ROSAT observations of the Chamaeleon star forming cloud

Two soft X-ray images of the Chamaeleon I star forming cloud obtained with the ROSAT Position Sensitive Proportional Counter are presented. Seventy reliable, and perhaps 19 additional, X-ray sources are found. Up to Ninety percent of these sources are certainly or probably identified with T Tauri stars formed in the cloud. Twenty to 35 are probably previously unrecognized ‘weak’ T Tauri (WTT) stars. T Tauri X-ray luminosities range from log , or 10² – 10⁴ times solar levels, with mean in the 0.2–2.5 keV band. The X-ray luminosities of well-studied Chamaeleon cloud members are correlated with a complex of four stellar properties: effective temperature, mass, radius and bolometric luminosity. The spatial distribution, H-R diagram locations of the stars indicate WTT and CTT are coeval. The total premain sequence population of the cloud is likely to be > 100 stars, with WTT stars outnumbering ‘classical’ T Tauri (CTT) stars by 2:1.     

Highly variable X-ray emitting objects in the Rho Oph dark cloud

The Rho Ophiuchi dark cloud region has been the subject of an extensive guest investigation using the $\text{Einstein}$ Observatory. The set of observations comprise 14 IPC fields and 3 HRI fields. The densest part of the cloud has been observed 6 times. Forty seven sources were detected at a level > 3.5 σ and twenty more above 2 σ. The majority of these sources have optical, IR, or even radio continuum counterparts; nine are identified with known T Tauri stars, while several others are identified with stars showing H α in emission. All show a high degree of time variability; flux variations reach factors of 5 in a few hours, or 25 in a day. Apparent luminosities are in the range 10(30) – 10(31)(1) erg.s^?1. The possibility that the X-ray variability is due to flares is examined. If this interpretation is correct, one source has been the seat of the largest stellar flare ever recorded in X rays [L_x = 10(32) erg.s^?1, E_x ?10(36) ergs^-].     

Evaluation of geomagnetic field models using magnetometer measurements for satellite attitude determination system at low earth orbits: Case studies

In this study, different geomagnetic field models are compared in order to study the errors resulting from the representation of magnetic fields that affect the satellite attitude system. For this purpose, we used magnetometer data from two Low Earth Orbit (LEO) spacecraft and the geomagnetic models IGRF-12 (Thébault et al., 2015) and T89 (Tsyganenko, 1989) models to study the differences between the magnetic field components, strength and the angle between the predicted and observed vector magnetic fields. The comparisons were made during geomagnetically active and quiet days to see the effects of the geomagnetic storms and sub-storms on the predicted and observed magnetic fields and angles. The angles, in turn, are used to estimate the spacecraft attitude and hence, the differences between model and observations as well as between two models become important to determine and reduce the errors associated with the models under different space environment conditions. We show that the models differ from the observations even during the geomagnetically quiet times but the associated errors during the geomagnetically active times increase. We find that the T89 model gives closer predictions to the observations, especially during active times and the errors are smaller compared to the IGRF-12 model. The magnitude of the error in the angle under both environmental conditions was found to be less than 1°. For the first time, the geomagnetic models were used to address the effects of the near Earth space environment on the satellite attitude.     

Blue light-dependent phosphorylations of cryptochromes are affected by magnetic fields in Arabidopsis

The blue light receptor cryptochrome that could form radical pairs after exposure to blue light was suggested to be a magnetoreceptor based on the proposition that radical pairs were involved in the magnetoreception. But the effects of magnetic fields on the function of cryptochrome are poorly understood. Phosphorylation of cryptochrome in Arabidopsis was closely associated with the function of this photoreceptor. Here, we grew Arabidopsis seedlings in a 500 μT magnetic field and a near-null magnetic field and found that the 500 μT magnetic field enhanced the blue light-dependent phosphorylations of CRY1 and CRY2, and the near-null magnetic field weakened the blue light-dependent phosphorylation of CRY2 but not CRY1. Dephosphorylations of CRY1 and CRY2 in the darkness were slowed down in the 500 μT magnetic field, whereas dephosphorylations of CRY1 and CRY2 were accelerated in the near-null magnetic field. These results suggest that magnetic field with strength higher or weaker than the local geomagnetic field affects the activated states of cryptochromes, which thus modifies the functions of cryptochromes.     

