Solar flare electron acceleration: Comparing theories and observations

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration.     

“无碰撞电子热传导"与多方关系

本文从矩方程出发,用多方关系假设导出了Hollweg根据定性分析提出的“无碰撞电子热传导”公式,确定了该公式中的无量纲常数与多方指数之间的联系,进一步讨论了“无碰撞电子热传导”公式的适用条件      

Substorms in the inner plasma sheet

Thin Current Sheets (TCS) are regularly formed prior to substorm breakup, even in the near-Earth plasma sheet, as close as the geostationary orbit. A self-consistent kinetic theory describing the response of the plasma sheet to an electromagnetic perturbation is given. This perturbation corresponds to an external forcing, for instance caused by the solar wind (not an internal instability). The equilibrium of the configuration of this TCS in the presence of a time varying perturbation is shown to produce a strong parallel thermal anisotropy (T T) of energetic electrons and ions (E>50keV) as well as an enhanced diamagnetic current carried by low energy ions (E<50keV). Both currents tend to enhance the confinement of this current sheet near the magnetic equator. These results are compared with data gathered by GEOS-2 at the geostationary orbit, where the magnetic signatures of TCS, and parallel anisotropics are regularly observed prior to breakup. By ensuring quasi-neutrality everywhere we find, when low frequency electromagnetic perturbations are applied, that although the magnetic field line remains an equipotential to the lowest order in T_e/T_i, a field-aligned potential drop exists to the next order in (T_e/T_i). Thus the development of a TCS implies the formation of a field-aligned potential drop ( few hundred volts) to ensure the quasi-neutrality everywhere. For an earthward directed pressure gradient, a field-aligned electric field, directed towards the ionosphere, is obtained, on the western edge of the perturbation (i.e. western edge of the current sheet). Thus field aligned beams of electrons are expected to flow towards the equatorial region on the western edge of the current sheet. We study the stability of these electron beams and show that they are unstable to “High Frequency” (HF) waves. These “HF” waves are regularly observed at frequencies of the order of the proton gyrofrequency (f_H+) just before, or at breakup. The amplitude of these HF waves is so large that they can produce a strong pitch-angle diffusion of energetic ions and a spatial diffusion that leads to a reduction of the diamagnetic current. The signature of a fast ion diffusion is indeed regularly observed during the early breakup; it coincides with the sudden development of large amplitude transient fluctuations, ballooning modes, observed at much lower frequencies (fH+). These results suggest that the HF waves, generated by field-aligned electron beams, provide the dissipation which is necessary to destabilize low frequency (ballooning) modes.     

Generation mechanism of the whistler-mode waves in the plasma sheet prior to magnetic reconnection

The whistler-mode waves and electron temperature anisotropy play a key role prior to and during magnetic reconnection. On August 21, 2002, the Cluster spacecrafts encountered a quasi-collisionless magnetic reconnection event when they crossed the plasma sheet. Prior to the southward turning of magnetospheric magnetic field and high speed ion flow, the whistler-mode waves and positive electron temperature anisotropy are simultaneously observed. Theoretic analysis shows that the electrons with positive temperature anisotropy can excite the whistler-mode waves via cyclotron resonances. Using the data of particles and magnetic field, we estimated the whistler-mode wave growth rate and the ratio of whistler-mode growth rate to wave frequency. They are 0.0016f_ce (Electron cyclotron frequency) and 0.0086f_ce, respectively. Therefore the whistler-mode waves can grow quickly in the current sheet. The combined observations of energetic electron beams and waves show that after the southward turning of magnetic field, energetic electrons in the reconnection process are accelerated by the whistler-mode waves.     

Energetic electrons in impulsive solar flares: Radio diagnostics

Radio emissions during and outside solar flares are tracers of energetic electrons from the bottom of the corona to the interplanetary space. This review focusses on impulsive flares, where joint analyses of radio, hard X-ray and γ-ray observations proved to be powerful probes of the properties of accelerated electrons and of the sites in the corona where they are accelerated. Evidence of electron acceleration and transport in the corona from microwave imaging and decimetre wave spectroscopy is reviewed and compared, and recent work on the interpretation of microwave spectra in terms of energetic electron spectra is discussed. The two directions for future instrumentation are the extension to shorter wavelengths, with the aim of probing relativistic electrons, and solar dedicated spectral imaging from centimetric to metric waves to provide a unified view of the acceleration signatures that stem so far from different instruments with either spectroscopic or imaging capabilities.     

Arbitrary amplitude ion-acoustic waves in a plasma with nonthermal electrons

We investigated the effect of the presence of a nonthermal electron population on the electrostatic nonlinear waves. We considered positively charged ions with two electron populations. Using the fluid equations for the unmagnetized case and the Sagdeev pseudo-potential approach the nonlinear ion-acoustic waves are studied. The cold electrons are in thermal equilibrium while the hot electron population follows a nonthermal distribution. Numerical investigation shows the importance of the presence of small amount of cold electrons that make it possible for the plasma to support nonlinear waves. We obtained the minimum cold electron density necessary to sustain these nonlinear waves. The relevant situation corresponds to the upper ionosphere where energetic electrons have been observed.     

