磁尾等离子体片中周期性高速流相伴随磁场超低频波

本文利用Cluster卫星2004年11月8日的观测数据,分析了磁尾等离子体片中与地向周期性高速离子流相伴随的ULF波.结果显示周期性高速流的速度波动与磁场和温度中的ULF波同时出现、同时增强、同时消失,而且波动的频率都集中在60~70 mHz.这说明磁场和温度ULF波与周期性高速流密切相关,周期性高速流是ULF波产生的来源.高速流波动的相位与磁场波动的相位大致反相关,与热离子温度波动的相位正相关,同时磁场波动与热离子温度波动呈相位反相关的特性.最小方差法分析的结果显示虽然波传播方向有地向分量,但其主要传播方向是向等离子体片中心传播,并与周期性高速流速度方向垂直.以上观测说明是高速流的周期性变化产生了磁场在Pi1频率范围内的ULF波.     

超低频波粒相互作用分析器

地球磁层的超低频(ULF)波是导致能量带电粒子快速加速的主要原因,同时这些粒子又可以使ULF波发生阻尼,它们之间的相互作用的过程可以通过星载仪器来定量观测.作者根据波与粒子相互作用的原理,结合中高轨道三维空间电场测量系统DC和低频部分的频率响应特点,设计了一种针对ULF波的波-粒相互作用分析器,并进行了模拟实验和验证.该仪器具有快速、精确、结构简单的特点,在中高轨道卫星观测时能对ULF波与粒子的相互作用的过程进行实时的定量测量.     

Magnetic Pulsations: Their Sources and Relation to Solar Wind and Geomagnetic Activity

Ultra low frequency (ULF) waves incident on the Earth are produced by processes in the magnetosphere and solar wind. These processes produce a wide variety of ULF hydromagnetic wave types that are classified on the ground as either Pi or Pc pulsations (irregular or continuous). Waves of different frequencies and polarizations originate in different regions of the magnetosphere. The location of the projections of these regions onto the Earth depends on the solar wind dynamic pressure and magnetic field. The occurrence of various waves also depends on conditions in the solar wind and in the magnetosphere. Changes in orientation of the interplanetary magnetic field or an increase in solar wind velocity can have dramatic effects on the type of waves seen at a particular location on the Earth. Similarly, the occurrence of a magnetospheric substorm or magnetic storm will affect which waves are seen. The magnetosphere is a resonant cavity and waveguide for waves that either originate within or propagate through the system. These cavities respond to broadband sources by resonating at discrete frequencies. These cavity modes couple to field line resonances that drive currents in the ionosphere. These currents reradiate the energy as electromagnetic waves that propagate to the ground. Because these ionospheric currents are localized in latitude there are very rapid variations in wave phase at the Earth’s surface. Thus it is almost never correct to assume that plane ULF waves are incident on the Earth from outer space. The properties of ULF waves seen at the ground contain information about the processes that generate them and the regions through which they have propagated. The properties also depend on the conductivity of the Earth underneath the observer. Information about the state of the solar wind and the magnetosphere distributed by the NOAA Space Disturbance Forecast Center can be used to help predict when certain types and frequencies of waves will be observed. The study of ULF waves is a very active field of space research and much has yet to be learned about the processes that generate these waves.     

High-latitude HF Doppler observations of ULF waves. 1. Waves with large spatial scale sizes

A quantitative study of observations of the ionospheric signatures of magnetospheric ultra low frequency (ULF) waves by a high-latitude (geographic: 69.6°N 19.2°E) high-frequency Doppler sounder has been undertaken. The signatures, which are clearly correlated with pulsations in ground magnetometer data, exhibit periods in the range 100–400 s and have azimuthal wave numbers in the range 3–8. They are interpreted here as local field line resonances. Phase information provided by O- and X-mode Doppler data support the view that these are associated with field line resonances having large azimuthal scale sizes. The relative phases and amplitudes of the signatures in the Doppler and ground magnetometer data are compared with a model for the generation of Doppler signatures from incident ULF waves. The outcome suggests that the dominant mechanism involved in producing the Doppler signature is the vertical component of an E × B bulk motion of the local plasma caused by the electric field perturbation of the ULF wave.     