Distribution and variability of accelerated electrons at Mars

We investigate accelerated electrons observed by Mars Global Surveyor (MGS), using data from the Electron Reflectometer (ER) instrument. We find three different types of accelerated electron events. Current sheet events occur over regions with weak or no crustal fields, have the highest electron energy fluxes, and are likely located on draped magnetotail fields. Extended events occur over regions with moderate crustal magnetic fields, and are most often observed on closed magnetic field lines. Localized events have the lowest energy fluxes, occur in strong magnetic cusp regions, and are the most likely kind of event to be found on open magnetic field lines. Some localized events have clear signatures of field-aligned currents; these events have much higher electron fluxes, and are preferentially observed on radially oriented open magnetic field lines. Electron acceleration events, especially localized events, are similar in many ways to events observed in the terrestrial auroral zone. However, physical processes related to those found in the terrestrial cusp and/or plasmasheet could also be responsible for accelerating electrons at Mars.     

Plasmoid formation in eruptive flares

One phenomena Yohkoh has observed is plasmoid eruption in flares. Thus this is a key factor that must be explained in any flare mechanism. In order to understand the dynamics of a plasmoid, we performed a numerical MHD simulation and investigated the evolution of the coronal magnetic field, which is initially a force-free configuration. The main results are as follows. At first, small amount of dissipation, induced by the initial perturbation, occurs in the current sheet where the plasmoid forms. This plasmoid is slowly going upward by magnetic tension force of the reconnected magnetic fields produced by initial dissipation. The crucial point comes when the perpendicular magnetic fields are washed away from the reconnection point, after that the reconnection proceeds effectively so that the magnetic tension force of the reconnected fields becomes strong, which make the plasmoid be rapidly erupted upward. These are consistent with the observational results, which say that before the main energy release the plasmoid slowly rises and when the flare sets in it is rapidly accelerated upward. In this paper, we emphasize on the role that the perpendicular magnetic fields play in the evolution of flare.     

Chromospheric dynamics

This review focuses on dynamics of the solar chromosphere, which serves as a good proxy for understanding processes in stellar chromospheres as well. In the quiet chromosphere it is useful to distinguish between the magnetic network on the boundaries of supergranulation cells, where strong magnetic fields are organized in mainly vertical magnetic flux tubes, and internetwork regions in the cell interior, where magnetic fields are weaker and dynamically less important. Recently, some progress has been made in understanding the physics of the non-magnetic chromosphere. On the other hand, the physical processes that heat the magnetic network have not been fully identified. Is the network heated by wave dissipation and if so, what are their physical characteristics? These and other aspects relating to the dynamics and energy transport mechanisms will be discussed in detail. In addition, some of the outstanding problems in the field such the driving mechanism for spicules and the nature of internetwork magnetic fields will also be highlighted. Furthermore, a critical assessment will be made on the challenges facing theory and the direction for future investigations, particularly in the light of the new space experiments, will be highlighted.     

太阳大气中磁场的扭转储能

本文从完整的磁流体动力学方程组出发, 研究太阳大气中磁力线管根部的扭转储存能量。通过数值方法, 研究了包含太阳表面过渡区在内的非线性无力场的动力学演化。假设初始磁场位形为势场, 根部等离子体运动使活动区磁场扭转。磁能蓬新分布, 在局部区域中储存大量的磁能。计算结果给出非线性无力场的定量关系, 可以解释太阳耀斑的储能过程。      

Topological and statistical properties of nonlinear force-free fields

We use our semi-analytic solution of the nonlinear force-free field equation to construct three-dimensional magnetic fields that are applicable to the solar corona and study their statistical properties for estimating the degree of braiding exhibited by these fields. We present a new formula for calculating the winding number and compare it with the formula for the crossing number. The comparison is shown for a toy model of two helices and for realistic cases of nonlinear force-free fields; conceptually the formulae are nearly the same but the resulting distributions calculated for a given topology can be different. We also calculate linkages, which are useful topological quantities that are independent measures of the contribution of magnetic braiding to the total free energy and relative helicity of the field. Finally, we derive new analytical bounds for the free energy and relative helicity for the field configurations in terms of the linking number. These bounds will be of utility in estimating the braided energy available for nano-flares or for eruptions.     

火星空间磁场结构特征

在火星空间模拟的单流体MHD模型的基础上, 研究了火星空间磁场结构及火星表面局部磁异常对磁场结构的影响. 在太阳风与火星相互作用的过程中, 形成弓激波和磁堆积区, 行星际磁场弯曲并向两极移动且被拖拽变形, 大部分磁力线从火星两极绕过, 通过火星之后在磁尾留下V字形结构. 火星表面附近局部磁异常也对火星磁场结构产生不可忽视的影响. 不同位置和强度的磁异常与太阳风相互作用形成结构及形态各异的微磁层, 如被拖拽的微磁层和存在开磁力线的微磁层等. 局部磁异常改变了近火磁场结构, 并可能改变等离子体的分布.      

