同步轨道相对论电子通量长期倒空事件的统计研究

地球辐射带中有"杀手"电子之称的相对论电子通量增强和损失过程一直是空间物理学和空间天气学研究的热点.本文通过对20002016年间,地球同步轨道相对论电子通量降低至背景通量水平并持续时间长达3天以上这一特殊现象进行了相关统计研究.从事件的时间分布角度,本文研究了约1.5个太阳活动周内共62例事件随太阳活动水平高低的分布情况,结果表明:在太阳活动周下降期有较少的事件发生,而在峰年、谷年这类事件的发生率与太阳活动水平的高低并没有直接联系.随后,我们对这62例事件在开始、持续、结束三个阶段分别做了一些相关参数的统计,探讨相对论电子通量长期倒空事件的客观规律和产生机制.研究结果表明:事件发生前,太阳风动压、密度的显著增加引起磁层顶向内收缩,等离子体层顶一直维持在高L区域,IMF Bz分量南向和磁暴过程使相对论电子通量通过绝热和非绝热等物理损失机制降至背景通量水平.当这些相对论电子达到背景通量水平后,较弱的太阳风条件和地磁活动水平不足以提供充分的可以使相对论电子通量增长的源;虽然有些相对论电子通量长期倒空事件期间存在中、小磁暴过程,但这些强度较弱的磁暴很可能不会显著地影响同步轨道相对论电子损失和增长的动态平衡,因此相对论电子仍然可以维持在背景通量水平.如果有长时间的亚暴活动和高强度的ULF (Ultra-Low Frequency)波活动发生,太阳风速度显著增加,那么这些物理过程能提供足够的种子电子和持续的加速条件,使得相对论电子通量打破倒空状态,进而呈现显著增长趋势.     

行星际扰动和地磁活动对GEO相对论电子影响

利用1988—2010年小时平均的GOES卫星数据,对地球同步轨道(GEO)相对论电子变化进行了统计分析,研究了相对论电子通量(Fe)增强事件的发展过程,探讨了利于相对论电子通量增强的太阳风和地磁活动条件.主要结论如下:(1)GEO相对论电子通量即使是峰值,也具有明显的地方时特性,最大电子通量出现在磁正午时.午/夜电子通量比率随着太阳风速度(Vsw)增加而增大;在Dst-50nT时相对论电子具有规则的地方时变化.在太阳活动下降相,电子通量与各参数的相关性较好,与其相关性最好的Vsw、Kp指数以及三次根号下的太阳风密度(N)分别出现在电子通量前39~57h、57~80h和12~24h.(2)强(日平均电子通量峰值Femax≥104 pfu)相对论电子事件,在距离太阳活动谷年前两年左右和春秋分期间发生率最高,较弱(104Femax≥103 pfu)事件无此特点;大部分强相对论电子事件中,电子通量在磁暴主相开始增加,较弱事件中则在恢复相开始回升.(3)太阳风密度变化对相对论电子事件的发展具有重要指示作用.电子通量在太阳风密度极大值后0~1天达到极小值,太阳风密度极小值后0~2天达到极大值.(4)90%以上相对论电子事件是在磁暴及高速太阳风的条件下发生的,与其伴随的行星际参数和地磁活动指数极值满足以下条件:Vswmax516km/s,Dstmin-31nT,Nmin2.8cm-3,Nmax14.1cm-3,Bzmin-2.9nT,AEmax698nT.(5)磁暴过程中,Dstmin后日平均电子通量大于103 pfu的发生概率为53%左右,强/弱相对论电子事件占总数比例分别为36%/64%左右,磁暴强度对其无影响.磁暴过程中的Vsw、N和AE指数大小对于能否引起相对论电子增强起着指示作用.     

