Seismological studies of the sun and other stars

Observations of a large number of different oscillation frequencies in the Sun provide an opportunity for detailed testing of the theory of stellar structure and evolution. At present highly significant discrepancies remain between observed and computed frequencies, and so our models of the solar interior have to be modified. With further improvements in the observations it might become possible to make a direct empirical determination of the density structure throughout the Sun.Similar oscillations have so far not been detected in other stars, but attempts to do so are under way. Theoretical estimates indicate that amplitudes somewhat greater than for the Sun might be expected for early F stars on the main sequence, and that the amplitude increases rapidly with decreasing gravity. Observation of such oscillations would enable investigations of the structure of these stars, and would in addition provide valuable information about the excitation mechanism of the oscillations.     

In situ Lunar Orientation Measurement (ILOM): Simulation of observation

The measurement of the rotation of the Moon is one of the key techniques to get the information of the internal structure. For this purpose, we proposed a small telescope experiment on the surface of the Moon in which motion of stars are utilized for the estimation of the rotation parameter. This paper describes results of simulation of observation, in which star trajectories observed are decomposed to librations, polar motion, and the precession and the amplitude and phase of each component are estimated. The standard deviation of the parameter estimation becomes nearly 1 ms of arc, which will be better than the Lunar Laser Ranging observation. From the viewpoints of accuracy of observation, thermal condition, and electric power generation, the instrument should be placed where the much sunshine is achieved on the lunar polar region.     

Solar and late-type dwarfs

The Einstein Observatory and the IUE satellite have provided the observational basis for a major restructuring in theories of coronal formation for late-type stars. For the first time, coronal and transition region emission from a large sample of low mass (1 M_o) dwarf stars has been directly observed, with the unexpected result that essentially all such stars are x-ray emitters. The Sun, which was previously assumed to be typical, is now known to be at the low end of the x-ray luminosity function for solar-type stars. K- and M-dwarfs are observed to have nearly the same luminosity distributions as G-dwarfs and all of these spectral types have a large spread in x-ray luminosity.Observationally, there is a strong correlation between the strength of coronal emission in stars with outer convective zones and the rotation rates of these stars. At the present time we have only the beginnings of a satisfactory theoretical explanation for this correlation; although we are beginning to understand the connection between coronal emission strength and the magnetic field, we do not yet understand the stellar dynamo which generates the magnetic field. Studies of the coronal emission of stars may lead to a better understanding of stellar dynamos.     

电波极化面旋转特性研究

推导了电波极化面旋转角表达式, 利用实测电子浓度、地磁场磁感应强度模与磁倾角剖面,对定高载体、不同天线侧视方向与地球法线夹角、不同地区、不同频段、不同时段、不同太阳活动情况下的电波极化面旋转角进行了仿真, 并对仿真结果进行了分析, 得出了电波极化面旋转角的时域、地域、频域等规律和特性. 为这一领域的深入研究提供了参考.      

Planetary radio astronomy from voyager

The Planetary Radio Astronomy instruments on Voyager 1 and 2 provided new, highly detailed measurements of several different kinds of strong, nonthermal radiation generated in the inner magnetospheres and upper ionospheres of Jupiter and Saturn. At Jupiter, an intense decameter-wavelength component (between a few tenths of a MHz and 39.5 MHz) is characterized by complex, highly organized structure in the frequency-time domain and by a strong dependence on the longitude of the observer and, in some cases, of Io. At frequencies below about 1 MHz there exists a (principally) kilometer-wavelength component of emission that is bursty, relatively broadbanded (typically covering 10 to 1000 kHz), and strongly modulated by planetary rotation. The properties of this component are consistent with a source confined to high latitudes on the dayside hemisphere of Jupiter. A second kilometric component is narrow-banded, relatively weak and exhibits a spectral peak near 100 kHz. The narrowband component also occurs periodically but at a repetition rate that is a few percent slower than that corresponding to the planetary rotation rate. This component is thought to originate at a frequency near the electron plasma frequency in the outer part of the Io plasma torus (8 to 10 R_J) and to reflect the small departures from perfect corotation experienced by plasma there.The Voyager instruments also detected intense, low frequency, radio emissions from the Saturn system. The Saturnian kilometric radiation is observed in a relatively narrow frequency band between 3 kHz and 1.2 MHz, is elliptically or circularly polarized, and is strongly modulated in intensity at Saturn's 10.66-hr rotation period. This emission is believed to be emitted in the right-hand extraordinary mode from regions near or in Saturn's dayside, polar, magnetospheric cusps. Variations in intensity at Saturn's rotation period may correspond to the rotation of a localized magnetic anomaly into the vicinity of the ionospheric footprint of the polar cusp. Variations in activity on time scales of a few days and longer seem to indicate that both the solar wind and the satellite Dione can also influence the generation of the radio emission.     