Transient ULF pulsations: time dependence of magnetic fields observed at the ground

The continuum oscillation of a latitudinal range of closed geomagnetic field lines or shells appears to be a basic feature of the magnetosphere. Such oscillations are observed at the ground, and have been termed transient ULF pulsations. Earlier modelling showed that the apparent mean damping rate at the ground should be much greater than that in the magnetosphere. This modelling is extended to examine the time dependence of the magnetic field of transient pulsations as seen by a latitudinal chain of magnetometers. It is found that there should be significant temporal variation of both period and damping decrement observed at a given latitude, which could help to identify transient events even when the period variation with latitude is not obvious. Time-frequency analysis and analytical signal analysis do not seem to be effective in determining temporal parameter variation for the short, highly damped data segments typical of transient events. Least squares fitting of two decaying sinusoids gives surprisingly good results, but seems to have no physical basis, is difficult to interpret, and may be misleading. Least squares fitting of a single sinusoid with time-varying period and damping rate gives reasonably good fits. The resulting parameter variations with latitude may help to determine the structures of ionospheric current systems associated with transient ULF events. In particular, the time change of the period at a single station can determine where that station is relative to the ionospheric current maximum.     

DEMETER卫星观测的超低频电场扰动 预处理方法及震例应用分析

基于法国DEMETER卫星观测的超低频(ULF)电场波形, 本文提出了更简单易实施的ULF电场扰动预处理方法. 利用该方法, 对文献中已报道的2010年2月27日智利M_S8.8地震前的ULF电场扰动进行了更全面的分析. 结果表明: 已报道的20°S—40°S地磁纬度带内的ULF电场扰动并非仅出现在最靠近震中的半轨道, 而是出现在较大的经度范围内. 大范围ULF电场扰动的区域正好位于南大西洋异常区, 很可能受到该区域异常磁场的影响. 南大西洋异常中心区域的ULF电场扰动表现出共轭特征, 即在南北地磁纬度20°—40°范围内ULF电场均出现约5 mV/m的扰动, 北纬地区的ULF电场扰动幅度相比南纬地区更小. 而在南大西洋异常中心区域外的ULF电场扰动则仅在20°S—40°S地磁纬度带内出现, 在其共轭区并未观测到ULF电场扰动. 这可能是由于南大西洋异常中心区域外的ULF电场扰动幅度比中心区域更小所致, 在电离层中由于碰撞效应更易被衰减, 因此ULF电场扰动无法传播至其共轭区. 本文的方法和结果可为后续更全面地分析卫星在电离层高度记录的ULF电场扰动提供参考.      

2001年1月26日高纬磁层顶通量管事件的观测研究

2001年1月26日11:10～11:40UT, ClusterⅡ卫星簇位于午后高纬磁鞘边界层和磁鞘区,此 时行星际磁场Bz为南向. 本文对在此期间观测到的多次磁通量管事件作了详细的研究 ,获得一系列的新发现：（1）高纬磁鞘边界层磁通量管的出现具有准周期性,周期约为78s ,比目前已知的磁层顶向阳面FTE的平均周期（8～11min）小得多. （2）这些通量管都具有 强的核心磁场;其主轴多数在磁场最小变化方向,少数在中间变化方向,有些无法用PAA判 定其方向（需要用电流管PAA确定）,这与卫星穿越通量管的相对路径有关. （3）每个事件 都存在很好的HT参考系,在HT参考系中这些通量管是准定常态结构;所有通量管都沿磁层顶 表面运动,速度方向大体相同,都来自晨侧下方. 通量管的径向尺度为1～2RE, 与通 常的FTE通量管相当. （4）起源于磁层的强能离子大体上沿着管轴方向由磁层向磁鞘运动; 起源于太阳风的热等离子体沿管轴向磁层传输. 通量管为太阳风等离子体向磁层输运和磁层 粒子向行星际空间逃逸提供了通道. （5）每个通量管事件都伴随有晨昏电场的反转,该电 场为对流电场.     

