基于Davis-Smith方法计算卫星磁强计零位补偿的误差特征研究

卫星所携带的磁通门磁强计会随时间发生零位漂移,因此需要对其进行补偿.基于行星际磁场的剪切阿尔芬波动特性,利用Davis-Smith方法可计算磁强计的零位磁场.本文利用数值模拟,分析了总磁场幅度B_T及压缩波动对Davis-Smith方法所产生零位补偿的误差特征.我们发现,B_T越大,零位补偿误差越大;且当压缩波动频率和剪切波动频率不同时,磁场各分量的零位补偿之间的比值与对应背景磁场的比值近似相等;压缩波动的幅度越小,零位补偿的误差趋于更小;此外,压缩波动的频率和初始相位等也会影响零位补偿的误差.这些误差特征可为提高零位补偿的可靠性提供重要的理论依据.     

基于行星际磁场阿尔芬特性的星载磁通门磁强计在轨标定新方法

准确的磁场测量对深入研究空间等离子体环境具有重要意义.星载磁通门磁强计的磁补偿随时间缓慢变化,因此,需要对其进行常规的在轨标定.现有的在轨标定方法都依赖于磁扰动或磁结构事件的筛选,导致标定结果的频次受限于事件的筛选,且差的事件还会增大计算误差.为此,本文研发出一种不依赖于磁场事件筛选的在轨标定新方法.根据行星际磁场强度的取值范围,我们可建构一个磁补偿取值空间.不同的磁补偿值会影响行星际磁场的阿尔芬特性;于是,我们通过在磁补偿空间中找到使被调整之后的行星际磁场的阿尔芬特性最强的点作为磁补偿值的最优解.测试结果表明,我们的新方法可以利用1～4 h时长的行星际磁场数据获得误差小于0.1 nT的标定结果.     

阿尔芬波总磁场存在漂移趋势时磁通门磁强计在轨标定的优化方法

高精度的磁场测量对深空探测具有重要意义,然而卫星上磁通门磁强计的磁补偿值会随着时间缓慢变化,需要进行在轨标定.近期,一种新的在轨标定方法被提出,即Wang-Pan方法.该方法发现阿尔芬波动存在一个线性度高的最优补偿曲线,利用多个波动的最优补偿曲线可计算出磁强计的磁补偿.然而,该方法没有考虑所选波动事件的总磁场存在漂移趋...     

用稳定高效的反Q滤波技术提高地震资料分辨率

地震波在地下传播时受到衰减影响,衰减会导致地震波场高频能量的损失和相位畸变.反Q滤波可补偿大地衰减效应.已有的反Q滤波方法存在下列不足:频率域的算法由于算子长度较长,所以计算效率较低;时间域的算法,或者对地震记录上到时较晚的同相轴进行了过度的补偿,或者为防止过度补偿后来的振幅而在最大增益处进行限制,导致振幅的多解性,而且还会影响滤波器的相位效应.本文给出一种通过直接求解时间域的Q模型方程来进行反Q滤波的算法.由于采用带状矩阵解算器,所以具有较高的计算效率,理论数据和实际地震资料的试算结果证明,本方法对地震波的吸收衰减进行了出色的补偿,提高了地震资料的分辨率.     

FHD分量质子磁力仪标定技术

FHD分量质子磁力仪利用补偿测量方法（Nelson方法）测量水平分量,利用偏置测量方法（Sensor方法）测量磁偏角的相对变化,由于装置存在误差和长期漂移,会对观测数据的长期稳定性产生影响,为了消除或校正这些影响,对观测误差进行分析,提出对相关影响因素进行校正的方法和步骤,并提出观测系统漂移量的判断和估算方法,以及在标定过程中应注意的问题,对FHD质子磁力仪的标定效果和质量起到较好的监控作用.     

慢太阳风中的小冕洞风和阿尔芬波

慢太阳风的起源是一个困扰空间物理领域多年的问题,一些研究人员认为慢太阳风中阿尔芬度比较高的部分起源于小冕洞或者是冕洞的边界,不过另外一些研究者发现高阿尔芬度慢太阳风在成分上和低阿尔芬度的慢太阳风并无显著差别.最近的一项研究中发现的慢太阳风中的小冕洞风并没有特别高的阿尔芬度.我们利用一个简单的二维太阳风模型计算了阿尔芬波和小冕洞风的传播;计算结果显示,由于阿尔芬波的传播特性,从小冕洞出发的等离子体和阿尔芬波动在传播到1AU的过程中会发生分离,形成了低阿尔芬度的小冕洞风和高阿尔芬度的普通慢风.这个结果定性地解释了慢太阳风中观测到的小冕洞风和高阿尔芬度慢风的特性,也为解开慢太阳风的起源之谜提供了新的思路.     

