and FAC models constructed by use of EISCAT data. The lack of decreased electron density in the night-time sector as observed by the EISCAT radar was found to be due to the spatial distribution of ionospheric convection resulting from electric fields of magnetospheric origin.  相似文献   

12.
Frequency-time analysis was applied to records of Rayleigh surface waves due to teleseismic earthquakes (M ≥ 6.0) to obtain dispersion curves of fundamental mode phase velocities for paths between 51 pairs of digital seismic stations in the Asian IRIS networks; the range of periods was 10–200 s. For each of these pairs we derived 1D-shear-velocity sections that fit the corresponding dispersion curves and image the integral earth structure down to depths of ∼650–700 km beneath each of the interstation seismic paths, which traverse the area of study in different directions. These structures were used by Yanovskaya (2001) to derive a 3D-shear-velocity model by 2D tomography, imaging the distribution of the larger horizontal inhomogeneities in the Central Asia mantle for depths of 50 to 600 km, and to construct 2D-velocity sections for five lines passing through the major tectonic features of Central Asia.  相似文献   

13.
In this work we examine simultaneous observations from the two geosynchronous satellites GOES-5 and GOES-6 located at 282°E and 265°E respectively, and from middle and low latitude ground observatories located within 250°E and 294°E geographic longitude, during isolated substorms of moderate activity. The spatial distribution of our observation points allows us to make a detailed study of the azimuthal expansion of the substorm current wedge. The data analysis shows evidence that the substorm initiation and development mechanism include the cross-tail current diversion/disruption, the substorm current wedge formation and the azimuthal expansion of the inner plasma sheet. The triggering mechanism is initially confined in a longitudinally narrow sector, estimated to be less than 15° and located very close to local midnight to the east or to the west. The current disruption region expands both eastward and westward in the magnetotail, so that the location of major field-aligned currents flowing into the ionosphere shifts successively eastward, and the location of the currents flowing out of the ionosphere shifts successively westward. Evidence was found that the perturbation travels toward the west with velocities greater than those expanding the wedge eastward. The drastic decrease of the velocity with the azimuthal distance from the location of the disturbance initiation, i.e., the onset sector, indicates that the energy release is a very localized phenomenon. Finally, the transient D perturbation observed by the geosynchronous satellites suggests that the field-aligned currents forming the wedge have a longitudunally limited extent.  相似文献   

14.
A model of alignment of the field-aligned current generator system in the disturbed magnetosphere is proposed. It has been found that Pedersen currents in the disturbed ionosphere of the auroral oval are meridional. They connect oppositely directed field-aligned currents (FACs) of two adjacent Iijima and Potemra (IP) regions. This supplements the dominant concept of currents in the literature, in which a substorm current wedge plays a major role. During the considered substorm, graphs have been obtained of changes in FAC intensities in each of the three Iijima and Potemra regions of the Northern Hemisphere. These new data suggest that during a disturbance, the Region 1 FAC and the sum of regions 2 and 0 FACs remain equal. The equality means that these three generators form a unified system. Inequalities of intensities between two FACs of different sign simultaneously observed in the dawn and dusk sectors have also been detected in each IP region. A conceptual model is proposed for the formation of this dawn-dusk asymmetry. In the model, in addition to FACs that close up in the ionosphere of the Northern Hemisphere, FACs connecting the two hemispheres are also presented. The conclusion is justified that the global magnetospheric dynamo operates as a voltage generator under relatively quiet (average) conditions, as during storms.  相似文献   

15.
Multichannel Analysis of Surface Waves (MASW) is one of the most widely used techniques in environmental and engineering geophysics to determine shear-wave velocities and dynamic properties, which is based on the elastic layered system theory. Wave propagation in the Earth, however, has been recognized as viscoelastic and the propagation of Rayleigh waves presents substantial differences in viscoelastic media as compared with elastic media. Therefore, it is necessary to carry out numerical simulation and dispersion analysis of Rayleigh waves in viscoelastic media to better understand Rayleigh-wave behaviors in the real world. We apply a pseudospectral method to the calculation of the spatial derivatives using a Chebyshev difference operator in the vertical direction and a Fourier difference operator in the horizontal direction based on the velocity–stress elastodynamic equations and relations of linear viscoelastic solids. This approach stretches the spatial discrete grid to have a minimum grid size near the free surface so that high accuracy and resolution are achieved at the free surface, which allows an effective incorporation of the free surface boundary conditions since the Chebyshev method is nonperiodic. We first use an elastic homogeneous half-space model to demonstrate the accuracy of the pseudospectral method comparing with the analytical solution, and verify the correctness of the numerical modeling results for a viscoelastic half-space comparing the phase velocities of Rayleigh wave between the theoretical values and the dispersive image generated by high-resolution linear Radon transform. We then simulate three types of two-layer models to analyze dispersive-energy characteristics for near-surface applications. Results demonstrate that the phase velocity of Rayleigh waves in viscoelastic media is relatively higher than in elastic media and the fundamental mode increases by 10–16% when the frequency is above 10 Hz due to the velocity dispersion of P and S waves.  相似文献   