The effects of IMF and convection on thermal ion outflow in magnetosphere-ionosphere coupling

We study the influence of the interplanetary magnetic field (IMF) and convection electric field on the rate and destination of polar wind and other thermal (low-energy) ion outflows, and its resulting effects on magnetosphere–ionosphere coupling, using single-particle trajectory simulations in conjunction with ion velocity distribution measurements on Akebono and IMF and ionospheric convection data. We find that the ions preferentially feed the dusk sector of the plasma sheet when the IMF is duskward (B_y>0), and are more evenly distributed in the plasma sheet when the IMF is dawnward. The flow of oxygen ions originating from the noon or dusk sectors of the polar cap has a higher probability of reaching the magnetosphere and beyond compared with that from the dawn or midnight sectors, due to the increased centrifugal acceleration associated with the larger magnetic field curvature near noon and the increased convection electric field in the dusk sector. The flow is enhanced and confined to lower L-shells at times of strongly southward IMF, compared with that at times of northward IMF. The outflow rate to both the plasma sheet and the magnetotail correlates strongly with the ion temperature. As a result, the IMF and the convection electric fields affect both the overall magnitude and the detailed distribution of mass transfer from the ionosphere to the magnetosphere in magnetosphere–ionosphere coupling.     

Response of the inner and outer magnetosphere to solar wind density fluctuations during the recovery phase of a moderate magnetic storm

We examine the geomagnetic field and space plasma disturbances developing simultaneously in the solar wind, in the inner and outer magnetosphere, and on the ground from 0730 to 2030 UT on April 11, 1997 during the recovery phase of a moderate magnetic storm. The fluctuations of the solar wind density, H-component of the geomagnetic field, and power of Pc1–2 (0.1–5 Hz) waves at middle and low latitudes evolve nearly simultaneously. These fluctuations also match very well with variations of density and flux of the magnetospheric plasma at the geosynchronous orbit, and of the geomagnetic field at the geosynchronous orbit and northern polar cap. The time delay between the occurrence of disturbances in different magnetosphere regions matches the time of fast mode propagation. These disturbances are accompanied by the generation of Pc1–2 waves at mid- and high-latitude observatories in nearly the same frequency range. A scenario of the evolution of wave phenomena in different magnetospheric domains is proposed.     

Outflowing ionospheric oxygen-ion motion in a reconfigurating magnetosphere

During substorms, large-scale changes of the topology of the Earths magnetosphere following the variation of the characteristics of the interplanetary medium are accompanied by the induction of the electric field. In this study a model of a time-dependent magnetosphere is constructed and the large-scale features of the induced electric field are described. Local-time sectors with upward or downward field-aligned component and with intense perpendicular component of the electric field are distinguished. The electric-field structure implies the existence of outflow regions particularly effective in ion energization. With the vector potential adopted in the study, the region from which the most energized ions originate is defined by the local-time sector near 2100 MLT and latitude zone near 71° MLAT. The motion of ionospheric oxygen ions of energy 0.3–3 keV is investigated during a 5-min reconfiguration event when the tail-like magneto-spheric field relaxes to the dipole-like field. As the characteristics of plasma in the regions near the equatorial plane affect the substorm evolution, the energy, pitch angle, and the magnetic moment of ions in these regions are analyzed. These quantities depend on the initial energy and pitch angle of the ion and on the magnetic and electric field it encounters on its way. With the vector potential adopted, the energy attained in the equatorial regions can reach hundreds of keV. Three regimes of magnetic-moment changes are identified: adiabatic, oscillating, and monotonous, depending on the ion initial energy and pitch angle and on the magnetic- and electric-field spatial and temporal scales. The implications for the global substorm dynamics are discussed.     