分量质子旋进磁力仪观测误差分析

分析了影响质子旋进磁力仪和分量质子旋进磁力仪观测质量的因素 ,给出了分量质子旋进磁力仪补偿线圈电流允许误差、补偿线圈定向 (特别是线圈磁南北方向 )应满足的关系式和定向允许误差。提出了分量质子旋进磁力仪的使用要点     

分量质子旋进磁力仪定向方法的研究

根据分量质子旋进磁力仪补偿线圈电流允许误差 ,补偿线圈轴定向应满足的关系式和给出的允许误差 ,研究并给出了实现分量质子旋进磁力仪各定向标准的实用方法。     

FHD 磁力仪分量线圈装置误差对磁偏角观测的影响分析

根据FHD 磁力仪观测磁偏角的原理, 计算了分量线圈不水平导致的磁偏角观测值误差,发现分量线圈仅在EW 方向的倾斜对磁偏角观测产生误差。文中将理论分析结果与实际观测结果进行了对比分析, 发现由于分量线圈底座水平的长期漂移, 仪器的垂直分量及磁偏角观测误差也会发生长期漂移, 长期漂移变化的周期约2 ~ 4 个月, 对垂直分量其幅度约3.5~ 11.0nT, 而磁偏角的幅度约0.1′~ 1.0′, 因此认为在将该仪器观测值用作绝对观测值进行地震异常分析时, 应注意回避这种长期漂移变化。     

基于遗传算法的磁通门磁力仪观测数据一致性校正

在对全国地磁台站磁通门磁力仪记录数据的质量效能评估中发现,多个台站存在同台观测仪器的记录数据曲线不一致问题.针对这类问题,研究人员对多种地磁观测数据影响因素进行了定性或定量的分析.本文通过分析格值系数及姿态角对磁通门磁力仪记录数据的影响规律,建立磁通门磁力仪三轴校正模型,构建目标函数,并利用遗传算法对模型进行求解,测定仪器姿态角和格值系数,根据测定结果对记录数据进行一致性校正.数值模拟计算和台站实测实验结果表明:利用该算法解算的格值系数精度优于0.002,姿态角精度优于0.15°.根据解算结果计算,日变幅为50 nT时引入的日变形态不一致误差小于0.13 nT,满足国家地磁台网对地磁相对记录数据日变形态不一致误差的要求.     

Subharmonic oscillations of a forced hydromagnetic cavity

Abstract

We study the propagation of nonlinear MHD waves in a highly magnetized plasma cavity. The cavity's moving boundaries generate Alfvén waves, which in turn drive and interact with slow magnetosonic waves. The interacting wave system is analyzed by a Galerkin and multiple-scale analyses leading to simple dynamical equations. When the frequency of the forcing provided by the moving boundaries and that of the fundamental Alfvén eigenmode are close, the cavity behaves like a Duffing oscillator. Application of the Melnikov function theory shows that the Alfvén wave's amplitude undergoes both flip and saddle-node bifurcations as the amplitude and the phase of the boundary forcing vary. Direct numerical integration confirms these results and provides an estimate of the amount of energy dissipated in the bifurcations.     

Formation of accelerated ion flows in Alfvén disturbances of the magnetotail

This paper shows that there exists a mechanism of longitudinal plasma acceleration which is inherent in the process of the process of resonant conversion of a fast magnetosonic wave freely propagating along the magnetic field into an Alfvén wave. This mechanism is caused by the Ampere force arising due to the interaction between the poloidal component of the current of the compressible disturbance and the generated toroidal disturbance. It is shown that plasma acceleration takes place at the stage of increase in the Alfvén wave amplitude and that the accelerated flow retains its velocity when the process of resonant conversion is over. We describe spatiotemporal structures of plasma flows arising with the transformation of fast magnetosonic waves into Alfvén waves. An interpretation of the presence of fast ion flows in the magnetotail as a consequence of the action of the plasma acceleration mechanism considered in this work is proposed.     

Nonlinear development of phase-mixed alfvén waves

Abstract

We derive an equation governing the nonlinear propagation of a linearly polarized Alfvén wave in a two-dimensional, anisotropic, slightly compressible, highly magnetized, viscous plasma, where nonlinearities arise from the interaction of the Alfvén wave with fast and slow magnetoacoustic waves. The phase mixing of such a wave has been suggested as a mechanism for heating the outer solar atmosphere (Heyvaerts and Priest, 1983).

We find that cubic wave damping dominates shear linear dissipation whenever the Alfvén wave velocity amplitude δv_y exceeds a few times ten metres per second. In the nonlinear regime, phase-mixed waves are marginally stable, while non-phase-mixed waves of wavenumber k_a are damped over a timescale k_uRe ₀|δ v_y/v_A |^?2, Re ₀ being the Reynolds number corresponding to the Braginskij viscosity coefficient η₀ and v_A the Alfvén speed. Dissipation is most effective where β = (v_s /v_A) ² ≈ 1, v_s being the speed of sound.     

