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.     

电离层舒曼共振的空间观测

通过功率谱分析和波阻抗函数计算,本文证实了Aureol 3卫星在电离层高度上(>600km)观测到的极低频(ELF)波场扰动是和舒曼共振相关的电磁振荡.与舒曼共振地面观测相比较,Aureol 3观测到的舒曼共振电场分量具有很好的谐振谱结构,峰值频率和各阶舒曼共振本征频率对应;磁场分量的高阶峰值频率偏离14, 20, 26Hz等舒曼共振本征频率;随着卫星高度的改变,电场与磁场谐振的一阶最大能量峰值并不会发生在同一频率,结合本文分析的数据,分别位于78Hz和10Hz;水平方向的磁场分量更接近南北方向的线极化而不是地球－电离层空腔中的椭圆极化;波阻抗随频率表现出不太规则的准正弦振荡,它会随着频率增加和飞行高度上升呈现减小的趋势.虽然舒曼共振信号和电离层密度梯度间的非线性作用可以解释舒曼共振空间观测的部分特征,但需加入其他机制,如电离层不稳定性,传播模式的耦合,进一步了解电离层高度上舒曼共振各种特征产生的原因.     

中、低纬度地区电离层不同层结间的耦合对夜晚F区不规则结构的影响

电离层不规则结构的形成和演化与电离层等离子体不稳定性密切相关 .长期以来 ,在中低纬区扩展 -F现象的研究中 ,没有考虑电离层上下层结之间的相互作用 .本文从理论上全面探讨了在中纬度地区 ,E区可变的Pedersen电导率和Hall电导率 ,与F区可变的Pedersen电导率的共同作用对F区梯度漂移不稳定性的影响 ,导出了存在这种耦合时电离层梯度漂移不稳定性的统一表达式 .这一耦合理论不但解释了实际观测中发现的 ,在某些地方电离层F区顶部的不稳定性发生率要高于F区底部这一现象 ;同时还表明 ,电离层E区与F区的耦合对F区夜晚梯度漂移不稳定性的形成不仅会有阻碍作用 ,同时还使得中、低纬度地区的扰动增长具有了方向的选择性 .该理论的一个重要结论是 ,F区中某一地区能否发展出梯度漂移不稳定性 ,并不完全由当地电离层F区的状态决定 ,同一根磁力线连接的、位于不同纬度地区的E区层结对其发展和演化也会有相当大的影响 .     

Alfven波在低纬电离层中的传播研究

Alfven波在低纬地区电离层的传播有其特殊性,一方面,低纬地区同样存在Alfven速度梯度的巨大变化,导致电离层Alfven谐振器(Ionospheric Alfven resonator, IAR)的形成;另一方面,由于在低纬地区磁倾角很小,所以剪切Alfven波在传播的过程中纬度方向跨度很大,不同纬度电离层参数将共同对其产生影响;并且,由于电离层水平分层,故磁力线与电离层不正交.本文选取双流体力学模型,在忽略场向电场的条件下,利用非正交坐标系,结合IRI07模型与MSISE00模型模拟低纬地区Alfven波的传播,得到其反射及耦合特性.结果表明,低纬地区同样存在电离层Alfven谐振现象,由耦合产生的压缩模有向磁赤道方向传播的趋势,夜间电离层状态相对于白天更适合IAR的形成,谐振频率沿磁力线L值增大单调递增.     

不同太阳活动及地磁条件下的电导率分布变化

电离层电导率在不同的太阳活动和地磁条件下会发生变化. 本文通过中性大气经验模式NRLMSISE_00（Neutral Atmosphere Empirical Model_2000,简称NRLMSISE_00）和电离层经验模式IRI_2001（International Reference Ionosphere_2001,简称IRI_2001）计算电离层的电子、离子碰撞频率以及电导率,并简要讨论了120 km和300 km高度上的电导率在不同季节、不同太阳活动和地磁指数下的经纬分布. 结果显示,电导率的分布与日照密切相关,且随太阳活动的变化而变化. 磁暴时电导率随地磁活动的变化相对于随太阳活动的变化要小,在120?km高度,磁暴期间电导率在低纬地区和高纬地区发生不同变化,且Pedersen电导率和Hall电导率变化趋势相反,向两极靠近,电导率变化幅度略有增长;在300?km高度上,磁暴对低纬地区和高纬地区电导率的影响要比120?km处大,Pedersen电导率和Hall电导率变化趋势相同,且越向两极靠近电导率的变化幅度越大.     