Spatial field structure and polarization of geomagnetic pulsations in conjugate areas

The ultra-low-frequency (ULF) geomagnetic pulsations observed at two nearly conjugate mid-latitude sites are examined to study their spatial structure and polarization, and learn about the role of ionospheric conductivity in forming their ground signatures. The data of 1999–2002 from Antarctica and New England (L of 2.4) are compared with the numerical results obtained in a simple plane model of ULF wave propagation through the ionosphere and atmosphere. The multi-layered model environment includes an anisotropic and parametrically time-dependent ionosphere, a uniform magnetosphere and a conducting Earth, all placed in a tilted geomagnetic field. The measured diurnal and seasonal variations in the orientation angle of the polarization ellipse are interpreted as effects of hydromagnetic wave propagation through the ionosphere and conversion to an electromagnetic field below. Essentially, the phase, amplitude and polarization of ULF waves observed at the ground are controlled by the wave's spatial structure in the magnetosphere and ionospheric transverse conductivities. The differences shown by the characteristics of simultaneous pulsations in conjugate areas arise mainly from different local ionospheric conditions, while the source waves of the pulsations are common to both sites.     

Ionospheric Signatures Of Bursty Bulk Flows

While the plasma convection in the Earth's magnetosphere was for a long timeconsidered to consist mostly of laminar flows with wide spatial extents, about adecade ago the phenomenon of bursty bulk flows (BBFs), which now could beunderstood as long, but narrow channels of fast earthward plasma flow in thecentral plasma sheet of the magnetospheric tail, was discovered. Soon after thisit became clear that such events are not exceptional, but a large portion of theearthward plasma flow in the inner part of the magnetospheric tail is organisedin this bursty, intermittent mode. Since the Earth's magnetosphere is connectedby highly conducting magnetic field lines with its ionosphere, the next logicalstep was the search for the ionospheric signatures of BBFs. We review the resultsobtained so far in this young field of Space Physics, in terms of theauroral and ground magnetic signatures caused by BBFs, the particle precipitationto the ionosphere, as well as of the ionospheric electrodynamics of the processesassociated with the BBFs in the magnetosphere. Finally, we briefly review somemodels of ionosphere-magnetosphere coupling with respect to their ability to explainthe ionospheric signatures observed.     

Alfven行波涌浪携带的场向电场──极光粒子加速机制

提出一个剪切Ａｌｆｖｅｎ波加速极光粒子的新模式。频率远小于离子回旋频率的Ａｌｆｖｅｎ波由磁层向电离层传播会演化成孤波，当场向电流超过离子声不稳定性的临界电流时，激发离子声不稳定性，波与粒子的相互作用产生反常阻尼使Ａｌｆｖｅｎ波演化成行波涌浪。它携带一个方向向上的平行电场，加速极光电子形成分立极光。对等离子体密度、电场及其对应的电势进行了数值计算，结果发现满足磁层加速区条件形成Ａｌｆｖｎ行波涌浪，提供足够强的加速粒子的电场。     

Ponderomotive effects in magnetospheric hydromagnetic waves

The ponderomotive force (PMF) is a ubiquitous nonlinear wave effect arising in plasma physics when applied wave fields or plasma parameters have significant spatial gradients. Some basic properties of the PMF are described as they relate to ULF hydromagnetic waves in the magnetosphere. Examples are given of recent results obtained using both analytical and numerical techniques for waves from the lowest frequencies (determined by the dimensions of the magnetosphere) up to the vicinity of the ion cyclotron frequency. These results include the possibilities that the PMF may transport plasma over large distances in the magnetosphere, and that the PMF may energize magnetospheric ions significantly. In particular the PMF may play a role in transporting and energizing O⁺ ions from the ionosphere into the body of the magnetosphere. The PMF can also generate nonlinear coupling between the slow magnetosonic mode and the other hydromagnetic modes. This should lead to limitation of density enhancements and, notably in the case of standing Alfvén waves, to spatial harmonic generation, secularly growing frequency shifts, and saturation of driven wave fields. Some implications of these results for the magnetosphere are discussed.Based on an invited review given in IAGA Symposium 3.07, Nonlinear and Kinetic Effects in ULF Waves, at the IUGG XXI General Assembly, Boulder, Colorado, July 2-14, 1995.     