高速滚动轴承保持架自由振动特性研究

通过对航空发动机主轴承保持架自由振动的计算分析,表明:保持架不存在小于751.12 Hz的固有频率;在整体按圆环的规律振动的同时,过梁和侧梁存在弯曲、剪切等局部振型;由于保持架的转动,存在不相等且与转动速度相关的前后行波频率,保持架可能共振的频率数目增多,而保持架又受到多种周期性的干扰,高速转动时更易激发共振,造成破坏,因此在设计中必须对保持架的振动问题给予重视.      

自旋稳定通信卫星姿态确定的矩阵方法

文章陈述了自旋稳定通信卫星姿态确定的矩阵方法及其工程应用的七种情况。这种方法是同时应用地球角θ_s、太阳角θ_s和从太阳到地中的转动角λ_s。确定地球同步自旋稳定卫星的姿态。这种方法可唯一确定卫星姿态,简单了计算,并仅受地球矢量和太阳矢量共线的几何限制条件,姿态确定精度较高,这种矩阵方法得到了中国成功发射的五颗地球同步通信卫星的满意的应用和验证。     

一种基于太阳自转轴观测角的新型天文导航方法

针对太阳探测器,提出一种基于太阳自转轴观测角的新型天文导航方法,通过光谱仪测量太阳圆盘面边缘上两组连线互相垂直点的速度差值,建立速度差值与太阳自转轴观测角的数学模型,并以该观测角作为量测量来提供探测器的位置信息。仿真结果表明:相较于传统以太阳视方向作为量测量的导航方法,以太阳自转轴观测角作为量测量的新型天文导航方法的导航精度提高了17.4%。此外,还分析了测速敏感器精度、滤波周期和轨道倾角对导航性能的影响,为深空探测自主导航提供了新的理论与方法。     

CoRoT and Kepler results: Solar-like oscillators

The space-borne observatories CoRoT (Convection Rotation and planetary Transits) and Kepler have provided photometric time series data of unprecedented precision for large numbers of stars. These data have revolutionized the fields of transiting exoplanets and asteroseismology. In this review some important asteroseismic results obtained using data from the CoRoT and Kepler space missions concerning stars that show solar-like oscillations are discussed. These results comprise, among others, measurements of the location of the base of the convection zone and helium second-ionization zone in main-sequence stars, the presence (or not) of core-helium burning in red-giant stars, as well as differential rotation in these stars.     

The search for extra-solar planetary systems.

I review the observational evidence for planetary systems around nearby stars and, using our own solar system as a guide, assess the stringent requirements that new searches need to meet in order to unambiguously establish the presence of another planetary system. Basically, these requirements are: 1 milliarcsecond or better positional accuracy for astrometric techniques, 9 orders of magnitude or better star to planet luminosity ratio discrimination at 0.5 to 1" separation in the optical for direct imaging techniques, 10 meters sec-1 or better radial velocity accuracy for reflex motion techniques and +/-1% or better brightness fluctuation accuracy for planet/star occultation measurements. The astrometric accuracy is in reach of HST, direct imaging will require much larger telescopes and/or a 50 times smoother mirror than HST while the reflex motion and occultation techniques best performed on the ground are just becoming viable and promise exciting new discoveries. On the other band, new indirect evidence on the existence of other planetary systems also comes from the observation of large dusty disks around nearby main sequence stars not too dissimilar from our sun. In one particular case, that of Beta Pictoris, a flattened disk seen nearly edge-on has been imaged in the optical and near IR down to almost 70 AU of the star. It probably represents a young planetary system in its clearing out phase as planetesimals collide, erode and are swept out of the inner system by radiation pressure. The hypothesized Kuiper belt around our solar system may be the analogous structure in a later evolutionary stage. Features of this type have been detected in the far IR and sub-millimeter wavelength regions around 50-100 nearby main sequence and pre-main sequence stars. I discuss a battery of new accurate observations planned in the near future of these objects some of which may actually harbour planets or planetesimals that will certainly dramatically improve our knowledge of planetary system formation processes and our peculiar position in this scheme.     