磁尾等离子体片区氧离子丰度对离子剪切流的开尔文-赫姆霍茨不稳定性的影响

考虑等离子体片区存在源于电离层的氧离子,研究了离子剪切流在低频表面波扰动情况下的开尔文-赫姆霍茨(K-H)不稳定性.在氧离子流与质子流具有近似相同的宏观流速假定下,采用磁流体力学(MHD)近似,并且考虑在磁场方程中保留速度场涡量项,推导出沿磁场方向传播的表面波线性扰动的色散关系.在等离子体片边界层区,发现随着氧离子相对丰度的增加,产生K-H不稳定性的最大临界扰动波长可增大到20RE(地球半径).对于给定的氧离子丰度,临界剪切相对扰动波长的变化存在一个最小值.氧离子丰度越高,最小临界剪切值越小,对应的扰动波长(称最不稳定波长)也越长.高氧离子丰度的不稳定性增长率随速度剪切增加而增加,快于低氧离子丰度.不稳定性增长率随速度剪切增加的最大饱和值接近对应的离子回旋频率.在地磁活动期间,由等离子体片中氧离子丰度增加而增大的沿磁场传播的表面波不稳定性对于理解低频磁脉动事件和磁层亚暴过程也有着十分重要的意义.     

极区Hall电流经度差异特征的统计学研究:CHAMP卫星观测

本文利用2001至2010年间CHAMP(CHallenging Minisatellite Payload)卫星标量磁力仪(Overhauser Magnetometer)观测的磁场数据,反演得到电离层霍尔(Hall)电流,并且对极区电离层Hall电流的特征进行了统计学研究.研究主要关注平静期,即重联电场小于2 mV·m~(-1)条件下,在磁纬60°至90°范围内的Hall电流在不同太阳活动、季节、磁经度、磁地方时等条件下的变化特征.研究发现:Hall电流具有明显的经度差异,在南北半球呈现显著一波经度结构,而且南北半球反相,即北半球电流密度呈现峰-谷-峰结构,而南半球呈现谷-峰-谷结构.Hall电流密度的经度结构与太阳活动紧密相关,太阳活动高年经度差异最大,太阳活动中年经度差异次之,太阳活动低年经度差异最小.研究发现,在磁地方时为10-14MLT的白天,影响Hall电流的因素主要是太阳辐射;而磁地方时为21-03MLT的夜晚,除了电导率的影响之外,可能存在其他的重要的物理过程影响着Hall电流的经度分布.本文还研究了与电流相关的焦耳热的经度分布情况,发现其在南北半球分别呈现单峰、单谷结构,经度差异亦十分明显.     

中纬电离层的低频等离子体不稳定性

本文研究了中纬电离层Rayleigh-Taylor不稳定性和漂移不稳定性的基本性质.如果不存在中性风和背景电场,中纬Rayleigh-Taylor不稳定性的增长率比赤道相应条件下的小.中性风和电场对中纬Rayleigh-Taylor不稳定性有重要影响.当中性风速度达到10m/s时,其作用就超过了重力.漂移不稳定性也受到重力、中性风和电场的影响,离子-中性粒子碰撞能降低漂移不稳定性的增长率.本文的分析表明,在赤道电离层容易产生大尺度的Rayleigh-Taylor不稳定性,在中纬电离层易产生小尺度的漂移不稳定性.     