Resonance properties of Psi5/Psc5 in geostationary orbit

It has been indicated that the magnetic field variations, registered by the geostationary satellites in the range 150–600 s after 14 SI/SC events, include transverse wave components that show the spectral—polarization properties of standing Alfvén waves. The amplitude of these waves increases with increasing pulse amplitude, the period increases with decreasing magnetic field magnitude, and the rotation direction and magnetic field vector orientation reverse their signs near the noon and midnight meridians.     

Thermal Generation of Alfvén Waves in Oscillatory Magnetoconvection: Diffusively Modified Modes

Thermal instabilities in the form of oscillatory magnetoconvection representing diffusively modified Alfvén waves in an electrically-conducting Bénard fluid layer of rigid walls in the presence of a vertical magnetic field are investigated. Emphasis of the article is on the transition from a nearly undamped Alfvén wave to diffusively modified Alfvén waves, and on the effect of physically realisable magnetic field boundary conditions on magnetoconvection. It is found that the extra magnetic dissipation in the magnetic Hartmann boundary layers can enhance oscillatory magnetoconvection in the form of strongly modified Alfvén waves. Oscillatory magnetoconvection produced solely by the Alfvén wave mechanism can be the most unstable mode even in the presence of a strong viscous effect. This article also represents the first study on the effect of an electrically conducting wall on magnetoconvection which is associated with a nonlinear eigenvalue problem. We find that the electrically perfectly conducting condition does not yield a good approximation for magnetoconvection with an electrically highly conducting wall. The size of oscillation frequency with an electrically highly conducting wall can be more than a factor of 2 larger than that obtained using the perfectly conducting condition.     

Interaction between the Alfvén wave and turbulent sheet

The interaction between the Alfvén wave and turbulent sheet (TS) with an anomalous conductivity has been considered. High frequency turbulence causes the appearance of not only anomalous field-aligned plasma conductivity but also cross-field conductivity. Alfvén waves can be partially reflect from TS, be absorbed in this sheet, and pass through TS. When field-aligned conductivity is predominant, the relative effectiveness of these processes strongly depends on a cross-field wave scale. If TS is thin as compared to the Alfvén wavelength, the resistive Alfvén wave (λ _A ) characterized by the field-aligned resistivity and Alfvén velocity above the sheet is the characteristic parameter responsible for the wave-sheet coupling. A comparison of the loss, estimated using the analytical relationships for a thin sheet and numerically calculated based on the complete formulas for a sheet with a finite thickness, indicates that the approximation of a thin sheet results in reasonable estimates at all wave scales except very small ones. The developed model has been applied to the interpretation of the results of the works on Pi2 pulsation damping during the substorm expansion phase, which indicated that the damping decrement increases at large substorm amplitudes. The estimates indicate that this increase in damping is related to the appearance of anomalous resistivity in the case when field-aligned currents exceed the threshold values necessary for excitation of high frequency turbulence.     

Dependence of the Pc4 magnetic pulsation parameters on the radiated power of the EISCAT HF heating facility

The interaction between the Earth’s ionosphere and magnetosphere in a situation when artificial disturbances are generated in the F region of the auroral ionosphere with the EISCAT/Heating facility is studied. An experiment was performed in the daytime when the facility effective radiated power changed in a stepwise manner. Wavelike disturbances with periods of (130–140) s corresponding to Pc4 pulsations were simultaneously registered by the method of bi-static backscatter and with ground magnetometers. The variations in the Doppler frequency shift were correlated with the changes in the facility power. Incoherent scatter radar measurements at a frequency of 930 MHz (Tromsö) and numerical calculations were used in an analysis. It has been indicated that the ionospheric drift of small-scale artificial ionospheric irregularities was modulated by magnetospheric Alfvén waves. The possible effect of powerful HF radioemission on the Alfvén wave amplitude owing to the modification of the magnetospheric resonator ionospheric edge reflectivity and the generation of an outgoing Alfvén wave above the region where the ionospheric conductivity is locally intensified has been considered.     

Alfén waves in a thermally stratified fluid

Abstract

The propagation of Alfvén waves along a uniform horizontal field in a highly conducting incompressible fluid, subject to the convective forces produced by a uniform vertical temperature gradient, is treated in a Boussinesq approximation. It is shown that there are exact solutions with large amplitude but restricted form. Their restricted form means that an arbitrary disturbing force produces other motions as well as Alfvén waves. An arbitrary initial disturbance of small amplitude produces waves whose state of polarization varies along the direction of propagation. For large amplitudes, however, any mixtures of polarization states causes scattering into new modes.     

Space-time structure of Alfvén resonant disturbances generated by transversally localized FMA wave

This study describes the space-time evolution of the Alfvén resonant disturbance generated by a transversally localized fast magnetoacoustic (FMA) wave induced by impulse action on its localization area. We determine the conditions for the formation of qualitatively different space-time structures of Alfvén resonant disturbances under different relationships between parameters characterizing both dispersion/absorption of Alfvén disturbance and resonant absorption of transversally localized FMA waves. The spatial structures of Alfvén resonant disturbances and their time evolution are described analytically.