A Portrait of the Nucleus of Comet 67P/Churyumov-Gerasimenko

In 2003, comet 67P/Churyumov–Gerasimenko was selected as the new target of the Rosetta mission as the most suitable alternative to the original target, comet 46P/Wirtanen, on the basis of orbital considerations even though very little was known about the physical properties of its nucleus. In a matter of a few years and based on highly focused observational campaigns as well as thorough theoretical investigations, a detailed portrait of this nucleus has been established that will serve as a baseline for planning the Rosetta operations and observations. In this review article, we present a novel method to determine the size and shape of a cometary nucleus: several visible light curves were inverted to produce a size–scale free three–dimensional shape, the size scaling being imposed by a thermal light curve. The procedure converges to two solutions which are only marginally different. The nucleus of comet 67P/Churyumov–Gerasimenko emerges as an irregular body with an effective radius (that of the sphere having the same volume) = 1.72 km and moderate axial ratios a/b = 1.26 and a/c = 1.5 to 1.6. The overall dimensions measured along the principal axis for the two solutions are 4.49–4.75 km, 3.54–3.77 km and 2.94–2.92 km. The nucleus is found to be in principal axis rotation with a period = 12.4–12.7 h. Merging all observational constraints allow us to specify two regions for the direction of the rotational axis of the nucleus: RA = 220°^+50° _−30° and Dec = −70^° ± 10^° (retrograde rotation) or RA = 40°^+50° _-30° and Dec = +70^°± 10^° (prograde), the better convergence of the various determinations presently favoring the first solution. The phase function, although constrained by only two data points, exhibits a strong opposition effect rather similar to that of comet 9P/Tempel 1. The definition of the disk–integrated albedo of an irregular body having a strong opposition effect raises problems, and the various alternatives led to a R-band geometric albedo in the range 0.045–0.060, consistent with our present knowledge of cometary nuclei. The active fraction is low, not exceeding ~ 7% at perihelion, and is probably limited to one or two active regions subjected to a strong seasonal effect, a picture coherent with the asymmetric behaviour of the coma. Our slightly downward revision of the size of the nucleus of comet 67P/Churyumov-Gerasimenko resulting from the present analysis (with the correlative increase of the albedo compared to the originally assumed value of 0.04), and our best estimate of the bulk density of 370 kg m⁻³, lead to a mass of ~ 8 × 10¹² kg which should ease the landing of Philae and insure the overall success of the Rosetta mission.     

The Rosetta Mission: Flying Towards the Origin of the Solar System

The ROSETTA Mission, the Planetary Cornerstone Mission in the European Space Agency’s long-term programme Horizon 2000, will rendezvous in 2014 with comet 67P/Churyumov-Gerasimenko close to its aphelion and will study the physical and chemical properties of the nucleus, the evolution of the coma during the comet’s approach to the Sun, and the development of the interaction region of the solar wind and the comet, for more than one year until it reaches perihelion. In addition to the investigations performed by the scientific instruments on board the orbiter, the ROSETTA lander PHILAE will be deployed onto the surface of the nucleus. On its way to comet 67P/Churyumov-Gerasimenko, ROSETTA will fly by and study the two asteroids 2867 Steins and 21 Lutetia.     

In-Situ Solar Wind and Magnetic Field Signatures of Interplanetary Coronal Mass Ejections

The heliospheric counterparts of coronal mass ejections (CMEs) at the Sun, interplanetary coronal mass ejections (ICMEs), can be identified in situ based on a number of magnetic field, plasma, compositional and energetic particle signatures as well as combinations thereof. We summarize these signatures and their implications for understanding the nature of these structures and the physical properties of coronal mass ejections. We conclude that our understanding of ICMEs is far from complete and formulate several challenges that, if addressed, would substantially improve our knowledge of the relationship between CMEs at the Sun and in the heliosphere.     