Field emission characteristics of gold dust grains

The space plasma environment usually contains charged dust grains. The grain charge is an important parameter determining its migration through the space, coagulation, formation of dust clouds and so on. The knowledge of its charge is thus one of the basic information we want to know. There are several emission processes leading to both positive and negative charges, among others photoemission, all kinds of secondary emissions, field emissions, etc. The present study is focused on a laboratory simulation of emissions caused by impacts by energetic ions. Our experiment is based on the Paul trap which gives us an opportunity to catch a single dust grain for several days inside the vacuum vessel and exposed it by electron/ion beams. This experimental approach allows us to separate an individual charging process. We have chosen spherically shaped gold grains and discussed the processes leading to the limitation of the grain charge. We suggested that the implantation of charging ions leads to deformations of the grain surface. The deformations enhance the local electric field strength that becomes sufficient for the field ionization.     

地球同步轨道高能电子通量预报方法研究

统计分析了GOES卫星测量得到的E > 2MeV能道电子通量与地磁Ap指数及太阳风数据的关系, 构建了基于径向基函数RBF的神经网络模型框架, 对GOES-12卫星所处的地球同步轨道高能电子通量进行提前1天的预报, 其对2008-2010年数据预测的效果较好. 另外, 发现在GOES-12卫星观测的E >2MeV能道高能电子达到108 cm-2·d-1·sr-1以上时, FY-2D卫星的测量数据同时达到108 cm-2·d-1·sr-1以上的比例达到90%左右. 通过对FY-2D卫星E >2MeV能道电子通量与GOES卫星E>2MeV电子通量的相关性分析, 建立了FY-2D卫星高能电子预报模型, 预报结果与实测通量符合较好.      

Acceleration of energetic particles by whistler waves in active space experiment with charged particle beams injection

In this paper the investigation of wave-particle interaction during simultaneous injection of electron and xenon ion beams from the satellite Intercosmos-25 (IK-25) carried out using the data of the double satellite system with subsatellite Magion-3 (APEX). Results of active space experiment devoted to the beam-plasma instability are partially presented in the paper Baranets et al. (2007). A specific feature of the experiment carried out in orbits 201, 202 was that charged particle flows were injected in the same direction along the magnetic field lines B₀ so the oblique beam-into-beam injection have been produced. Results of the beam-plasma interaction for this configuration were registered by scientific instruments mounted on the station IK-25 and Magion-3 subsatellite. Main attention is paid to study the electromagnetic and longitudinal waves excitation in different frequency ranges and the energetic electron fluxes disturbed due to wave-particle interaction with whistler waves. The whistler wave excitation on the 1st electron cyclotron harmonic via normal Doppler effect during electron beam injection in ionospheric plasma are considered.     

由高能电子引发的风云二号C星跳变事件概率分析

通过对地球同步轨道高能电子监测数据(来自GOES)与风云二号卫星跳变事件的对比分析发现, 跳变事件均发生在高能电子增强事件即所谓高能电子暴期间, 因此初步断定, 跳变事件与高能电子引起的卫星介质深层充放电事件有关. 通过对不同通量高能电子增强事件期间所发生的跳变事件发生率进行量化计算, 给出跳变事件发生概率的计算方法, 为卫星在轨运行管理及防护提供参考.      

Escaping near-relativistic electron beams from the solar corona

Processes in the solar corona are prodigious accelerators of energetic ions, and electrons. The angular distribution, composition, and spectra of energetic particles observed near Earth gives information on the acceleration mechanisms. A class of energetic particle observations particularly useful in understanding the solar acceleration is the near-relativistic impulsive beam-like electron events. During five years of operation the Advanced Composition Explorer (ACE) has measured well over 400 electron events. Approximately 25% of these electron events are impulsive beam-like events that are released onto interplanetary field lines predominantly from western solar longitudes. We extend our initial 3 year study during the rise to solar maximum (Haggerty and Roelof, 2002; Simnett et al., 2002) to a five year statistical analysis of these beam-like energetic electron events in relationship to optical flares, microwave emission, soft X-ray emission, metric and decametric type-III radio bursts, and coronal mass ejections.     