Large-scale LF electromagnetic waves in the Earth’s magnetosheath

The model equations describing the dynamics of the solar wind and interplanetary magnetic field in the dayside Earth’s magnetosheath have been studied. The large-scale flow structure near the critical point of the magnetosphere is determined in an approximation of the Chaplygin stagnation zone identified with the magnetosheath focal part. It has been indicated that magnetic gradient waves (MGWs), which represent a special branch of ULF electromagnetic oscillations of the magnetospheric resonator, can be generated in a magnetized plasma in the case when the magnetic field distribution is spatially inhomogeneous. The characteristic frequencies, periods, phase velocities, wavelengths, and amplitudes of MGW magnetic pulsations have been determined.     

电离层Alfven谐振器对地面观测到的地磁信号的影响初步研究

本文研究了0.1~10 Hz频率范围内的ULF波从磁层到地面的传播,得到了解析解,分析了电离层Alfven谐振器、磁倾角、电离层电导率、以及波频率对地面观测到的地磁信号的影响.数值结果表明:在磁层中剪切波在竖直方向有明显的谐振结构;地面观测到的信号在IAR谐振频率出现极大值,其谐振频率随磁倾角的增大而增大;电离层电导率的变化可以改变IAR的谐振频率,并能改变波的透射,从而影响地面地磁信号的频谱.     

On the relationship of SAPS to storm-enhanced density

We use magnetic field-aligned mapping between the ionosphere and the magnetosphere to intercompare ground-based observations of storm enhanced density (SED), and plasmasphere drainage plumes imaged from space by the IMAGE EUV imager, with the enhanced inner-magnetosphere/ionosphere SAPS electric field which develops during large storms. We find that the inner edge of the SAPS electric field overlaps the erosion plume and that plume material is carried sunward in the SAPS overlap region. The two phenomena, SED in the ionosphere and the erosion plume at magnetospheric heights, define a common trajectory for sunward-propagating cold plasma fluxes in the midnight—dusk–postnoon sector. The SAPS channel at ionospheric heights and its projection into the equatorial plane serve to define the sharp outer boundary of the erosion plume. The SAPS electric field abuts and overlaps both the plasmasphere boundary layer and the plasmasphere erosion plume from pre-midnight through post-noon local times.     

Observations of PC5 pulsations near field-aligned current regions

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.     

Ultra-low-frequency (ULF) and total electron content (TEC) anomalies observed at Agra and their association with regional earthquakes

Employing a three-component search coil magnetometer (f = 0.01–30 Hz) the ultra-low-frequency (ULF) magnetic field observations have been taken in a noise free rural site at Bichpuri, Agra, India since 20 April 2007. We have analysed 7 months of night time data from 01 May to 30 November 2007. The data show seven occurrences of ULF bursts with periods ranging from a few minutes to an hour in a wide frequency range up to 15 Hz. The occurrence of such ULF bursts has been statistically analysed. The polarization parameter analysis shows that two of these bursts occurred from sources in the ionosphere and magnetosphere (Z/X < 1). It is further seen that four bursts correlated very well with some regional earthquakes (M > 4.5), which occurred at distances less than 628 km from the observation site at Agra. In some cases the bursts occurred 1–3 days prior to the main shocks.Since earthquakes are also known to perturb the ionosphere, we have analysed total electron content (TEC) data obtained from a GPS receiver stationed at Agra since June 2006. Analysis of the data for the same period as the magnetic field observations shows that there existed TEC anomalies on all the days when ULF bursts occurred. The TEC anomalies show enhancement and depletion in the data in all cases. While the mechanisms of the ULF and TEC anomalies are not well known, it is expected that the ULF bursts occurred due to microfracturing of the earth's crust during stress accumulation and that the TEC anomalies occurred as a result of the electric field induced in the ionosphere during such processes.