Reversal of Heliospheric Magnetic Field Polarity: Theoretical Model

A simple analytical model of the reversal of the heliospheric magnetic field is suggested. The shape of the heliospheric current sheet is found for each instant of time using a kinematic approximation. Calculation results are illustratively presented in graphic and animated forms, showing a 3-D dynamic picture of the reversal of the heliospheric magnetic field throughout a 22-year solar cycle.     

Effects of the Polarity States of the Heliospheric Magnetic Field and Particle Drifts in Cosmic Radiation

Forbush decrease (FD) events recorded at the ground-based neutron monitors (NMs) during the period 1961 – 1999, have been selected and recovery characteristic of these events have been analyzed. The average profile of FDs observed during different polarity states of the heliosphere is obtained by superposed epoch analysis separately for the periods 1961 – 1969 (A < 0), 1971 – 1979 (A > 0), 1981 – 1989 (A < 0) and 1991 – 1999 (A > 0). Hourly count rate of neutron monitors of different cut-off rigidities have been utilized. The results are compared with model predictions including drifts. No marked difference is observed in the amplitudes of FDs during A < 0 and A > 0. Rigidity spectrum fitted with a power law yields the values of spectral exponent that are closer to values predicted by two-dimensional models including drifts. The recovery rate of FDs varies with the polarity of HMF and the rate is higher (recovery time smaller) during A > 0 than during A < 0 epoch, consistent with the model predictions including the drift effects in the HMF. This difference in recovery time of FDs during A > 0 and A < 0 polarity conditions provides experimental evidence that drift plays an important role in cosmic ray modulation.     

磁场的对流再产生和扩散

根据双尺度平均场电动力学,研究一种特定的周期对流的磁场再产生和扩散效应。数值计算运动学发电机方程,给出旋转对流元中感应电动势图象,得出无量纲磁场再产生系数α′≈-0.4～+0.4;对流扩散系数β′≈0.05.所提供的α效应能满足符合太阳活动周期的α-ω发电机模型的要求,若取对流元特征旋转速度u_1为100m/s,对流元特征长度α为1000km计算得到的对流扩散效应较发电机理论和观测的要求约小20倍。需计入脉动磁场和平均流场的相互作用,进一步研究旋转对流元的扩散效应。     

Magnetic Flux Imbalance of the Solar and Heliospheric Magnetic Fields

A comparative analysis of solar and heliospheric magnetic fields in terms of their cumulative sums reveals cyclic and long-term changes that appear as a magnetic flux imbalance and alternations of dominant magnetic polarities. The global magnetic flux imbalance of the Sun manifests itself in the solar mean magnetic field (SMMF) signal. The north – south asymmetry of solar activity and the quadrupole mode of the solar magnetic field contribute the most to the observed magnetic flux imbalance. The polarity asymmetry exhibits the Hale magnetic cycle in both the radial and azimuthal components of the interplanetary magnetic field (IMF). Analysis of the cumulative sums of the IMF components clearly reveals cyclic changes in the IMF geometry. The accumulated deviations in the IMF spiral angle from its nominal value also demonstrate long-term changes resulting from a slow increase of the solar wind speed over 1965 – 2006. A predominance of the positive IMF B _z with a significant linear trend in its cumulative signal is interpreted as a manifestation of the relic magnetic field of the Sun. Long-term changes in the IMF B _z are revealed. They demonstrate decadal changes owing to the 11/22-year solar cycle. Long-duration time intervals with a dominant negative B _z component were found in temporal patterns of the cumulative sum of the IMF B _z .     

Magnetic and Velocity Shear Driven Instabilities in the Heliospheric Plasma

We have addressed the problem of combined magnetic and velocity shear driven instabilities in the context of the heliospheric plasma. New high-order numerical methods have been used to analyze the instability dynamics of the heliospheric current-sheet interacting with the structure determined by the slow component of the solar wind on the solar equatorial plane above the helmet streamers. Preliminary results are presented.     

Systematically Asymmetric Heliospheric Magnetic Field: Evidence for a Quadrupole Mode and Non-Axisymmetry with Polarity Flip-Flops

