A comparison of field-line resonances observed at the Goose Bay and Wick radars

Previous observations with the Goose Bay HF coherent-scatter radar have revealed structured spectral peaks at ultra-low frequencies. The frequencies of these spectral peaks have been demonstrated to be extremely consistent from day to day. The stability of these spectral peaks can be seen as evidence for the existence of global magneto spheric cavity modes whose resonant frequencies are independent of latitude. Fieldline resonances occur when successive harmonics of the eigenfrequency of the magnetospheric cavity or waveguide match either the first harmonic eigenfrequency of the geomagnetic field lines or higher harmonics of this frequency. Power spectra observed at the SABRE VHF coherent-scatter radar at Wick, Scotland, during night and early morning are revealed to show similarly clearly structured spectral peaks. These spectral peaks are the result of local field-line resonances due to Alfvén waves standing on magnetospheric field lines. A comparison of the spectra observed by the Goose Bay and Wick radars demonstrate that the frequencies of the field-line resonances are, on average, almost identical, despite the different latitudinal ranges covered by the two radars. Possible explanations for the similarity of the signatures on the two radar systems are discussed.     

Comparison between two theoretical mechanisms for the formation of the plasmapause and relevant observations

The plasmapause formation physical mechanisms are recalled: (i) the MHD convection mechanism, based on the original idea that the plasmapause coincides with the last closed equipotential (LCE) of the magnetospheric convection electric field or with the last closed streamline (LCS) of the equatorial plasma, and (ii) the interchange mechanism, which is based on peeling off the plasmasphere as a result of substorm associated enhancements of the night side convection velocity, leading to an enhanced centrifugal acceleration in the outermost layers of the plasmasphere. The plasmapause positions, predicted by these alternative theories, were numerically determined for two different magnetospheric empirical electric field models: (i) the Volland-Stern-Maynard-Chen (VSMC) and (ii) McIlwain E5D models, both of which are Kp-dependent. The predicted positions and overall shape of the equatorial plasmapause cross-sections are confronted to those derived from decades of whistler and satellite observations including the EUV observations during the substorm of June 27, 2001. It is found that the VSMC electric field model and the LCS plasmapause formation theory less correspond to whistler measurements and in-situ satellite observations than the E5D model and the interchange plasmapause formation mechanism. Published in Russian in Geomagnetizm i Aeronomiya, 2008, Vol. 48, No. 5, pp. 579–597. The article was translated by the authors.     

Morning polar substorms and variations in the atmospheric electric field

The effects of morning magnetospheric substorms in the variations in near-Earth atmospheric electricity according to the observations of the electric field vertical component (E _z ), at Hornsund polar observatory (Spitsbergen). The E _z, data, obtained under the conditions of fair weather (i.e., in the absence of a strong wind, precipitation, and fog), are analyzed. An analysis of the observations indicated that the development of a magnetospheric substorm in the Earth’s morning sector is as a rule accompanied by positive deviations in E _z, independently of the Hornsund location: in the polar cap or at its boundary. In all considered events, Hornsund was located near the center of the morning convection vortex. In the evening sector, when Hornsund fell in the region of evening convection vortex, the development of a geomagnetic substorm was accompanied by negative deviations in E _z., It has been concluded that the variations in the atmospheric electric field E _z), at polar latitudes, observed during the development of magnetospheric substorms, result from the penetration of electric fields of polar ionospheric convection (which are intensified during a substorm) to the Earth’s surface.     

Effect of magnetospheric convection on the energy distribution of protons from the Earth radiation belts

The structure of energetic protons from the Earth radiation belts, averaged for a magnetically quiet period, can be explained by the equilibrium between the radial diffusion transfer, loss due to Coulomb collisions, charge exchange with ambient neutral hydrogen of the geocorona, and drift of protons under the influence of magnetospheric convection. By transfer we mean diffusion owing to fluctuations related to substorms in the large-scale electric and magnetic fields. Equatorially mirroring protons with energies of 1–750 keV have been considered, and the theoretical predictions of the proton energy spectra for L = 1.0−6.6 have been compared with the observations on several satellites.     

Topology of currents in the high-latitude magnetosphere and magnetospheric response to variations in solar wind parameters

We consider a number of new approaches that arise when the topology of currents in the high-latitude magnetosphere is investigated. We note that the high correlation between magnetospheric processes and solar wind parameters is a well-known feature of the magnetospheric dynamics. The proposed explanations of the observed dependences run into difficulties related to the high level of observed turbulence in the magnetosheath and inside the magnetosphere. The topology of the high-latitude magnetosphere in the transition region from dipole magnetic field lines to those extending into the tail is also poorly known. We consider the topology of transverse magnetospheric currents using satellite measurements of the plasma pressure distribution. The currents of the nearest plasma sheet are shown to be closed inside the magnetosphere. The generation of field-aligned currents in Iijima and Potemra region currents 1 and large-scale magnetospheric convection are discussed.     