16.
The construction of S-wave velocity models of marine sediments down to hundreds of meters below the seafloor is important in a number of disciplines. One of the most significant trends in marine geophysics is to use interface waves to estimate shallow shear velocities which play an important role in determining the shallow crustal structure. In marine settings, the waves trapped near the fluid–solid interface are called Scholte waves, and this is the subject of the study. In 1998, there were experiments on the Ninetyeast Ridge (Central Indian Ocean) to study the shallow seismic structure at the drilled site. The data were acquired by both ocean bottom seismometer and ocean bottom hydrophone. A new type of seafloor implosion sources has been used in this experiment, which successfully excited fast and high frequency (>500 Hz) body waves and slow, intermediate frequency (<20 Hz) Scholte waves. The fundamental and first higher mode Scholte waves have both been excited by the implosion source. Here, the Scholte waves are investigated with a full waveform modeling and a group velocity inversion approach. Shear wave velocities for the uppermost layers of the region are inferred and results from the different methods are compared. We find that the full waveform modeling is important to understand the intrinsic attenuation of the Scholte waves between 1 and 20 Hz. The modeling shows that the S-wave velocity varies from 195 to 350 m/s in the first 16 m of the uppermost layer. Depths levels of high S-wave impedance contrasts compare well to the layer depth derived from a P-wave analysis as well as from drilling data. As expected, the P- to S-wave velocity ratio is very high in the uppermost 16 m of the seafloor and the Poisson ratio is nearly 0.5. Depth levels of high S-wave impedance contrasts are comparable to the layer depth derived from drilling data.  相似文献   

17.
—?During 1997 and 1998, twelve chemical explosions were detonated in boreholes at the former Soviet nuclear test site near the Shagan River (STS) in Kazakhstan. The depths of these explosions ranged from 2.5 to 550 m, while the explosive yield varied from 2 to 25 tons. The purpose of these explosions was for closure of the unused boreholes at STS, and each explosion was recorded at local distances by a network of seismometers operated by Los Alamos National Laboratory and the Institute of Geophysics for the National Nuclear Center (NNC). Short-period, fundamental-mode Rayleigh waves (Rg) were generated by these explosions and recorded at the local stations, resultingly the waves exhibited normal dispersion between 0.2 and 3 seconds. Dispersion curves were generated for each propagation path using the Multiple Filter Analysis and Phase Match Filtering techniques. Tomographic maps of Rg group velocity were constructed and show a zone of relatively high velocities for the southwestern (SW) region of the test site and slow propagation for the northeastern (NE) region. For 0.5?sec Rg, the regions are separated by the 2.1?km/sec contour, as propagation in the SW is greater than 2.1?km/sec and less in the NE region. At 1.0 sec period, the 2.3?km/sec contour separates the two regions. Finally, for 1.5 and 2.0 sec, the separation between the two regions is less distinct as velocities in the NE section begin to approach the SW except for a low velocity region (<2.1?km/sec) near the center of the test site. Local geologic structure may explain the different regions as the SW region is composed predominantly of crystalline intrusive rocks, while the NE region consists of alluvium, tuff deposits, and Paleozoic sedimentary rocks. Low velocities are also observed along the Shagan River as it passes through the SW region of the test site for shorter period Rg (0.5–1.0?sec). Iterative, least-squares inversions of the Rg group velocity dispersion curves show shear-wave velocities for the southwestern section that are on average 0.4?km/sec higher than the NE region. At depths greater than 1.5?km the statistical difference between the models is no longer significant. The observed group velocities and different velocity structures correlate with P-wave complexity and with spatial patterns of magnitude residuals observed from nuclear explosions at STS, and may help to evaluate the mechanisms behind those observations.  相似文献   