Recent studies of the heliospheric magnetic field (HMF) have detected interesting, systematic hemispherical and longitudinal asymmetries which have a profound significance for the understanding of solar magnetic fields. The in situ HMF measurements since the 1960s show that the heliospheric current sheet (HCS) is systematically shifted (coned) southward during solar minimum times, leading to the concept of a bashful ballerina. While temporary shifts can be considerably larger, the average HCS shift (coning) angle is a few degrees, less than the 7.2^∘ tilt of the solar rotation axis. Recent solar observations during the last two solar cycles verify these results and show that the magnetic areas in the northern solar hemisphere are larger and their intensity weaker than in the south during long intervals in the late declining to minimum phase. The multipole expansion reveals a strong quadrupole term which is oppositely directed to the dipole term. These results imply that the Sun has a symmetric quadrupole S0 dynamo mode that oscillates in phase with the dominant dipole A0 mode. Moreover, the heliospheric magnetic field has a strong tendency to produce solar tilts that are roughly opposite in longitudinal phase. This implies is a systematic longitudinal asymmetry and leads to a “flip-flop” type behaviour in the dominant HMF sector whose period is about 3.2 years. This agrees very well with the similar flip-flop period found recently in sunspots, as well as with the observed ratio of three between the activity cycle period and the flip-flop period of sun-like stars. Accordingly, these results require that the solar dynamo includes three modes, A0, S0 and a non-axisymmetric mode. Obviously, these results have a great impact on solar modelling.     

Understanding the Behavior of the Heliospheric Magnetic Field and the Solar Wind During the Unusual Solar Minimum Between Cycles 23 and 24

The properties of the heliospheric magnetic field and the solar wind were substantially different in the unusual solar minimum between Cycles 23 and 24: the magnetic-field strength was substantially reduced, as were the flow properties of the solar wind, such as the mass flux. Explanations for these changes are offered that do not require any substantial reconsideration of the general understandings of the behavior of the heliospheric magnetic field and the solar wind that were developed in the minimum of Cycle 22?–?23. Solar-wind composition data are used to demonstrate that there are two distinct regions of solar wind: solar wind likely to originate from the stalk of the streamer belt (the highly elongated loops that underlie the heliospheric current sheet), and solar wind from outside this region. The region outside the streamer-stalk region is noticeably larger in the minimum of Cycle 23?–?24; however, the increased area can account for the reduction in the heliospheric magnetic-field strength in this minimum. Thus, the total magnetic flux contained in this region is the same in the two minima. Various correlations among the solar-wind mass flux and coronal electron temperature inferred from solar-wind charge states were developed for the Cycle 22?–?23 solar minimum. The data for the minimum of Cycle 23?–?24 suggest that the correlations still hold, and thus the basic acceleration mechanism is unchanged in this minimum.     

地球磁场对带电人造卫星轨道根数的摄动影响

研究了地球磁场对带电的非赤道卫星的轨道根数的摄动影响。理论结果表明，地球磁场对带电卫星的轨道半长轴没有摄动影响，既无周期摄动，也无长期摄动，但对轨道偏心率、轨道倾角、升交点赤经、近地点经度和历元平近点角均有周期摄动，且对升交点和近地点经度还有长期摄动效应。通过算例表明，当卫星带有大量电荷时，地球磁场对卫星轨道的摄动影响必须加以考虑。     

Supersonic Ambipolar Diffusion: Estimating the Magnetic Field Strength in Protostellar Outflows

The magnetic field plays a crucial role in star formation. It is involved in rotational braking, collapse braking, outflow formation and jet collimation. Direct observations of the field are difficult. However, the field can be indirectly estimated through the field-cushioned C-shocks which produce strong infrared molecular emission lines. In particular, a high field in the outflows will generate the ‘shock absorber’ signature: very broad H₂lines. Such lines are indeed observed. Here we summarise recent progress in C-shock formation and stability. We demonstrate numerically that the Shock Absorbers are evolutionary and stable. The widths of H₂lines then limit the magnetic field strength. A field of 6 mG is suggested for HH 212. This revised version was published online in July 2006 with corrections to the Cover Date.     

Acceleration and Transport of Energetic Charged Particles in Space

Recent advances in constructing stellar evolution models of hydrogen-deficient post-asymptotic giant branch (AGB) stars are presented. Hydrogen-deficiency can originate from mixing and subsequent convective burning of protons in the deeper layers during a thermal pulse on the post-AGB (VLTP). Dredge-up alone may also be responsible for hydrogen-deficiency of post-AGB stars. Models of the last thermal pulse on the AGB with very small envelope masses have shown efficient third dredge-up. The hydrogen content of the envelope is diluted sufficiently to produce H-deficient post-AGB stars (AFTP). Moreover, dredge-up alone may also cause H-deficiency during the Born-again phase (LTP). During the second AGB phase a convective envelope develops. A previously unknown lithium enrichment at the surface of Born-again stellar models may be used to distinguish between objects with different post-AGB evolution. The observed abundance ratios of C, O and He can be reproduced by all scenarios if an AGB starting model with inclusion of overshoot is used for the post-AGB model sequence. An appendix is devoted to the numerical methods for models of proton capture nucleosynthesis in the He-flash convection zone during a thermal pulse.     