大气重力波产生的大尺度赤道电离层扰动

本文研究了大气重力波产生的大尺度赤道电离层扰动的性质．当重力波的传播方向与磁场方向倾斜相交时，重力波在Ｆ区产生行进电离层扰动．当重力波垂直于磁场传播时，能触发等离子体Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性，形成大尺度赤道扩展Ｆ不均匀体．重力波引起的扩展Ｆ主要出现于晚上，行进电离层扰动则可能出现于任何时间．本文建立了行进电离层扰动和大尺度赤道扩展Ｆ的统一理论模型，深入全面地揭示了电离层扰动的性质．     

暴时场向电流及磁层—电离层耦合过程

本文综述了不同类型电离层场向电流(亦称为Birkeland电流)及其在地磁扰动(磁暴和亚暴)期间的时空分布特征.首先简述了场向电流近70年的研究历程,总结了各种与太阳风—行星际磁场(interplanetary magnetic field,IMF)、电离层电导率空间分布不均匀相关的1区和2区场向电流,北向、晨昏向IM...     

用传播矩阵法研究层状交错地层中的多分量感应测井

为了实现交错沉积等复杂环境中的电磁场数值模拟,本文在常规横向同性模型的基础上引入了电导率主轴坐标系相对地层坐标系的层理方位角和倾角,建立了交错地层模型.并利用传播矩阵法建立了一维层状交错地层模型中的多分量感应测井仪器响应的正演模拟算法.首先将频率-波数域中的电磁场分解为上行和下行模式波,给出了任意朝向的磁偶极子在无限大地层中模式波的解析解.进一步通过引入地层界面上的透射、局部反射以及广义反射系数矩阵,推导了一维层状地层中的模式波表达式.在此基础上,利用二维Gauss-Legendre积分实现了Fourier逆变换,得到了可用于多分量感应测井模拟的频率-空间域磁场并矢格林函数.最后,通过多个数值模拟结果考察了井眼倾角、层理方位角和倾角变化对多分量感应测井响应的影响.     

赤道电离层R-T不稳定性发展的控制因素分析

本文从质量和电荷守恒方程出发,分析了控制电离层等离子体R-T不稳定性线性增长的各种因素,重点研究了热层风和背景电场的空间梯度对R-T不稳定性线性增长的影响.结果表明,热层风场和背景电场两者的空间梯度对R-T不稳定性线性增长有不可忽视的促进或抑制作用;对R-T不稳定性线性增长起促进作用还是起抑制作用,依赖热层风场和背景电场及其空间梯度的方向;对R-T不稳定性线性增长影响的显著程度主要依赖于热层风场和背景电场两者空间梯度的大小.数值计算结果表明,对典型的背景电离层条件,磁力线顶点高度为330 km时,对线性增长率的影响最高达到120%.     

Excitation of Alfvén waves and vortices in the ionospheric Alfvén resonator by modulated powerful radio waves

A review of the artificial excitation of Alfvén waves and vortices in the ionospheric Alfvén resonator (IAR) is presented. In the framework of simplified models of the IAR and the Alfvén vortex instability, we discuss the main physical phenomena arising under the periodic heating of the ionosphere by a powerful HF radio signal with a modulation frequency F which lies in the range of short-period geomagnetic pulsations (F = 0.1–10 Hz). The amplitudes, frequency spectra, and polarization characteristics of artificial pulsations on the ground are found, and a brief comparison with experimental data is made. The Alfvén vortex instability in the IAR is analysed from the point of view of its artificial triggering. Two ways of such a triggering are discussed. The first suggests the use of two spaced transmitter antenna which produce an ionospheric current being in resonance with Alfvén vortices. Existing heating facilities are suitable for this experiment. The second method is based on the change of macroscopic parameters of the ionosphere, such as the conductivity in the instability region. This method is simple, but requires more powerful heaters.     