廊坊、富克地磁台观测数据分析

通过将廊坊、富克地磁台站数据与国内地磁台站十三陵、三亚台以及国际标准地磁台站MMB台的观测数据对比分析,发现各地磁台站数据变化幅度相近、相关性高,对地磁暴有明显响应,且磁暴期间各时段扰动幅度相近。同时,以高质量地磁数据为基础,采用快速傅里叶变换,对廊坊与富克地磁台站的地磁观测数据开展地磁脉动提取,研究发现,两个台站在地磁暴期间均能观测到明显的Pi1、Pi2、Pc5地磁脉动,且Pc5的持续时间、平均强度、变化幅度和Pi1的持续时间、变化幅度及Pi2的变化幅度均有明显增加,为今后开展地磁脉动与高能电子增强事件相关性研究奠定了基础。     

磁层相对论电子通量变化与磁暴/亚暴的关系

本文分析了1 AU处的行星际磁场、太阳风速度、Kp指数、Dst和AE的变化关系,以及它们和地球同步轨道附近相对论电子通量的变化关系.分析说明,当行星际磁场Bz分量出现南向扰动和太阳风速度增大超过500 km/s时,地球磁层中常常发生磁暴/亚暴活动.在磁暴主相期间,相对论电子(能量E≥1 MeV)通量下降;而在磁暴恢复相期间,相对论电子通量恢复上升.但是,只有在伴随有高强度（AE≥500 nT）的持续性亚暴活动的磁暴恢复相期间,相对论电子的通量才能增长到超过暴前通量值,且能量低于300 keV的亚暴电子的通量越高,相对论电子的通量越高,反之则越低.亚暴注入电子数的多少很大程度上决定了磁暴恢复相期间相对论电子数的多少,这说明亚暴活动注入能量低于300 keV的亚暴电子是磁层相对论电子的一个重要来源.     

1998年4-5月地球同步轨道MeV电子通量增强事件

采用GOES9卫星观测的能量大于2MeV和大于4MeV电子通量和行星际飞船ACE太阳风参数的高时间分辨率资料,以及磁暴指数Dst资料,分析了1998年4-5月期间地球同步轨道电子通量增强事件的时间和能量响应特征及其与行星际太阳风参数、磁暴和亚暴等扰动条件的对应关系.结果表明,地球同步轨道相对论性（MeV）电子通量增强事件有明显的周日变化,中午极大和午夜极小.4月22日和5月5日开始的两次大事件中,能量大于2MeV电子通量中午极大值上升到最大值的时间尺度分别约为4天和1天,中午极大值高于背景水平的持续时间分别为13天（4月22日-5月4日）和16天（5月4日-20日）以上.每次MeV电子通量增强事件的能量范围不完全相同.两次大事件的上升段都对应于磁暴的恢复相,与太阳风动压脉冲、高速流脉冲和负Bz分量关系密切.     

地球磁层对磁云边界层的大尺度响应分析——个例研究

主要分析了WIND飞船2004年11月9日探测的磁云边界层引起的大尺度地球磁层活动.磁层响应主要包括以下3个方面:(1)磁云边界层内本身持续较强南向磁场驱动了一个强磁暴的主相.(2)由于磁云边界层内部较强南向磁场持续一段时间后发生向北偏转触发了一个典型磁层亚暴.文中详细分析了亚暴膨胀相发生时夜侧磁层各区域的观测现象,包括极光观测、高纬地磁湾扰、地球同步轨道无色散粒子注入现象、Pi2脉动突然增强以及等离子体片偶极化现象等.(3)磁云边界层和前面鞘区组成一个动压增强区,此动压增强区强烈压缩磁层,致使磁层顶进入地球同步轨道以内;当磁云边界层扫过磁层时,位于向阳侧地球同步轨道上的两颗GOES卫星大部分时间位于磁层磁鞘中,以致很长时间内直接暴露在太阳风中.利用Shue(1998)模型计算得到当磁云边界层扫过磁层时磁层顶日下点的位置被压缩至距地心最近距离为5.1RE,磁云边界层的强动压结构以及强间断面决定了磁云边界层对磁层的强压缩效应.强动压结构、多个强间断结构以及持续较长时间的强南向磁场是许多磁云边界层的共性,这里以此磁云边界层事件为例分析了磁云边界层的地球磁层响应.     