Interstellar magnetic fields

For three decades, magnetic fields have been known to permeate interstellar space. Each decade focussed attention on a different problem concerning the role of magnetic fields in star formation, and developed distinct techniques for the solution of the respective problem. A historical perspective of this period is given first. Then theoretical studies of the role of magnetic fields in star formation are reviewed critically, with emphasis on the dynamical processes occuring in collapsing interstellar clouds and on the interplay between theory and observation. A synthesis in the form of a scenario, albeit incomplete, for star formation in magnetic clouds is given. Prospects for the solution, during the 1980's, for remaining fundamental problems are discussed, and the need for certain kinds of observations is emphasized.     

On the heating mechanism of magnetic flux loops in the solar atmosphere

Hα filtergrams and magnetograms indicate that bright features (such as plages and granulation boundaries) correspond to areas of strong vertical magnetic fields and dark features (such as fibrils and filaments) are associated with strong horizontal magnetic field. It was suggested by /1/ that there is an excess dissipation of waves, available for heating, in regions of vertical magnetic fields. With this suggestion in mind, we have investigated the physical heating mechanism due to ponderomotive forces exerted by turbulent waves along curved magnetic flux loops. Results show that the temperature difference (ΔT) between the inside and outside of the flux loop can be classified into three parts; ΔT = ΔT₁ + ΔT₂ + ΔT₃; in which ΔT₁ and ΔT₃ represent the heating or cooling effect from the ponderomotive force, and ΔT₂ is the heating effect due to conversion of turbulent energy from the localized plasma. The specific physical mechanism (i.e., the ponderomotive forces exerted by turbulent waves), is used to illustrate solar atmospheric heating via an example leading to the formulation of plages.     

Coronal heating mechanisms

As a result of the large body of data available from solar and stellar coronae, our understanding of the mechanisms responsible for the heating of coronal plasmas to temperatures of the order of ～ 10⁸ K has changed. The solar corona is highly structured by magnetic fields and the acoustic shocks which, according to early theories, should have acted as the coronal energy source have not been observed. Einstein Observatory data show moreover that coronae are present in most regions of the H-R diagram. The observed relationship between X-ray luminosity and rotational velocity in dwarf stars from spectral types F to M again suggests an active role for the magnetic fields.The basic picture which is emerging is that coronae in stellar types from F to M are produced because of the interaction of the magnetic field with the convective velocity fields generated in the photosphere resulting in MHD waves or currents which dissipate in the corona. X-ray emission in early type stars cannot be explained with this mechanism and the models which have been proposed for these stars are not yet completely satisfactory.     

Solar flares: an overview.

This is a survey of solar phenomena and physical models that may be useful for improving forecasts of solar flares and proton storms in interplanetary space. Knowledge of the physical processes that accelerate protons has advanced because of gamma-ray and X-ray observations from the Solar Maximum Mission telescopes. Protons are accelerated at the onset of flares, but the duration of any subsequent proton storm at 1 AU depends on the structure of the interplanetary fields. X-ray images of the solar corona show possible fast proton escape paths. Magnetographs and high-resolution visible-band images show the magnetic field structure near the acceleration region and the heating effects of sunward-directed protons. Preflare magnetic field growth and shear may be the most important clues to the physical processes that generate high energy solar particles. Any dramatic improvement in flare forecasts will require high resolution solar telescopes in space. Several possibilities for improvements in the art of flare forecasting are presented, among them: the use of acoustic tomography to probe for subsurface magnetic fields; a satellite-borne solar magnetograph; and an X-ray telescope to monitor the corona for eruptions.     

Biological effects due to weak magnetic field on plants.

Throughout the evolution process, Earth's magnetic field (MF, about 50 microT) was a natural component of the environment for living organisms. Biological objects, flying on planned long-term interplanetary missions, would experience much weaker magnetic fields, since galactic MF is known to be 0.1-1 nT. However, the role of weak magnetic fields and their influence on functioning of biological organisms are still insufficiently understood, and is actively studied. Numerous experiments with seedlings of different plant species placed in weak magnetic field have shown that the growth of their primary roots is inhibited during early germination stages in comparison with control. The proliferative activity and cell reproduction in meristem of plant roots are reduced in weak magnetic field. Cell reproductive cycle slows down due to the expansion of G1 phase in many plant species (and of G2 phase in flax and lentil roots), while other phases of cell cycle remain relatively stable. In plant cells exposed to weak magnetic field, the functional activity of genome at early pre-replicate period is shown to decrease. Weak magnetic field causes intensification of protein synthesis and disintegration in plant roots. At ultrastructural level, changes in distribution of condensed chromatin and nucleolus compactization in nuclei, noticeable accumulation of lipid bodies, development of a lytic compartment (vacuoles, cytosegresomes and paramural bodies), and reduction of phytoferritin in plastids in meristem cells were observed in pea roots exposed to weak magnetic field. Mitochondria were found to be very sensitive to weak magnetic field: their size and relative volume in cells increase, matrix becomes electron-transparent, and cristae reduce. Cytochemical studies indicate that cells of plant roots exposed to weak magnetic field show Ca2+ over-saturation in all organelles and in cytoplasm unlike the control ones. The data presented suggest that prolonged exposures of plants to weak magnetic field may cause different biological effects at the cellular, tissue and organ levels. They may be functionally related to systems that regulate plant metabolism including the intracellular Ca2+ homeostasis. However, our understanding of very complex fundamental mechanisms and sites of interactions between weak magnetic fields and biological systems is still incomplete and still deserve strong research efforts.     