EISCAT雷达观测到的极区电离层等离子体反常对流

综合分析EISCAT雷达与卫星当地测量数据,并利用磁层磁场模式对磁力线进行追踪,研究了发生在极光椭圆朝极盖边界附近电离层中,一例反常的背离太阳流动的强等离子体对流事件,及相关的太阳风-磁层-电离层耦合过程.结果表明,磁暴期间IMFBz指向南时观测到这一反常高速对流,及其相应的等离子体性态特征,很可能是向阳侧磁层顶磁重联过程在电离层中的印记.     

Upward acceleration of magnetospheric ions by oscillatory centrifugal force

The paper addresses the issue of upward acceleration of ions along geomagnetic field lines. It has been shown that ion acceleration by electric field oscillations (formerly known as magnetic moment “pumping” or MMP) may be treated as a centrifugal acceleration mechanism. More precisely, the case in point is oscillatory centrifugal acceleration; this brings up the question on comparing the MMP with the centrifugal acceleration caused by the quasi-static magnetospheric convection field. It has been found that at high geomagnetic latitudes, the oscillatory centrifugal force is weaker or stronger than the centrifugal force of magnetospheric convection if the ratio of the electric field oscillation amplitude to the mean field is correspondingly lower or higher than \(\sqrt 2 \). Analysis of data from measurements and calculations of magnetospheric electric fields suggests that, contrary to current opinion, the oscillatory centrifugal force may be comparable to the centrifugal force of magnetospheric convection and even exceed it when strong global Pc5 pulsations are excited in the magnetosphere.     

EISCAT雷达观测到的极区电离层等离子体反常对流

综合分析EISCAT雷达与卫星当地测量数据,并利用磁层磁场模式对磁力线进行追踪,研究了发生在极光椭圆朝极盖边界附近电离层中,一例反常的背离太阳流动的强等离子体对流事件,及相关的太阳风-磁层-电离层耦合过程.结果表明,磁暴期间IMFBz指向南时观测到这一反常高速对流,及其相应的等离子体性态特征,很可能是向阳侧磁层顶磁重联过程在电离层中的印记.     

Magnetospheric-ionospheric convection in an open model of the magnetosphere

Magnetospheric-ionospheric convection has been calculated for an open model of the magnetosphere with an ellipsoidal magnetopause in an approximation of uniform IMF. It is assumed that only 0.1 part of IMF falls in the magnetosphere as a result of the effect of IMF shielding by the magnetopause. The modeling of convection has been performed for the cases when the IMF B _z component is directed southward and the B _y component is westward or eastward. A Tsyganenko 96 model has been used as a magnetospheric model. The model calculations are compared with the data on the ion drift in the ionosphere. A certain disagreement between the experimental and calculated data has been found in the pattern of convection on the dayside of the ionosphere for the case of B _y >0, which manifested itself in the dimensions of a convection “tongue” and in the position of the convection throat on the dayside. It has been indicated that the convection pattern agrees with the results of observations if the azimuthally inhomogeneous magnetospheric conductivity is taken into account.     

Solar-wind–magnetosphere coupling,including relativistic electron energization,during high-speed streams

High geomagnetic activity occurs continuously during high-speed solar wind streams, and fluxes of relativistic electrons observed at geosynchronous orbit enhance significantly. High-speed streams are preceded by solar wind compression regions, during which time there are large losses of relativistic electrons from geosynchronous orbit. Weak to moderate geomagnetic storms often occur during the passage of these compression regions; however, we find that the phenomena that occur during the ensuing high-speed streams do not depend on whether or not a preceding storm develops. Large-amplitude Alfvén waves occur within the high-speed solar wind streams, which are expected to lead to intermittent intervals of significantly enhanced magnetospheric convection and to thus also lead to repetitive substorms due to repetitively occurring reductions in the strength of convection. We find that such repetitive substorms are clearly discernible in the LANL geosynchronous energetic particle data during high-speed stream intervals. Global auroral images are found to show unambiguously that these events are indeed classical substorms, leading us to conclude that substorms are an important contributor to the enhanced geomagnetic activity during high-speed streams. We used the onsets of these substorms as indicators of preceding periods of enhanced convection and of reductions in convection, and we have used ground-based chorus observations from the VELOX instrument at Halley station as an indicator of magnetospheric chorus intensities. These data show evidence that it is the periods of enhanced convection that precede substorm expansions, and not the expansions themselves, that lead to the enhanced dawn-side chorus wave intensity that has been postulated to cause the energization of relativistic electrons. If this inference is correct, and if it is chorus that energizes the relativistic electrons, then high-speed solar wind streams lead to relativistic electron flux enhancements because the embedded large-amplitude Alfvén waves give multi-day periods of intermittent significantly enhanced convection.     

