哨声模波对高能电子槽区和外辐射带的调节作用

本文利用磁层哨声模嘶声和合声波的幅度分布模型、近赤道背景电子(能量在eV量级)的数密度分布模型和IGRF10磁场模型建立了一个高能电子(能量大于50 keV)准线性扩散模型.模型的数值结果表明,在不同的地磁条件下,等离子体层顶位置的变化改变了磁层背景电子数密度的空间分布,从而改变了哨声模嘶声对高能电子有效的投掷角扩散（损失）区域,同时也改变了哨声模合声波对高能电子有效的动量扩散（加速）区域.哨声模嘶声对电子投掷角扩散区域的变化和RRES卫星探测到的高能电子的槽区变化是一致的,而合声波对电子的动量扩散区域的变化和卫星探测到外辐射带的变化相同.这种对应关系说明：在不同的地磁条件下,哨声模波对高能电子扩散区域的变化是造成高能电子槽区和外辐射带的空间位置和大小变化的一个重要因素.在一些强磁暴期间,由于嘶声对部分能量范围电子的投掷角扩散作用消失,这些电子的槽区也随之消失,从而使内外辐射带连接在一起.     

Relativistic electron losses related to EMIC waves during CIR and CME storms

The losses of radiation belt electrons to the atmosphere due to wave–particle interactions with electromagnetic ion-cyclotron (EMIC) waves during corotating interaction region (CIR) storms compared to coronal mass ejections (CME) storms is investigated. Geomagnetic storms with extended ‘recovery’ phases due to large-amplitude Alfvén waves in the solar wind are associated with relativistic electron flux enhancements in the outer radiation belt. The corotating solar wind streams following a CIR in the solar wind contain large-amplitude Alfvén waves, but also some CME storms with high-speed solar wind can have large-amplitude Alfvén waves and extended ‘recovery’ phases. During both CIR and CME storms the ring current protons are enhanced. In the anisotropic proton zone the protons are unstable for EMIC wave growth. Atmospheric losses of relativistic electrons due to weak to moderate pitch angle scattering by EMIC waves is observed inside the whole anisotropic proton zone. During storms with extended ‘recovery’ phases we observe higher atmospheric loss of relativistic electrons than in storms with fast recovery phases. As the EMIC waves exist in storms with both extended and short recovery phases, the increased loss of relativistic electrons reflects the enhanced source of relativistic electrons in the radiation belt during extended recovery phase storms. The region with the most unstable protons and intense EMIC wave generation, seen as a narrow spike in the proton precipitation, is spatially coincident with the largest loss of relativistic electrons. This region can be observed at all MLTs and is closely connected with the spatial shape of the plasmapause as revealed by simultaneous observations by the IMAGE and the NOAA spacecraft. The observations in and near the atmospheric loss cone show that the CIR and CME storms with extended ‘recovery’ phases produce high atmospheric losses of relativistic electrons, as these storms accelerate electrons to relativistic energies. The CME storm with short recovery phase gives low losses of relativistic electrons due to a reduced level of relativistic electrons in the radiation belt.     

The dual role of ELF/VLF chorus waves in the acceleration and precipitation of radiation belt electrons

This paper provides a brief review of the role that chorus waves play in controlling the dynamics of the Earth's outer radiation belt. Three major topics are discussed: (i) the morphology, characteristics, and properties of chorus waves themselves, with special emphasis on more recent results, (ii) the role that chorus waves play in the loss of radiation belt particles, showing initial results from modeling of relativistic electron microbursts, and estimated lifetimes based on microburst occurrence rates during the main phase of storms, and (iii) the role that chorus waves play in the acceleration of electrons to relativistic energies in the recovery phase of storms, based on a new quasilinear diffusion based calculation.     

Effects of energy and pitch angle mixed diffusion on radiation belt electrons

Understanding the dynamics of the Earth’s radiation belts is important for modeling and forecasting the intensities of energetic electrons in space. Wave diffusion processes are known to be responsible for loss and acceleration of electrons in the radiation belts. Several recent studies indicate pitch angle and energy mixed-diffusion are also important when considering the total diffusive effects. In this study, a two-dimensional Fokker Planck equation is solved numerically using the Alternating Direction Implicit method. Mixed diffusion due to whistler-mode chorus waves tends to slow down the total diffusion in the energy-pitch angle space, particularly at smaller equatorial pitch angles. We then incorporate the electron energy and pitch angle mixed diffusions due to whistler-model chorus waves into the 4-dimensional Radiation Belt Environment (RBE) model and study the effect of mixed diffusion during a storm in October 2002. The 4-D simulation results show that energy and pitch angle mixed diffusion decrease the electron fluxes in the outer belt while electron fluxes in the slot region are enhanced (up to a factor of 2) during storm time.     