2000年7月14日太阳高能粒子事件引起地球同步轨道区相对论电子通量巨幅增加

2000年7月14日10:24UT一个X5.6级的耀斑暴发生在太阳中心子午线附近(AR 9077), 同时伴随着一个朝向地球的CME事件及太阳高能粒子(Solar Energetic Particle, SEP)事件. 这次耀斑暴发及CME事件引起了地球磁层、电离层及高层大气的强烈扰动. 中国“风云二号(B)”卫星上的高能粒子探测器(EPD)观测到SEP事件期间, 同步轨道区高能质子、相对论电子有非常剧烈的增加. SEP期间, 高能质子对相对论电子通量的探测造成严重的污染. 结合“风云二号(B)”卫星上的高能粒子探测器(EPD)的特性, 建立了一种从相对论电子通量探测中“清除”高能质子“污染”的方法, 并对相对论电子通量的探测数据实施“清洁”处理. “纯净的”相对论电子通量探测结果显示, 当行星际磁场南向时, 上游太阳风中的高能电子使同步轨道区相对论电子通量有大幅度的增加.     

地球磁层对磁云边界层的大尺度响应分析——个例研究

主要分析了WIND飞船2004年11月9日探测的磁云边界层引起的大尺度地球磁层活动．磁层响应主要包括以下3个方面：（1）磁云边界层内本身持续较强南向磁场驱动了一个强磁暴的主相．（2）由于磁云边界层内部较强南向磁场持续一段时间后发生向北偏转触发了一个典型磁层亚暴．文中详细分析了亚暴膨胀相发生时夜侧磁层各区域的观测现象,包括极光观测、高纬地磁湾扰、地球同步轨道无色散粒子注入现荆、Pi2脉动突然增强以及等离子体片偶极化现象等．（3）磁云边界层和前面鞘区组成一个动压增强区,此动压增强区强烈压缩磁层,致使磁层顶进入地球同步轨道以内;当磁云边界层扫过磁层时,位于向阳侧地球同步轨道上的两颗GOES卫星大部分时间位于磁层磁鞘中,以致很长时间内直接暴露在太阳风中．利用Shue（1998）模型计算得到当磁云边界层扫过磁层时磁层顶日下点的位置被压缩至距地心最近距离为5．1RE,磁云边界层的强动压结构以及强间断面决定了磁云边界层对磁层的强压缩效应．强动压结构、多个强间断结构以及持续较长时间的强南向磁场是许多磁云边界层的共性,这里以此磁云边界层事件为例分析了磁云边界层的地球磁层响应．     

2005年8月24日磁暴主相期间亚暴过程的多卫星联合观测分析

本文根据OMNI、TC-2卫星、LANL系列卫星、Cluster星簇卫星(C1-C4)以及加拿大的8个中高纬地磁台站的观测数据,研究了2005年8月24日强磁暴(SYM-H_min~ -179 nT)主相期间的强亚暴(AL_min~ -4046 nT)事件特征.该强磁暴在大振幅(IMF B_{z min}~ -55.57 nT)、短持续时间(~90 min)的行星际磁场条件下产生,有明显的磁暴急始(SSC),强度较大且持续时间较短.发生在磁暴主相期间的亚暴发展的主要特征如下:亚暴增长相期间,C1-C4卫星先后穿越中心等离子体片;亚暴膨胀相触发后,在近地磁尾(X~-6R_E)可观测到磁场偶极化现象;等离子体无色散注入区在亚暴onset开始后迅速沿经向扩展,但被限制在有限的经度范围;磁纬60°附近,Pi2地磁脉动振幅超过了100 nT.膨胀相开始后,在中、高磁纬地磁台站可观测到负湾扰,近地磁尾可观测到Pi2空间脉动,中磁尾区域可观测到尾向流、磁重联以及O⁺/H⁺数密度比值在亚暴onset之后增大等现象.分析表明该强磁暴主相期间的强亚暴现象发生时序是自内向外:X~-6R_E处TC-2观测到磁场偶极化(~09:42:30 UT),同步轨道卫星LANL1994-084观测到等离子体无色散注入(~09:44:30 UT),X~-17.8R_E处C1观测到磁场重联(~09:45:30 UT),由此推断该亚暴事件很可能是近地磁尾不稳定性触发产生,其发生区域距离地球很近.     