Solar wind effects on ionospheric convection: a review

The solar wind, magnetosphere, and ionosphere are intrinsically coupled through magnetic field lines. The electrodynamic state of the high-latitude ionosphere is controlled by several geophysical processes, such as the location and rate of magnetic reconnection at the magnetopause and in the magnetotail, and the energisation and precipitation of solar wind and magnetospheric plasmas. Amongst the most observed ionospheric manifestation of solar wind/magnetospheric processes are the convection bursts associated with the so-called flux transfer events (FTEs), magnetic impulse events (MIEs), and travelling convection vortices (TCVs). Furthermore, the large-scale ionospheric convection configuration has also demonstrated a strong correspondence to variations in the interplanetary medium and substorm activity. This report briefly discusses the progress made over the past decade in studies of these transient convection phenomena and outlines some unsettled questions as well as future research directions.     

Whistler–electron interactions in the magnetosphere: new results and novel approaches

Attention is focused here on the quasilinear and nonlinear physics of cyclotron interactions between magnetospheric whistler mode waves and energetic electrons on dipolar geomagnetic flux tubes. These interactions can lead to the generation of noise-like emissions or phase-coherent discrete signals in the frequency-time domain. In the magnetosphere noise-like emissions called hiss are accompanied by a smooth electron precipitation pattern. Examples of discrete emissions are ELF/VLF chorus or VLF emissions triggered by whistlers from lightning or by radio transmitters on the ground. The rapid temporal variations of these signals are associated with fine structure of the distribution function of the radiation belt electrons, such as a transient step-like deformation or a well-organized beam, which are prepared by initial noise-like emissions or by a quasimonochromatic whistler–wave packet, respectively. These cause the properties of the electrons, which may be observed on a satellite, to evolve rapidly in time and on relatively short spatial scales. Bursts of precipitating electrons occur, and can contribute significantly to depleting the radiation belts. Recent results on improvements in the theoretical understanding of such processes and on new observations of magnetospheric electrons and whistler-mode waves are presented.     

Role of magnetospheric convection and precipitation in the formation of the “dusk effect” during the main phase of a magnetic storm

This paper studies the role of magnetospheric factors, such as convection and energetic electron precipitation during the formation of positive disturbances in the total electron content under the conditions of the summer evening ionosphere. à numerical model of the ionosphere and plasmasphere, where time variations in the magnetospheric convection velocity and electron precipitation parameters correspond to the main phase of a magnetic storm, has been used for this purpose. It has been indicated that the total electron content sharply increases (the “dusk effect”) in the eastern and western sectors at approximately the same geomagnetic latitudes corresponding to the subauroral zone provided that a sudden storm commencement is registered in the morning hours. local time. This peak of the total electron content is formed as a result of joint reconstruction of the magnetospheric convection pattern and energetic electron precipitation during the main phase of a storm. In this case, magnetospheric convection plays the main role, raising the F2 layer by 40–80 km into the region with a lower recombination rate.     

Weimer电场模式在地球磁层内的特征

地球磁层中的电场是研究磁层物理的重要参数,目前常用的对流电场有均匀晨昏电场和投影电场.电离层电场可以看做磁层电场沿磁力线在电离层的投影,本文选取的电离层电场模型为Weimer(2001模式)电场.利用T96磁场模式,沿磁力线将电离层电场投影到磁层空间,得到一个新的磁层电场模式,并讨论了磁暴、行星际磁场(IMF)、太阳风参数和亚暴等对磁层电场的影响.利用该模型计算的电场结果与卫星探测结果相符.     

基于正则化方法的高频地波雷达海浪方向谱反演

提出了一种从高频地波雷达海面回波谱中提取海浪方向谱的新方法:基于高频电波海洋探测基本原理将线性化后的非线性积分方程离散为矩阵方程组,通过引入正则化数学模型将不适定方程转化为正规方程,然后采用奇异值分解法求解.对于正则化方法中正则化参数,采用L曲线法来确定.为了验证本文算法的有效性,分别在单部雷达和双部雷达探测下进行了不同噪声水平下的数值模拟,结果表明了该正则化反演方法的有效性.文中还给出了实测数据分析初步结果.该算法的工程性应用还需进一步研究.对于实际的地波雷达海浪反演具有良好的应用前景.     