18.
Regions of the detection of electromagnetic ion-cyclotron (EMIC) waves on the THEMIS satellites near the equatorial plane and the precipitation of energetic protons on POES low Earth orbiting satellites are compared with the magnetospheric magnetic field model. It is confirmed that low Earth orbiting satellites detect the precipitation of energetic protons in the regon associated with observations of EMIC waves in the magnetosphere. This is consistent with the idea that protons are scattered in the loss cone as a result of ioncyclotron interaction. Thus, observations of fluxes of energetic protons in low Earth orbits can be used to monitor ion-cyclotron instability regions in the magnetosphere. Simultaneous observations at high and low Earth orbits contribute to the construction of a spatiotemporal pattern of the interaction region of EMIC waves and energetic protons. In addition, it is shown that proton precipitation associated with EMIC waves can cause errors in determining the latitude of the isotropic boundary (the equatorial boundary of isotropic fluxes of energetic protons), which is an indicator of the configuration of the magnetic field in the magnetosphere.  相似文献   

19.
We present two case studies in the night and evening sides of the auroral oval, based on plasma and field measurements made at low altitudes by the AUREOL-3 satellite, during a long period of stationary magnetospheric convection (SMC) on November 24, 1981. The basic feature of both oval crossings was an evident double oval pattern, including (1) a weak arc-type structure at the equatorial edge of the oval/polar edge of the diffuse auroral band, collocated with an upward field-aligned current (FAC) sheet of ≈1.0 μA m−2, (2) an intermediate region of weaker precipitation within the oval, (3) a more intense auroral band at the polar oval boundary, and (4) polar diffuse auroral zone near the polar cap boundary. These measurements are compared with the published magnetospheric data during this SMC period, accumulated by Yahnin et al. and Sergeev et al., including a semi-empirical radial magnetic field profile BZ in the near-Earth neutral sheet, with a minimum at about 10–14 RE. Such a radial BZ profile appears to be very similar to that assumed in the “minimum B/cross-tail line current” model by Galperin et al. (GVZ92) as the “root of the arc”, or the arc generic region. This model considers a FAC generator mechanism by Grad-Vasyliunas-Boström-Tverskoy operating in the region of a narrow magnetic field minimum in the near-Earth neutral sheet, together with the concept of ion non-adiabatic scattering in the “wall region”. The generated upward FAC branch of the double sheet current structure feeds the steady auroral arc/inverted-V at the equatorial border of the oval. When the semi-empirical BZ profile is introduced in the GVZ92 model, a good agreement is found between the modelled current and the measured characteristics of the FACs associated with the equatorial arc. Thus the main predictions of the GVZ92 model concerning the “minimum-B” region are consistent with these data, while some small-scale features are not reproduced. Implications of the GVZ92 model are discussed, particularly concerning the necessary conditions for a substorm onset that were not fulfilled during the SMC period.  相似文献   

20.
Geotail energetic particle, magnetic field data and plasma observations (EPIC, MGF and CPI experiments) have been examined for a number of energetic particle bursts in the distant tail (120Re < |XGSM| < 130 Re), associated with moving magnetic field structures, following substorm onsets. The features obtained from this data analysis are consistent with the distant magnetotail dynamics determined first by ISEE3 observations and explained in terms of the neutral line model. At the onset of the bursts, before plasma sheet entrance, energetic electrons appear as a field-aligned beam flowing in the tailward direction, followed by anisotro-pic ions. Within the flux rope region, suprathermal ions exhibit a convective anisotropy, which allows determination of the plasma flow velocity, assuming that the anisotropy arises from the Compton-Getting effect. The velocities thus determined in the plasma sheet are estimated to be 200–650 km/s, and compare favourably with the velocities derived from the CPI electron and proton experiment. The estimated length of magnetic field structures varies between 28 and 56 Re and depends on the strength of the westward electrojet intensification. Finally, the three structures reported here show clear magnetic field signatures of flux rope topology. The existence of a strong magnetic field aligned approximately along the Y-axis and centred on the north-to-south excursion of the field, and the bipolar signature in both By and/or Bz components, is consistent with the existence of closed field lines extending from Earth and wrapping around the core of the flux rope structure.  相似文献   

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