低纬Pi2地磁脉动的多台观测

本文分析了1988年12月和1989年1月在长春、北京、兰州3个台站上观测的地磁脉动资料,研究了3个台站同时记录到的Pi2地磁脉动的频谱特征和偏振特性,进而对低纬Pi2地磁脉动的产生和传播机制做了理论研究.     

The relation between geomagnetic pulsations and an increase in the fluxes of geosynchronous relativistic electrons during geomagnetic storms

The dynamics of the Pc5 and Pi1 pulsation characteristics and relativistic electron fluxes at geostationary orbit were comparatively analyzed for three nine-day intervals, including quiet periods and periods of geomagnetic storms. It was shown that relativistic electron fluxes increase considerably when the power of global Pc5 pulsations and the index of midlatitude irregular Pi1 pulsations increase simultaneously. The correlation between the characteristics of Pi1 and Pc5 geomagnetic pulsations and the level of the relativistic electron flux at geostationary orbit during the magnetic storm recovery phase were studied. It was shown that the correlation coefficient of the relativistic electron maximal fluxes during the magnetic storm recovery phase with the parameter of midlatitude Pi1 pulsations is slightly higher than such a correlation coefficient with the solar wind velocity.     

Prediction of maximal daily average values of relativistic electron fluxes in geostationary orbit during the magnetic storm recovery phase

The relation of the maximal daily average values of the relativistic electron fluxes with an energy higher than 2 MeV, obtained from the measurements on GOES geostationary satellites, during the recovery phase of magnetic storms to the solar wind parameters and magnetospheric activity indices has been considered. The parameters of Pc5 and Pi1 geomagnetic pulsations and the relativistic electron fluxes during the prestorm period and the main phase of magnetic storms have been used together with the traditional indices of geomagnetic activity (A _E, K _p, D _st). A simple model for predicting relativistic electron fluxes has been proposed for the first three days of the magnetic storm recovery phase. The predicted fluxes of the outer radiation belt relativistic electrons well correlate with the observed values (R ∼ 0.8–0.9).     

Indicators of an increase in the flux of relativistic electrons in geostationary orbit during geomagnetic storms

The relation of the fluxes of relativistic electrons in geostationary orbit during magnetic storms to the state of the magnetosphere and variations in the solar wind parameters is studied based on the GOES satellite data (1996–2000). It has been established that, in ～52–65% of all storms, the fluxes of electrons with energies higher than 0.6 and 2 MeV during the storm recovery phase are more than twice as high as the electron fluxes before a storm. It has been indicated that the probability of such cases is closely related to the prestorm level of fluxes and to a decrease in fluxes during the storm main phase. It has been found that the solar wind velocity on the day of the storm main phase and the geomagnetic activity indices at the beginning of the storm recovery phase are also among the best indicators of occurrence of storms with increased fluxes at the storm recovery phase.     

地磁场结构对静止轨道≥2 MeV高能电子分布的影响

基于AE8电子辐射带模式和各地磁场模式,本文系统分析了地磁场模式、太阳风、地磁扰动、地磁轴指向对静止轨道≥2 MeV高能电子分布的影响以及静止轨道不同经度位置≥2 MeV高能电子分布的差异,并与GOES系列卫星实测结果进行了对比分析.结果表明,AE8+IGRF+T96模式所得静止轨道≥2 MeV高能电子分布结果优于AE8+IGRF+OPQ77模式或AE8+IGRF+T89模式结果,其大部分定性结果与GOES系列卫星观测结果较为一致,AE8+IGRF+T96模式所得静止轨道≥2 MeV高能电子分布与磁壳参量Lm、局地磁场B分别呈较好的负和正相关性.基于AE8+IGRF+T96模式得到在相同太阳风及地磁扰动条件下2010年每分钟静止轨道≥2 MeV高能电子通量分布结果,经分析得到：1年内每个时刻静止轨道上≥2 MeV高能电子通量最大值和最小值比值变化范围为2.50~7.51,变化主周期为1天,每天比值变化量都超过3;1年内静止轨道各经度位置每天内≥2 MeV高能电子通量最大值和最小值比值变化范围为2.98~6.00,比值随着时间和经度位置而变化;1年内同一天静止轨道各经度位置≥2 MeV高能电子日积分通量最大值出现在170°W附近,最小值出现在70°W附近,最大值与最小值的比值分布在1.86~2.13之间.以上所得静止轨道≥2 MeV高能电子分布变化主要来自Lm变化,B/B₀的影响小于5%,其中B₀为磁力线上磁场最小值.因此,在构建≥2 MeV高能电子分布模式时,需要考虑地磁场结构的影响,特别是Lm参数.     