High-latitude HF Doppler observations of ULF waves: 2. Waves with small spatial scale sizes

The DOPE (Doppler Pulsation Experiment) HF Doppler sounder located near Tromsø, Norway (geographic: 69.6°N 19.2°E; L = 6.3) is deployed to observe signatures, in the high-latitude ionosphere, of magnetospheric ULF waves. A type of wave has been identified which exhibits no simultaneous ground magnetic signature. They can be subdivided into two classes which occur in the dawn and dusk local time sectors respectively. They generally have frequencies greater than the resonance fundamentals of local field lines. It is suggested that these may be the signatures of high-m ULF waves where the ground magnetic signature has been strongly attenuated as a result of the scale size of the waves. The dawn population demonstrate similarities to a type of magnetospheric wave known as giant (Pg) pulsations which tend to be resonant at higher harmonics on magnetic field lines. In contrast, the waves occurring in the dusk sector are believed to be related to the storm-time Pc5s previously reported in VHF radar data. Dst measurements support these observations by indicating that the dawn and dusk classes of waves occur respectively during geomagnetically quiet and more active intervals.     

Flow angle dependence for the asymmetry of broad 50-MHz coherent echoes at large magnetic aspect angles

The skewness of broad Type 2-like spectra has been studied using data collected by two orthogonal CW 50-MHz radio links with co-located scattering volumes. Geometrical aspect angles of observations were about 10. One short event was considered. For this event, the electron flow direction was changing periodically (period about 9 minutes) presumably due to the passage of a magnetospheric MHD wave through the ionosphere. It was found that for the radar observations along the electrojet flow, the skewness had the same sign as the mean Doppler shift with average absolute values in between 0.5–1.0. For observations perpendicular to the electrojet flow, spectra were more symmetrical (average skewness was around 0) and the sign of the skewness was sometimes opposite to the sign of the mean Doppler shift. These observations are interpreted in terms of contribution from both the Farley-Buneman and gradient-drift instabilities to the resultant spectrum. Differences with radar observations at small aspect angles are discussed.     

Generation of electromagnetic oscillations at frequencies of the lower hybrid and bounce resonance in mirror magnetic traps: Laboratory and rocket experiments

The generation of electromagnetic oscillations in the vicinity of the lower hybrid resonance and at the frequency of bounce oscillations of fast electrons in the plasma resonator of the magnetospheric type, formed by an HF discharge in a mirror magnetic trap, has been detected and studied. The results of the field rocket experiments aimed at stimulating maser effects, including such effects at frequencies of bounce oscillations of 40-keV electrons in the magnetospheric flux tube of the geomagnetic field, are discussed.     

Naturally Enhanced Ion-Acoustic Spectra And Their Interpretation

Incoherent scatter radars are designed to detect scatter from thermalfluctuations in the ionosphere. These fluctuations contain, among other things,features associated with ion-acoustic waves driven by random motions within theplasma. The resulting spectra are generally broad and noisy, but neverthelessthe technique can, through a detailed analysis of spectra, be used to measure arange of physical parameters in the Earth's upper atmosphere, and provides apowerful diagnostic in studies of magnetosphere-ionosphere coupling,thermosphere dynamics and the geospace environment in general. In recent yearsthere has been much interest in naturally occurring (as opposed to artificiallystimulated) enhanced ion-acoustic spectra seen in the auroral zone andcusp/cleft region. A study of the plasma instability processes that lead tosuch spectra will help us to better understand auroral particle acceleration,wave-particle and wave-wave interactions in the ionosphere, and theirassociation with magnetospheric processes. There is now a substantial body ofliterature documenting observations of enhanced ion-acoustic spectra, but thereremains controversy over generation mechanisms. We present a review ofliterature documenting observations of naturally enhanced ion-acoustic spectra,observed mainly along the geomagnetic field direction, along with a discussionof the theories put forward to explain such phenomena.     

MHD evolutionary processes in the Earth’s model magnetic resonator

Specific features of the spatial-temporal dynamics of LF disturbances of the Earth’s magnetosphere have been analyzed by the method of numerical simulation of magnetic hydrodynamic equations taking into account plasma temperature variations. The effects related to the appearance of long-living magnetospheric jumps of density and temperature, MHD wave scattering on such inhomogeneities, Alfvén wave reflection from the near-Earth region, and magnetospheric plasma heating as a result of dissipative processes at a repeated propagation of MHD waves between magnetically conjugate regions have been considered. The problems of conformity of the discrete mathematical model with continuous equations of magnetic hydrodynamics are discussed.