Solar Wind Sources and Their Variations Over the Solar Cycle

In this paper I will briefly summarize the present status of our knowledge on the four different sorts of solar wind, their sources and their short- and long-term variations. First: the fast solar wind in high-speed streams that emerges from coronal hole regions. Second: the slow solar wind emerging from the non-active Sun near the global heliospheric current sheet above helmet streamers and underlying active regions. Third: the slow solar wind filling most of the heliosphere during high solar activity, emerging above active regions in a highly turbulent state, and fourth: the plasma expelled from the Sun during coronal mass ejections. The coronal sources of these different flows vary dramatically with the solar activity cycle.     

Solar Energetic Particle Events: Phenomenology and Prediction

Space Science Reviews - Solar energetic particle events can cause major disruptions to the operation of spacecraft in earth orbit and outside the earth's magnetosphere and have to be considered...     

活性碳毡对称振子阵列／树脂复合吸波材料的研究

研究了含活性碳毡对称振子阵列吸波复合材料的微波吸收特性.结果表明:含振子阵列吸波材料的吸波性能与入射电场方向、振子两臂间的电阻、振子间的距离和阵列的位置密切相关.对称振子阵列对电磁波的吸收呈各向异性,振子与入射电场平行时,吸波性能较好;随振子两臂间电阻或振子间距的增大,材料的吸波性能先提高后降低,电阻和振子间距均存在最佳值;随振子阵列和试样下表面距离的减小,材料的吸波性能提高,振子与入射电场平行时,本实验条件下可获得有效带宽13GHz,最大吸收峰值-30.3dB的反射衰减.     

基于微型压力传感器阵列的边界层分离点检测方法

为了检测边界层的分离点,探讨了根据表面压力分布变化判定分离点的检测方法,提出了基于微机电系统(MEMS)技术和柔性衬底的微型压力传感器阵列的结构方案,可实现在线实时测量,并满足非平面表面的流体测量要求;同时提出了1种分离点检测的判定方法,并通过二维圆柱绕流仿真验证了该方法的正确性和有效性。微型压力传感器阵列的引入,拓宽了边界层分离点检测的解决途径。     

On the Lyman α and O VI Line Profiles in Streamers and Coronal Holes

The profiles of the Lyα line at 1215.6 Å and of the O VI doublet at 1031.9 Å and 1037.6 Å in the extended solar corona have been analyzed vs. latitude and radial direction, performing observations with the Ultraviolet Coronagraph Spectrometer (UVCS) on board the ESA-NASA solar satellite SOHO (Solar and Heliospheric Observatory). The results show that these lines behave differently with latitude: the H I Ly α line has larger full width at half maximum (FWHM) values in the streamer region and narrower values towards the pole, while the FWHM of O VI lines has a minimum at the center of the streamer and slightly increases towards the polar regions. We briefly discuss the impact of the results on coronal heating theories.     

The Transition Between Fast and Slow Solar Wind from Composition Data

The transition between coronal hole associated fast solar wind and slow solar wind is studied using data from the high resolution mass spectrometer SWICS on ACE. We discuss the data in the framework of a recent theory about the global heliospheric magnetic field and conclude that the data are consistent with magnetic connections between field-lines in the fast and in the slow wind. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Theory of Anomalous Cosmic Rays

Anomalous cosmic rays are a heliospheric phenomenon in which interstellar neutral atoms stream into the heliosphere, are ionized by either solar radiation or the solar wind, and are subsequently accelerated to very high energies, greater than 1 GeV. Current thinking has the bulk of the acceleration to very-high energies taking place, by the mechanism of diffusive shock acceleration, at the termination shock of the solar wind. Detailed two-dimensional numerical simulations and models based on this picture show broad agreement with a number of the observed properties of anomalous cosmic rays. Recent improvements to this picture include the observation of multiply charged cosmic rays and the suggestion that some "preacceleration" of the initially ionized particles occurs in the inner heliosphere.