Investigation of magnetospheric processes with the use of a source of strong magnetic field in the ionosphere

More than 20 years ago V.P. Shabansky suggested that the magnetic system installed aboard the satellite, could be used as a physical instrument for studying the processes which occur in the near Earth space. The corresponding space scales of an artificial “magnetosphere”—“magnisphere”—are 10 m in the experiment with relatively small magnets in the ionosphere and 100 m in the solar wind. The corresponding similarity criteria are estimated. The possible scheme of the experiment with a superconducting magnet (magnetic moment 10⁵ A · m²) installed aboard the satellite is considered. The experimental complex includes a number of systems for measuring the fluxes of charged particles in a wide energy range, DC electric and magnetic fields, the electromagnetic fields in different frequency bands (from X-rays to radio). The scientific objectives are discussed in detail.     

利用TIE-GCM模式计算电离层电流及夜间电离层磁场

电离层电流产生的磁场是地磁场卫星测绘时需要剔除的干扰源.利用电离层热层模式TIE-GCM计算电离层中的中性风、重力驱动和压强梯度等形成的电离层电流的全球分布,分析电流在特定位置产生的磁场及磁场三分量随纬度的变化规律.结果表明,E层尤其是磁赤道和极区的电流密度较大,可达103nA·m-2量级,F层电流密度量级约为10nA·m-2.在磁静日（Kp≤ 1）夜间22:00LT-04:00LT,电离层电流在中低纬度（南北纬50°之间）产生的磁场量级为几个nT,且磁场的南北向分量和径向分量基本大于东西向分量.通过与CHAMP卫星磁测数据分析比较,发现TIE-GCM模式计算电离层干扰磁场在中低纬度可以取得较好的结果,但在高纬度地区的效果不理想,还需进一步改进模式以提高计算精度.      

基于电磁力与力矩的航天器运动控制的研究进展

电磁力与力矩是一种非接触式的相互作用力与力矩.将通电线圈或磁铁放在磁场中会受到电磁力与力矩的作用.多个通电线圈或磁铁相互作用时，在彼此构成的磁场中也会受到电磁力与力矩的作用.作为一种非接触的力与力矩，电磁力与力矩在航天器的姿态与相对位置控制系统中已有了广泛和深入的应用.本文主要介绍近年来由航天器上电磁装置受到地磁场作用产生电磁力矩在姿态控制中的应用，和带有电磁机构的多航天器间相互作用控制多航天器间相对位姿的应用等.     

行星际磁场与磁尾磁通量绳形成的关系

收集了Cluster卫星在2001-2005年间观测到的磁尾磁通量绳事件,并对磁通量绳(magnetic flux rope)形成及其内部磁场结构与行星际磁场(IMF)的关系作了统计研究.考虑磁通量绳被观测到时行星际磁场的条件,在所有73个磁通量绳事件中,行星际磁场By分量占有主导地位的事件有80%,且78%的事件具有与行星际磁场By分量相同方向的核心场.行星际磁场通过在磁层顶与地球磁场相互作用改变南北等离子体片内磁场相对方向,形成有利于磁通量绳形成的磁场位形,并且行星际磁场By分量的方向对磁通量绳内部核心场的方向具有决定性影响.从统计结果来看,磁通量绳的形成并不会依赖于行星际磁场Bz分量的方向.     

Magnetic field measurements in the ionosphere-magnetosphere region

An analysis of the differences between the measured by the Intercosmos Bulgaria 1300 satellite and the computed from the IGRF-1980 values of the magnetic field intensity is presented. In a global scale the residuals are negative. A 75 nT spacecraft remanent magnetic field along the X-axis is determined. Partially the negative residuals could be explained by the model itself which results in a stronger field than the actual one. The spacecraft inductive magnetization may be a reasonable explanation for the rest. It is difficult to explain the negative residuals in the polar region by a magnetization along any of the axes. The complex character of the differences in this region is conditioned by the external fields, in particular, by the ionospheric current systems. The authors believe that the bias fields from the spacecraft have a static character and they are restricted mainly along the X-axis.