地球同步轨道高能电子增强事件预报方法

分析了地球同步轨道高能电子通量增强事件的发生规律及其与太阳风和行星际磁场参数的关系，并在此基础上建立了基于人工神经网络的高能电子增强事件模式，经实测数据检验，预报模式可以对未来1天的高能电子通量进行预报，误差为8．2％，达到了较高水平．     

同步轨道遥感器热设计和热分析

为解决遥感器在同步轨道环境温度场分析中热传导与热辐射的综合处理问题,热传导模型温度计算采用控制容积方法建立有限差分方程;热辐射模型采用奥本海姆方法计算设备表面单元之间辐射换热;根据是否被遮挡,辐射换热中表面单元角系数的计算分别采用积分和数值方法。计算结果表明,采用上述方法进行温度场分析能够有效解决热传导模型与热辐射模型的耦合,求解精度较高,遥感器设备部件稳态分析温度分布和在轨瞬态分析温度曲线变化清楚,可作为进一步精密热控设计的依据。     

从空间飞行器入射进电离层的调制电子束产生的高频波辐射

在主动束-等离子体试验中,调制电子束从空间飞行器入射进电离层等离子体将会产生电磁波辐射,在不同试验条件下电磁波辐射机理也不一样,由电子束纵向约束性产生电磁波辐射是其中之一.对半无界稀薄调制电子束从空间飞行器入射进电离层等离子体时所产生的波现象进行了理论分析和数值计算.结果表明,当调制电子束沿磁力线入射时,会在电离层等离子体中产生高频电磁波辐射,该辐射主要集中在垂直于入射电子束运动方向的平面内.      

三维点阵结构等效热分析与优化方法

为了实现三维点阵结构高效、准确的热传导分析,基于热力学原理,推导了点阵结构的热传导等效计算公式,提出了热传导等效分析模型的建模方法。运用等效建模方法,建立了点阵结构的等效有限元模型,通过对比非等效有限元模型的计算结果,证明了等效有限元模型热传导分析的高效性与准确性。针对功能与性能要求下的点阵结构优化设计问题,结合所提出的热传导等效分析方法给出了点阵结构优化方法并建立优化数学模型,利用混合整数序列二次规划（MISQP）算法进行迭代计算,得到了最优设计方案,使点阵结构在满足均热性能约束的同时质量得到了降低。      

Transport modeling of energetic electrons in the inner magnetosphere with synchrotron energy losses

The resulting L-distributions and energy spectra of energetic magnetospheric electrons obtained from numerical solution of the radiation belt transport equation with and without accounting for electron synchrotron energy losses are compared. It is demonstrated that synchrotron losses play an important role in formation of the space and energetic distributions of electrons in the inner magnetosphere.     

X-ray, γ-emission and energetic particles in near-Earth space as measured by CORONAS-F satellite: From maximum to minimum of the last solar cycle

The Russian solar observatory CORONAS-F was launched into a circular orbit on July 31, 2001 and operated until December 12, 2005. Two main aims of this experiment were: (1) simultaneous study of solar hard X-ray and γ-ray emission and charged solar energetic particles, (2) detailed investigation of how solar energetic particles influence the near-Earth space environment. The CORONAS-F satellite orbit allows one to measure both solar energetic particle dynamics and variations of the solar particle boundary penetration as well as relativistic electrons of the Earth’s outer radiation belt during and after magnetic storms. We have found that significant enhancements of relativistic electron flux in the outer radiation belt were observed not only during strong magnetic storms near solar maximum but also after weak storms caused by high speed solar wind streams. Relativistic electrons of the Earth’s outer radiation belt cause volumetric ionization in the microcircuits of spacecraft causing them to malfunction, and solar energetic particles form an important source of radiation damage in near-Earth space. Therefore, the present results and future research in relativistic electron flux dynamics are very important.     

大型空间折展机构简化模型的热传导分析

基于热传导效应和等效的热学参数,对空间伸展臂结构及所处热环境进行了简化处理。将边界温度条件进行傅里叶级数展开,利用一维热传导模型,分别计算了在边界上施加各阶展开分量作用时,简化伸展臂结构的瞬态温度响应,进而利用叠加原理得到了结构总体的近似热响应。建立了伸展臂热学分析的有限元模型,计算了伸展臂在相应条件下的动态热响应,验证了本文分析结果的合理性,为伸展臂结构的分析设计尤其是概念设计阶段的热性能评估提供了一定的工程参考。      

PBGA封装芯片热环境适应性仿真分析

针对多数塑料焊球阵列（PBGA）封装芯片仅依据美军MIL标准进行高低温交变测试致使预测服役寿命偏差较大的情况，将微控制芯片任务状态时间谱转化为环境温度载荷谱，在综合考虑热传导、热对流的情形下，利用icepak完成芯片热电耦合仿真分析，并借助于Transient Thermal及Transient Structural完成芯片结温的获取及焊点应力、应变的计算求解。同时，依据Arrhenius模型及修正Coffin-Manson热疲劳模型分别预测芯片本身及焊点的寿命，从而实现对其热环境适应能力的定量分析。仿真结果表明:芯片的预测寿命约为6.26年，寿命预测偏差约为13.4%，符合GJB 4239-2001中单个关键环境因素预测寿命偏差标准，能够较为精确地反映其热环境适应性。