Probing solar and stellar interior dynamics and dynamo

Solar and stellar activity is a result of complex interaction between magnetic field, turbulent convection and differential rotation in a star’s interior. Magnetic field is believed to be generated by a dynamo process in the convection zone. It emerges on the surface forming sunspots and starspots. Localization of the magnetic spots and their evolution with the activity cycle is determined by large-scale interior flows. Thus, the internal dynamics of the Sun and other stars hold the key to understanding the dynamo mechanism and activity cycles. Recently, significant progress has been made for modeling magnetohydrodynamics of the stellar interiors and probing the internal rotation and large-scale dynamics of the Sun by helioseismology. Also, asteroseismology is beginning to probe interiors of distant stars. I review key achievements and challenges in our quest to understand the basic mechanisms of solar and stellar activity.     

基于联合滤波器的高精度卫星定姿系统设计

设计了由陀螺、GPS姿态敏感器、红外地平仪和太阳敏感器构成的太阳同步极轨卫星姿态确定系统。提出联邦滤波器结构和算法,推导了各子系统的量测方程和姿态确定系统的误差状态方程。为规避对量测值进行非相关处理,采用减小GPS姿态敏感器输出的姿态滤波值作为系统滤波器量测值的频率的方法。仿真结果表明,采用联邦滤波器对多敏感器卫星姿态确定系统进行信息融合,具有计算量小、精度高、可靠性好等优点。     

I.U.E. high resolution observations of hot stars emitting coronal x-rays

The far UV resonance lines of a sample of 21 early-type stars, which were observed in the soft X-Ray band with the $\text{Einstein}$ satellite, are examined using I.U.E. high resolution spectra to search for possible correlation between the X-Ray coronal emission and far UV spectral properties. In particular, those quantities that can give information on the structure of the outer envelope (such as wind terminal velocities, emission-absorption ratios) are measured and compared with the observed X-Ray flux.     

A triangle voting algorithm based on double feature constraints for star sensors

A novel autonomous star identification algorithm is presented in this study. In the proposed algorithm, each sensor star constructs multi-triangle with its bright neighbor stars and obtains its candidates by triangle voting process, in which the triangle is considered as the basic voting element. In order to accelerate the speed of this algorithm and reduce the required memory for star database, feature extraction is carried out to reduce the dimension of triangles and each triangle is described by its base and height. During the identification period, the voting scheme based on double feature constraints is proposed to implement triangle voting. This scheme guarantees that only the catalog star satisfying two features can vote for the sensor star, which improves the robustness towards false stars. The simulation and real star image test demonstrate that compared with the other two algorithms, the proposed algorithm is more robust towards position noise, magnitude noise and false stars.     

Proposal for ionospheric tomography technique using a new HF radio source

Measurements of the line integral of the electron density along satellite-to-ground ray paths (i.e. TEC) using differential phase or Doppler of two coherent VHF/UHF signals transmitted from NNSS or GPS satellite networks have been used in the ionospheric tomography for mapping large-scale ionospheric images over region of interest. In this paper, we present theoretical studies of using a new signal source HF frequency in tomographic imaging. The initial phase problem inherent in the phase measurement can be eliminated by measuring Faraday rotations. Relative rotation on two adjacent HF frequencies is used to solve the ambiguity problem. A second-order approximation of the Faraday rotation incorporated with ray-tracing technique improves the reconstruction degradation due to ionospheric refractions. CASSIOPE is a multi-purpose small satellite that receives HF signals from ground radar facilities and it is scheduled for launch in early 2008. Simulations have demonstrated the potential applications of the ionospheric tomography in CASSIOPE/e-POP satellite experiment.     