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.     

Outer radiation belt of relativistic electrons during the minimum of the 23rd solar cycle

The data on fluxes of electrons with energy Ee > 1 MeV and on radiation doses under the Al shielding of about 2 g/cm² measured on the GLONASS satellite (circular orbit with altitude 20000 km and inclination 65°) for the period from December 2006 through May 2010 are analyzed. The minimum of the 23rd solar cycle turned out to be the longest for all over the space exploration age. Consequently, average semiannual electron fluxes and daily radiation doses are showing the decrease by more than an order of magnitude in comparison with the levels observed in 2007. We present an example of a diffusion wave of relativistic electrons; the wave develops in a period between magnetic storms. This process may result in a significant increase of the radiation dose measured in the orbit, even under the conditions of weak geomagnetic disturbances. The dynamics of variations in relativistic electron fluxes during the magnetic storm of April 5?C6, 2010, is discussed so far as this is the first strong flux enhancement in the 24th solar cycle.     

Variations in the ULF index of daytime geomagnetic pulsations during recurrent magnetic storms

A new index of wave activity (ULF index) is applied to analyze daytime magnetic pulsations in the Pc5 range (f = 2–7 mHz) during ten successive recurrent magnetic storms (CIR (corotating interaction region) storms) of 2006. The most intense daytime geomagnetic Pc5 pulsations on the Earth’s surface in all phases of CIR storms are predominantly observed in the pre-noon sector at latitudes higher than 70°, while those in CME storms (storms initiated by coronal mass ejection (CME)) are observed at latitudes lower than 70°. A comparison of wave activity during CIR and CME storms has shown that the amplitude of Pc5 pulsations in CIR storms is much smaller than that in CME storms and the spectrum maximum is observed at lower frequencies and higher latitudes. At the same time, the mechanism of ULF wave generation during both types of magnetic storms seems to be similar, namely, resonance of magnetic field lines due to the development of the Kelvin-Helmholtz instability caused by an approach of a high-velocity solar wind stream to the Earth’s magnetosphere. Since resonance oscillations are excited only in the closed magnetosphere, the higher-latitude position of the Pc5 pulsation intensity maximum in CIR storms points to larger dimensions of the daytime magnetosphere during CIR storms as compared to CME storms.     

基于NOAA/POES卫星观测的磁层相对论电子起源的初探

本文利用低高度极轨卫星NOAA/POES的观测数据，并结合ACE卫星和Polar卫星的观测结果，研究分析了磁层相对论电子的起源. NOAA/POES卫星对于不同地磁活动时期相对论电子的分布和起源进行了较为详细观测, 分析结果表明（1） 亚暴期间注入磁层的能量电子可以为与磁暴相关的磁层高能电子暴提供种子电子；（2）太阳质子事件期间太阳风中的能量电子也可以为磁层中的相对论电子提供所需要的源.     

Nonlinear character of ion cyclotron waves (Pc1 pulsations) with a spreading dynamical spectrum

New types of Pc1 geomagnetic pulsations with a narrow dynamical spectrum at the onset of an event and with a very broad spectrum during the development of the process have been analyzed. The observed phenomena can be divided into three types according to the evolution of their dynamical spectra: (1) events with a spectrum spreading toward low frequencies, (2) events with a symmetrically spreading spectrum, and (3) events with a spectrum spreading toward high frequencies. All events were mainly observed at local night during the recovery phase of moderate magnetic storms. It has been proposed to interpret spreading Pc1 pulsation spectra as manifestations of ion cyclotron wave nonlinearity.