Approximate Solutions of the Equations of Motion of a Charged Particle in the Field of a Magnetic Dipole

Three differential equations of the first order (26)—(28) which determine the trajectory are deduced. An approximate integration by means of elementary functions yields a set of formulas (in chapt. 13. Conclusions) that give a „window”︁ in the sky inside of which the radiation point of a high energy cosmic ray (monotonely bent in the magnetic field) must lie.     

Generation of Magnetic Field and Fast Particles During Collision of Electron-Positron Plasma Clouds

We present results of analytical studies and 2D3V PIC simulations of electron-positron plasma cloud collisions. We concentrate on the problem of quasi-static magnetic field generation. It is shown from linear theory, using relativistic two-fluid equations for electron-positron plasmas, that the generation of a quasi-static magnetic field can be associated with the counter-streaming instability. A two-dimensional relativistic particle simulation provides good agreement with the above linear theory and that, in the nonlinear stage of the instability, about 5.3% of the initial plasma flow energy can be converted to magnetic field energy. It is also shown from the simulation that the quasi-static magnetic field undergoes a collision-less change of structure, leading to large scale, long living structures and the production of high-energy particles. These processes may be important for understanding of production of high-energy particles in the region where two pulsar winds collide. This revised version was published online in July 2006 with corrections to the Cover Date.     

Orbits of a Charged Particle in a Skew Uniform Magnetic Field on Kerr Back Ground Geometry

In this paper we discuss the particle trajectories in a uniform magnetic field superimposed on Kerr geometry. The rotation axis of the Kerr black hole makes an angle with the asymptotic magnetic field. We find that the particle is trapped for suitable initial conditions.     

On the Influence of a Large-scale Magnetic Field on Turbulent Motions in an Electrically Conducting Medium

This paper deals with turbulent motions in a homogeneous incompressible electrically conducting medium in the presence of a magnetic field which is on average homogeneous and stationary. Using a model in which the turbulence is produced by a stochastic body force, and supposing a weak interaction between motion and magnetic field, a method is developed for calculating the pair correlation tensor of the velocity field from that occuring in a zero magnetic field. As an example, the pair-correlation tensor for a homogeneous stationary turbulence, which is isotropic and mirror-symmetric for zero magnetic field, is determined. With obvious assumptions on the correlation for zero field, two results are obtained. Firstly, the turbulent velocity is reduced by the magnetic field, the component parallel to the field, however, less than those perpendicular to it. Secondly, the correlation length parallel to the field turns out to be greater than the one perpendicular to it, indicating a tendency towards two-dimensional motion. Finally, the possibility of special situations is briefly discussed in which the turbulent velocity is enhanced by the magnetic field, and the anisotropies of the velocity components and the correlation lengths are opposite to those above.     

Modeling of Proton Acceleration in a Magnetic Island Inside the Ripple of the Heliospheric Current Sheet

Crossings of the heliospheric current sheet (HCS) at the Earth’s orbit are often associated with observations of anisotropic beams of energetic protons accelerated to energies from hundreds of keV to several MeV and above. A connection between this phenomenon and the occurrence of small-scale magnetic islands (SMIs) near reconnecting current sheets has recently been found. This study shows how pre-accelerated protons can be energized additionally due to oscillations of multiple SMIs inside the ripple of the reconnecting HCS. A model of the electromagnetic field of an oscillating 3D SMI with a characteristic size of ~0.001 AU is developed. A SMI is supposed to be bombarded by protons accelerated by magnetic reconnection at the HCS to energies from ~1keV to tens of keV. Numerical simulations have demonstrated that the resulting longitudinal inductive electric fields can additionally reaccelerate protons injected into a SMI. It is shown that there is a local “acceleration” region within the island in which particles gain energy most effectively. As a result, their average escape energies range from hundreds of keV to 2 MeV and above. There is almost no particle acceleration outside the region. It is shown that energies gained by protons significantly depend on the initial phase and the place of their entry into a SMI but weakly depend on the initial energy. Therefore, low-energy particles can be accelerated more efficiently than high-energy particles, and all particles can reach the total energy limit upon their escape from a SMI. It is also found that the escape velocity possesses a strong directional anisotropy. The results are consistent with observations in the solar wind plasma.

     

Properties of Magnetic Neutral Line Gradients and Formation of Filaments

We investigate the gradients of magnetic fields across neutral lines (NLs) and compare their properties for NLs with and without chromospheric filaments. Our results show that there is a range of preferred magnetic field gradients where the filament formation is enhanced. On the other hand, a horizontal gradient of the magnetic field across an NL alone does not appear to be a single factor that determines if a filament will form (or not) in a given location.     