Daytime VLF emissions at the Sodankyla Observatory (L ∼ 5.3) at the front of high-speed solar wind streams

We discuss the results of an analysis of digital high-sensitivity ground-based observations of very low frequency (VLF) emissions, carried out in Northern Finland (L = 5.3) in May–June 2012. During this period of time, we found that three high-speed solar wind streams approached the Earth’s magnetosphere and at the front of these fluxes long-lasting intense daytime bursts of VLF emissions were generated in two frequency bands: above and below ～2.5 kHz. At frequencies above ～2.5–3.0 kHz, there were VLF hiss waves, the temporal structure of which consisted of a quasi-periodic sequence of separate stronger spots of noise signals. The low-frequency band was represented by chorus waves, superimposed on intense hiss emissions at frequencies below ～1.5 kHz. The high-frequency (f > 2.5 kHz) waves were elliptic and, predominately, left-hand polarized and the low-frequency waves were right-hand polarized. It was supposed that high-frequency VLF hiss waves were generated at L < 5 and VLF chorus waves were generated at L > 5. We discuss a possible scenario of the generation and propagation of the VLF emissions observed.     

波粒相互作用导致环电流质子沉降的卫星共轭观测

波粒相互作用是环电流损失的重要机制之一,但波粒相互作用导致的环电流离子沉降而损失迄今为止缺乏直接的观测证据.基于磁层及电离层卫星的协同观测,本文报道了发生在2015年9月7日,由电磁离子回旋波(EMIC波)导致环电流质子沉降的共轭观测事件.在等离子体层的内边界,Van Allen Probe B卫星观测到,存在EMIC波的区域和不存在EMIC波的区域相比,离子通量的投掷角分布的各向异性变弱.我们将Van Allen Probe B卫星沿着磁力线投影到电离层高度,同时在该投影区域内DMSP 16卫星在亚极光区域观测到环电流质子沉降.而且,通过从理论上计算质子弹跳平均扩散系数,我们进一步证实观测的EMIC波确实能将环电流质子散射到损失锥中.本文的研究工作为EMIC波导致环电流质子沉降提供了直接的观测证据,揭示了环电流衰减的重要物理机制:EMIC波将环电流质子散射到损失锥中,从而沉降到低高度大气层中而损失.     

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.     

地球电子外辐射带的数据同化建模与分析

地球电子外辐射带对太阳与地磁活动呈现高度动态变化的响应,了解外辐射带的全球动态变化过程对于近地空间粒子辐射环境的理解认知和预测预报具有重要意义.基于卡尔曼滤波数据同化方法,本文利用范阿伦A星、B星和GOES-13和GOES-15四颗卫星的辐射带电子观测数据,分别利用三种不同维度的辐射带物理模型,将观测结果与数值结果有机融合,对2013年3月地球外辐射带电子通量的径向分布与变化进行数据同化分析.结果表明,考虑了磁层波动与辐射带电子共振作用引起的径向扩散、投掷角扩散以及能量扩散过程的三维同化模型可有效、合理地重现外辐射带电子通量的径向分布.本文进一步利用该三维同化模型对2013年一整年外辐射带电子的相空间密度分布进行重构与分析,得到了不同绝热不变量和不同地磁活动条件下电子辐射带的时空演化过程,从而为深入理解外辐射带电子的变化过程和动力学机制提供了强有力信息.通过分析同化过程中的新息矢量以及度量同化过程中观测数据在多大程度上修改了物理模型结果,还有助于定量分析现有辐射带物理模型中的源项和损失项的相对贡献以及可能忽略的物理机制或过程.     