Seasonal and solar cycle variations of the auroral electrojet indices

Using hourly mean auroral electrojet indices for the past 20 years, we examine the seasonal and solar cycle variations of the AU and AL indices as well as the smaller time-scale fluctuations in these indices. The AU and AL indices maximize during summer and equinoctial months, respectively. By removing the effects of the solar conductance from the AU index, it is found that the electric field contribution to the AU index exhibits the same semiannual variation pattern as the AL index, indicating that the semiannual magnetic variations are controlled by the electric field. Since the auroral electrojets are mostly Hall currents flowing in the east–west direction, the fluctuations of the auroral electrojet indices can be interpreted in terms of fluctuations in the north–south component of the electric field and the Hall conductance. The AU fluctuation is largely due to that of the electric field, while the AL fluctuation is attributed to both the electric field and Hall conductance with their contributions being comparable. The high fluctuation of AL compared to that of AU is attributed to particle precipitation associated with substorm activity. However, the fluctuations of the electric field and conductance do not show any noticeable seasonal dependence. The variation pattern of the yearly mean AL index follows the mirror image of the AU index during the past 20 years, indicating that the absolute values of the two indices are proportional to each other. This suggests again that the electric field is the main modulator of magnetic disturbance. On the other hand, they show a tendency to become higher during the declining phase of the solar cycle. This is the same variation pattern confirmed from the aa index. However, the fluctuations of the electric field and the Hall conductance do not show any apparent dependence on the solar cycle.     

Hall effect on magnetic reconnection at the Earth's magnetopause

We analyze in situ observations of magnetic reconnection at the Earth magnetopause to estimate the importance of the Hall current during the merging of interplanetary and magnetospheric magnetic field lines. The reconnection process is studied through numerical simulations, integrating the Hall MHD equations in 2.5 dimensions. A large influence of the Hall effect is found, which can be measured by a significant increase of the reconnection rate.     

Possible role of magnetosphere-ionosphere coupling in auroral arc generation

Three models for the magnetosphere-ionosphere coupling feedback instability are considered. The first model is based on demagnetization of hot ions in the plasma sheet. The instability takes place in the global magnetosphere-ionosphere system when magnetospheric electrons drift through a spatial gradient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetospheric electrons move earthward through a radial gradient of hot ions. This leads to the formation of field-aligned currents. The effect of upward field-aligned current on particle precipitation and the magnitude of ionospheric conductivity leads to the instability of this earthward convection and to its division into convection streams oriented at some angle with respect to the initial convection direction. The growth rate of the instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such arcs, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow arcs. For such arcs another type of instability must be considered. In the other two models the instability arises because of the generation of Alfven waves from growing arc-like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfven wave. The other model takes into consideration the reflection of Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000 km. The growth of structures in both models takes place when the ionization function associated with upward field-aligned current is shifted from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different from the E × B drift. Although both models may work, the growth rate for the model, based on the modulation of the precipitating accelerated electrons, is significantly larger than that of the model based on the Alfven wave reflection. This mechanism is suitable for generation of auroral arcs with widths of about 1 km and less. The growth rate of the instability can be as large as 1 s^-1, and this mechanism enables us to justify the development of auroral arcs only in one ionosphere. It is hardly suitable for excitation of wide and conjugate auroral arcs, but it may be responsible for the formation of small-scale structures inside a wide arc.Polar Geophysical Institute, Apatity, Russia     

NUMERICAL MODELLING OF A NEW MULTI-FREQUENCY ELECTROMAGNETIC SYSTEM*

The amplitude of the horizontal magnetic field in the ground between two parallel wires, both carrying an alternating current in the same direction, is likely to have a saddle point if the separation between the wires is small and the frequency is low. The amplitude has a maximum in the vertical direction and a minimum in the horizontal. Rectangular geological structures in the ground which are centered between the wires have a varying effect on the magnetic fields at the surface. In general, the vertical magnetic field “crosses over” at the center of the structure. A shallow and flat lying conductor displays a broad flat type of profile when the horizontal magnetic field between the wires is measured. Changing the structure to a narrower but more conducting one at depth will provide a more pointed but still broad profile. The phase of the horizontal field is also increased. When the structure is a thin vertical dyke, the amplitude of the horizontal magnetic field anomaly due to the dyke rapidly decreases as the depth of the dyke is increased. The phase of the horizontal field is less sensitive to changes in depth of the dyke but is more sensitive to the conductivity ratio of the dyke and the half-space. The amplitude of the vertical magnetic field anomaly due to the dyke is only slightly influenced by conductivity contrast or the depth of the dyke. The phase of the vertical magnetic field, however, is strongly influenced by the conductivity contrast, particularly if the conductivity frequency product is greater than hundred. In essence, the field behaves like that of the conventional vertical loop source, but the fields are uniform over much larger areas. This suggests the possibility of using dip angle measurements for rapid reconnaissance.