以太阳为射电源直接测量天线G／T

介绍以太阳为射电源,直接测量扩充后中国陆地卫星地面接收站,３个数据通道天线的Ｇ／Ｔ性能．对固定通道,测量了Ｌａｎｄｓａｔ—５下行频率的Ｇ／Ｔ～α（天线俯仰角）性能曲线;对可变通道,在α＝５°、３０°时,测量了Ｌａｎｄｓａｔ—６、ＪＥＲＳ—１、ＥＲＳ—１、ＳＰＯＴ卫星、ＺＹ—１卫星下行频率的Ｇ／Ｔ性能,以及可变通道的Ｇ／Ｔ～ｆ性能曲线,最后进行了误差分析．     

Supernova remnants with magnetars: Clues to magnetar formation

In this paper I discuss the lack of observational evidence that magnetars are formed as rapidly rotating neutron stars. Supernova remnants containing magnetars do not show the excess of kinetic energy expected for such a formation scenario, nor is there any evidence for a relic pulsar wind nebula. However, it could be that magnetars are formed with somewhat slower rotation periods, or that not all excess rotational energy was used to boost the explosion energy, for example as a result of gravitational radiation. Another observational tests for the rapid initial period hypothesis is to look for statistical evidence that about 1% of the observed supernovae have an additional 10⁴⁰–10⁴⁴ erg/s excess energy during the first year, caused by the spin down luminosity of a magnetar.     

星载SAR电离层探测及误差补偿

工作于低频波段的星载合成孔径雷达(SAR)信号会受到电离层的显著影响,因此在系统设计时必须考虑相应的补偿方法。基于此,首先开展了适用于低频大宽带SAR模式的电离层影响评估研究,建立了基于勒让德展开的五阶误差分析模型,可有效解决传统模型各阶次耦合问题。其次,针对背景电离层色散问题,开展了基于双频自聚焦算法的补偿研究,利用ALOS PALSAR数据进行了半物理仿真验证,电离层反演精度优于0.4TECU,可有效提升图像聚焦质量;针对法拉第旋转角误差,利用ALOS PALSAR回波散射矩阵信息开展了补偿研究,结果显示,相比官网提供的补偿参数,误差可进一步降低27%。上述基于回波数据本身的补偿研究,可避免第三方数据精度差、分辨率低、需地面接收机、传播路径不一致等问题,同时降低了载荷成本。最后,结合SAR高分辨率特性,基于回波的电离层反演可有效提高现有电离层探测能力,开展了PALSAR-垂测仪联合反演电子密度研究,其结果比仅垂测仪数据精度普遍提高了30%以上。研究成果可为未来低频星载SAR的系统设计提供技术支撑。     

超空泡水下航行体的结构动力响应特性

通过综合有关文献的理论,得到作用于超空泡水下航行体尾部的冲击载荷与航行速度的关系,然后利用有限元法研究了在不同速度条件下航行体的结构动力响应和变形特性.得到了加速度响应的主频率及其取值区间、航行体内的最大应变值及其分布位置;并给出了加速度响应主频率以及航行体内最大应变值与运动速度的关系.计算结果表明:存在2个加速度响应主频率,当航行体航速相对较低时,应采用第1个主频率作为结构设计时的参考频率,速度较高时应采用第2个主频率;计算还表明:航行体内最大应变值随着航速的增加而显著增加.结果对超空泡水下航行体的结构强度分析和结构优化设计有着指导意义.     

空间碎片天基监测图像仿真研究

空间碎片天基光学监测可以有效弥补地基监测的时空覆盖缺陷.目前天基观测数据不足甚至没有数据,这给监测系统研制、数据处理算法研究及软件系统研发和验证造成了一定困难.因此,研发能够为相关研究提供空间监测图像的空间碎片天基监测图像仿真软件系统具有重要意义.本文根据天基监测图像的特点,研究基本算法,设计仿真流程,给出仿真图像并对结果进行分析.为得到符合实际观测场景的高精度仿真图像,软件系统采用瞬时惯性坐标系;基于Tycho2星表提出了采用基于索引文件的快速星表查询方法来生成背景星图;利用两行根数（Two-Line Element,TLE）数据集仿真空间碎片观测值;并对结果图像中目标的运行轨迹进行分析,得出天基监测空间目标的成像特性.仿真结果表明,该仿真系统能够针对不同观测条件和观测平台,在兼顾精度与时效性的情况下,获得较为真实的空间碎片天基光学系列观测图像.