Energetic proton precipitation related to ion–cyclotron waves

This brief review summarizes recent findings related to particle precipitation associated with electromagnetic ion–cyclotron (EMIC) waves seen on the ground as geomagnetic Pc1 and IPDP pulsations.Particle precipitation signatures of ion–cyclotron interaction are described as revealed from low-altitude satellite measurements of the energetic proton fluxes as well as from observations of the proton aurora. As a result, localized proton precipitation patterns situated equatorward of the isotropy boundary are disclosed. One of the patterns is a proton precipitation spot in the morning sector, presumably mapped onto plasmapause; another one is an elongated region of the precipitation, presumably mapped onto the plasmaspheric plume.Clear evidence of the pitch-angle scattering associated with the ion–cyclotron wave activity is found near the equatorial plane in the region conjugated with the localized proton precipitation at low altitude.Thus, the revealed precipitation patterns determine the location of the region of intense pitch-angle scattering of energetic protons, and, therefore, their observations can be used to monitor the region of the ion–cyclotron interaction and to study its origin and properties. Some examples of such application of the low-altitude observations of energetic particles are described.     

Amplitude–frequency characteristics of ion–cyclotron and whistler-mode waves from Van Allen Probes data

Using two-hour (from 2300 UT January 25, 2013 to 0100 UT January 26, 2013) measurement data from Van Allen Probes on fluxes of energetic particles, cold plasma density, and magnetic field magnitude, we have calculated the local growth rate of electromagnetic ion–cyclotron and whistler-mode waves for field-aligned propagation. The results of these calculations have been compared with wave spectra observed by the same Van Allen Probe spacecraft. The time intervals when the calculated wave increments are sufficiently large, and the frequency ranges corresponding to the enhancement peak agree with the frequency–time characteristics of observed electromagnetic waves. We have analyzed the influence of variations in the density and ionic composition of cold plasma, fluxes of energetic particles, and their pitch-angle distribution on the wave generation. The ducted propagation of waves plays an important role in their generation during the given event. The chorus VLF emissions observed in this event cannot be explained by kinetic cyclotron instability, and their generation requires much sharper changes (“steps”) for velocity distributions than those measured by energetic particle detectors on Van Allen Probes satellites.     

Responses of geostationary orbit energetic electron fluxes to chorus waves under different geomagnetic conditions

We investigate the flux evolution of geostationary orbit energetic electrons during a strong storm on 24 August 2005(event A,the storm index Dst<200 nT,the average substorm index AE=436 nT)and a weak storm on 28 October 2006(event B,Dst>50 nT,average AE=320 nT).Data collected by LANL and GOES-12 satellites show that energetic electron fluxes increase by a factor of 10 during the recovery phase compared to the prestorm level for both events A and B.As the substorm continued,the Cluster C4 satellite observed strong whistler-mode chorus waves(with spectral density approaching 10 5nT2/Hz).The wave amplitude correlates with the substorm AE index,but is less correlated with the storm Dst index.Using a Gaussian distribution fitting method,we solve the Fokker-Planck diffusion equation governing the wave-particle interaction.Numerical results demonstrate that chorus waves efficiently accelerate~1 MeV energetic electrons,particularly at high pitch angles.The calculated acceleration time scale and amplitude are comparable to observations.Our results provide new observational support for chorus-driven acceleration of radiation belt energetic electrons.     

Quantitative description of electron precipitation during auroral absorption events in the morning/noon local-time sector

Flux-energy spectra of precipitating electrons are derived from electron density profiles measured by the EISCAT radar during auroral absorption events in the morning/noon local-time sector. The inversion technique uses effective recombination coefficient profiles computed on the basis of a previously validated theoretical model of the lower ionosphere. It is shown that flux-energy spectra for the energy range 30–200 keV are in reasonable agreement with those derived for the same events using trapped flux-energy spectra from geosynchronous satellite data and a model for diffusion of trapped electrons into the loss cone by scattering on whistler waves. During individual events, strongly varying precipitating fluxes are found to be due primarily to varying pitch-angle diffusion.     

Polar PWI and CEPPAD observations of chorus emissions and radiation belt electron acceleration: Four case studies

We investigated the role of whistler-mode chorus in accelerating outer radiation belt electrons during four moderate geomagnetic storms when data from the Polar Plasma Wave Investigation (PWI) were available. The storm time periods we examined included two storms associated with coronal mass ejections (CMEs), the well-studied January 10–13, 1997 International Solar Terrestrial Physics event and the May 12–15, 1997 event. We compared these two storms with two geomagnetically active periods that were not associated with CMEs. Although strong chorus emissions were observed during all four events, the association of electron acceleration with chorus emissions is not clear. During all four events, the Polar Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD) experiment observed increases in the fluxes of energetic electrons (0.8<E<6.4 MeV). The two events associated with CMEs featured a sudden increase in the electron fluxes above 0.8 MeV that may have been related to the impact of the CME shock wave upon the magnetosphere. The other two events featured more gradual increases in the electron fluxes over a period of several days. The data from these events indicate that the role played by resonant interactions with chorus in accelerating electrons may depend on the upstream solar wind conditions driving the storm.     