带电测试粒子在磁化史瓦西黑洞中的混沌运动

当史瓦西黑洞周围存在渐近均匀的外部磁场时, 描述带电粒子在史瓦西黑洞附近运动的哈密顿系统会变为不可积系统. 类似于这样的相对论哈密顿系统不存在有显式分析解的2部分分离形式, 给显式辛算法的构建和应用带来困难. 近一年以来的系列工作提出将相对论哈密顿系统分解为具有显式分析解的2个以上分离部分形式, 成功解决了许多相对论时空构建显式辛算法的难题. 最近的工作回答了哈密顿系统显式可积分离数目对长期数值积分精度有何影响、哪种显式辛算法有最佳长期数值性能这两个问题, 指出哈密顿有最小可积分离数目即3部分分裂解形式并且应用于优化的4阶分段龙格库塔显式辛算法可取得最好精度. 由此选择上述数值积分方法并利用庞加莱截面、最大李雅普诺夫指数和快速李雅普诺夫指标研究在磁化史瓦西黑洞附近运动的带电粒子轨道动力学. 结果显示: 针对某特定的粒子能量和角动量, 较小的外部磁场很难形成混沌轨道; 较大的正磁场参数容易使轨道产生混沌, 并且随着磁场的增大, 轨道的混沌程度也随之加强; 粒子能量适当变大也可以加剧混沌程度, 但负磁场参数和粒子角动量变大都会减弱混沌.     

太阳黑子发展中的磁扩散不稳定性

根据太阳发电机理论中的ω-效应,在太阳对流层内将产生纬向磁场,它的磁浮力要促使流团上浮。在文[5]中讨论了在流团上浮过程中,流团表面的磁扩散率梯度将对纬向磁场产生扰动,这一扰动使纬向磁场集积在流团表面磁扩散率梯度大的地方,围绕流团表面形成了黑子磁环。 本文进一步从磁流力学方程组的小扰动方程出发探讨了太阳黑子磁环发展的不稳定性问题。结果表明:在扰动方程中存在着不稳定模式。这一不稳定性产生的原因是由于当温度(或者说磁扩散率)受到小扰动时,纬向磁场要集积在磁扩散率(或温度)梯度大的地方,而磁场的集积将导致磁压增强及气压减低。在绝热条件下,这将使温度减低,而温度的减低又加强了温度梯度的增大,这又进一步促使磁场在梯度大的地方集积。这种磁场与温度发展的相互促进关系可以称它为磁扩散不稳定性。本文认为太阳黑子磁环和它低温的形成正是由于这种不稳定性发展起来的。     

Physics and Gasdynamics of the Heliospheric Interface

During 30 years, a big theoretical effort to understand the physical processes in the heliospheric interface has followed the pioneer papers by Parker (1961) and Baranov et al. (1971). The heliospheric interface is a shell formed by the solar wind interaction with the ionized component of the circumsolar local interstellar medium (LISM). For fully ionized supersonic interstellar plasma two-shocks (the termination shock and the bow shock) and a contact discontinuity (the heliopause) are formed in the solar wind/LISM interaction. However, LISM consists of at least of three components additional to plasma: H-atoms, galactic cosmic rays and magnetic field. The interstellar atoms that penetrate into the solar wind, are ionized there and form pickup ions. A part of the pickup ions is accelerated to high energies of anomalous cosmic rays (ACRs). ACRs may modify the plasma flow upstream the termination shock and in the heliosheath. In this short review I summarize current understanding of the physical and gasdynamical processes in the heliospheric interface, outline unresolved problems and future perspectives. This revised version was published online in July 2006 with corrections to the Cover Date.     

耀斑大尺度电流片的湍流耗散和能谱分析

磁重联在宇宙的许多动力学现象中都是非常核心的过程.磁流体动力学(MHD)数值模拟是研究磁重联过程以及相应物理图像的一种很有效的手段.通过不同的参数组合,来研究MHD数值模拟中磁雷诺数和空间分辨率对磁重联率、数值耗散和能谱分布的影响.对得到的数据进行分析后,发现磁雷诺数对磁重联率和能谱分布有一定的影响.磁雷诺数越大,磁重联过程进入非线性阶段所需的特征时间越短,磁重联率就越早发生跃升.磁雷诺数R_m对耗散开始发挥作用的Kolmogorov微观尺度l_ko有明显影响:R_m越大,l_ko就越小.研究了磁重联过程中包括数值耗散在内的额外耗散对重联过程的影响.结果表明,撕裂模不稳定性开始之前的额外耗散以纯数值耗散为主,撕裂模不稳定性出现之后,额外耗散出现同步跃升,说明不稳定性导致的湍流明显增强了耗散的效果,相当于在局部湍流区引入了超电阻.能谱分析进一步表明,大尺度电流片的l_ko完全可能出现在宏观的MHD尺度上.