Resonant enhancement of relativistic electron fluxes during geomagnetically active periods

The strong increase in the flux of relativistic electrons during the recovery phase of magnetic storms and during other active periods is investigated with the help of Hamiltonian formalism and simulations of test electrons which interact with whistler waves. The intensity of the whistler waves is enhanced significantly due to injection of 10–100 keV electrons during the substorm. Electrons which drift in the gradient and curvature of the magnetic field generate the rising tones of VLF whistler chorus. The seed population of relativistic electrons which bounce along the inhomogeneous magnetic field, interacts resonantly with the whistler waves. Whistler wave propagating obliquely to the magnetic field can interact with energetic electrons through Landau, cyclotron, and higher harmonic resonant interactions when the Doppler-shifted wave frequency equals any (positive or negative) integer multiple of the local relativistic gyrofrequency. Because the gyroradius of a relativistic electron may be the order of or greater than the perpendicular wavelength, numerous cyclotron, harmonics can contribute to the resonant interaction which breaks down the adiabatic invariant. A similar process diffuses the pitch angle leading to electron precipitation. The irreversible changes in the adiabatic invariant depend on the relative phase between the wave and the electron, and successive resonant interactions result in electrons undergoing a random walk in energy and pitch angle. This resonant process may contribute to the 10–100 fold increase of the relativistic electron flux in the outer radiation belt, and constitute an interesting relation between substorm-generated waves and enhancements in fluxes of relativistic electrons during geomagnetic storms and other active periods.     

伴随等离子体密度下降的磁声波与辐射带电子的波粒相互作用及其散射效应

利用范阿伦卫星的高质量观测数据,我们报道了伴随等离子体密度下降的磁声波现象.通过选取分别发生于2013年7月26日(事件A)和2013年9月19日(事件B)的两个相应事件进行细致分析,我们开展试验粒子模拟计算了磁声波对辐射带电子的散射系数,并求解二维福克-普朗克扩散方程量化了磁声波散射导致的辐射带电子动态变化.结果表明,事件A中的磁声波的散射作用主要发生于投掷角范围为60°～80°、能量范围为20～200keV的辐射带电子,而事件B中的磁声波的散射作用主要发生于投掷角范围为50°～80°、能量范围为20～400keV的辐射带电子;两个事件中的磁声波均能导致辐射带电子的蝴蝶状投掷角分布,但是由于事件B的磁声波幅度更强,形成的电子蝴蝶状分布更明显.     

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.     

基于Van Allen Probes EMFISIS波动仪器观测的内磁层下频带哨声合声波全球分布的统计分析

基于Van Allen Probes近三年的EMFISIS仪器波动观测数据,对内磁层下频带哨声模合声波幅度的全球分布特性对地磁活动水平的依赖性进行了详细的统计分析,着重研究下频带合声波平均场强幅度随磁壳值L、磁地方时、地磁纬度的分布特征及不同强度区间的合声波的发生概率.结果表明,下频带合声波的波动强度与地磁活动密切正相关,处于强磁扰期间的合声波具有更大的振幅,其发生率与地磁活动强度具有同样的正相关特性.下频带合声波主要发生于午夜至下午的磁地方时区间,其余的磁地方时时段下频带合声波较弱.赤道面附近的下频带合声波主要分布在夜侧至黎明这一时段内,随着磁纬度的增加逐步向日侧扩展.下频带合声波在午夜侧(21-03 MLT)主要出现在15°的磁纬范围内,在晨侧(03-09 MLT)可以到达15°磁纬甚至更高纬度.下频带合声波主要发生于L=~4.5的附近区域.随着地磁活动的增加,下频带合声波所覆盖的L-shell空间区域增大,趋势为向高、低L值区域同时扩展.建立的下频带哨声合声波的全球分布模型将有助于进一步深入理解该重要磁层波动对辐射带电子的波粒作用散射效应和对辐射带动力学过程的定量贡献.     

Generation of EMIC Waves in the Magnetosphere and Precipitation of Energetic Protons: Comparison of the Data from THEMIS High Earth Orbiting Satellites and POES Low Earth Orbiting Satellites

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.