MESSENGER: Exploring Mercury’s Magnetosphere

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only ∼1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. Magnetic reconnection at the dayside magnetopause may erode the subsolar magnetosphere, allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, ∼1–2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic tail. Because of Mercury’s proximity to the sun, 0.3–0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the solar wind–Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multi-polar decomposition of Mercury’s intrinsic magnetic field. MESSENGER’s highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.     

D/H and Nearby Interstellar Cloud Structures

Analysis of UV spectra obtained with the HST, FUSE and other satellites provides a new understanding of the deuterium abundance in the local region of the galactic disk. The wide range of gas-phase D/H measurements obtained outside of the Local Bubble can now be explained as due to different amounts of deuterium depletion on carbonaceous grains. The total D/H ratio including deuterium in the gas and dust phases is at least 23 parts per million of hydrogen, which is providing a challenge to models of galactic chemical evolution. Analysis of HST and ground-based spectra of many lines of sight to stars within the Local Bubble have identified interstellar velocity components that are consistent with more than 15 velocity vectors. We have identified the structures of 15 nearby warm interstellar clouds on the basis of these velocity vectors and common temperatures and depletions. We estimate the distances and masses of these clouds and compare their locations with cold interstellar clouds.     

The Grain Impact Analyser and Dust Accumulator (GIADA) Experiment for the Rosetta Mission: Design, Performances and First Results

The Grain Impact Analyser and Dust Accumulator (GIADA) onboard the ROSETTA mission to comet 67P/Churyumov–Gerasimenko is devoted to study the cometary dust environment. Thanks to the rendezvous configuration of the mission, GIADA will be plunged in the dust environment of the coma and will be able to explore dust flux evolution and grain dynamic properties with position and time. This will represent a unique opportunity to perform measurements on key parameters that no ground-based observation or fly-by mission is able to obtain and that no tail or coma model elaborated so far has been able to properly simulate. The coma and nucleus properties shall be, then, clarified with consequent improvement of models describing inner and outer coma evolution, but also of models about nucleus emission during different phases of its evolution. GIADA shall be capable to measure mass/size of single particles larger than about 15 μm together with momentum in the range 6.5 × 10⁻¹⁰ ÷ 4.0 × 10⁻⁴ kg m s⁻¹ for velocities up to about 300 m s⁻¹. For micron/submicron particles the cumulative mass shall be detected with sensitivity 10⁻¹⁰ g. These performances are suitable to provide a statistically relevant set of data about dust physical and dynamic properties in the dust environment expected for the target comet 67P/Churyumov–Gerasimenko. Pre-flight measurements and post-launch checkouts demonstrate that GIADA is behaving as expected according to the design specifications. The International GIADA Consortium (I, E, UK, F, D, USA).     

The Genesis Solar Wind Concentrator Target: Mass Fractionation Characterised by Neon Isotopes

The concentrator on Genesis provided samples of increased fluences of solar wind ions for precise determination of the oxygen isotopic composition. The concentration process caused mass fractionation as a function of the radial target position. This fractionation was measured using Ne released by UV laser ablation and compared with modelled Ne data, obtained from ion-trajectory simulations. Measured data show that the concentrator performed as expected and indicate a radially symmetric concentration process. Measured concentration factors are up to ∼30 at the target centre. The total range of isotopic fractionation along the target radius is 3.8%/amu, with monotonically decreasing ²⁰Ne/²²Ne towards the centre, which differs from model predictions. We discuss potential reasons and propose future attempts to overcome these disagreements.     

MESSENGER Mission Overview

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched on August 3, 2004, is nearing the halfway point on its voyage to become the first probe to orbit the planet Mercury. The mission, spacecraft, and payload are designed to answer six fundamental questions regarding the innermost planet: (1) What planetary formational processes led to Mercury’s high ratio of metal to silicate? (2) What is the geological history of Mercury? (3) What are the nature and origin of Mercury’s magnetic field? (4) What are the structure and state of Mercury’s core? (5) What are the radar-reflective materials at Mercury’s poles? (6) What are the important volatile species and their sources and sinks near Mercury? The mission has focused to date on commissioning the spacecraft and science payload as well as planning for flyby and orbital operations. The second Venus flyby (June 2007) will complete final rehearsals for the Mercury flyby operations in January and October 2008 and September 2009. Those flybys will provide opportunities to image the hemisphere of the planet not seen by Mariner 10, obtain high-resolution spectral observations with which to map surface mineralogy and assay the exosphere, and carry out an exploration of the magnetic field and energetic particle distribution in the near-Mercury environment. The orbital phase, beginning on March 18, 2011, is a one-year-long, near-polar-orbital observational campaign that will address all mission goals. The orbital phase will complete global imaging, yield detailed surface compositional and topographic data over the northern hemisphere, determine the geometry of Mercury’s internal magnetic field and magnetosphere, ascertain the radius and physical state of Mercury’s outer core, assess the nature of Mercury’s polar deposits, and inventory exospheric neutrals and magnetospheric charged particle species over a range of dynamic conditions. Answering the questions that have guided the MESSENGER mission will expand our understanding of the formation and evolution of the terrestrial planets as a family.     

Solar and Solar-Wind Composition Results from the Genesis Mission

The Genesis mission returned samples of solar wind to Earth in September 2004 for ground-based analyses of solar-wind composition, particularly for isotope ratios. Substrates, consisting mostly of high-purity semiconductor materials, were exposed to the solar wind at L1 from December 2001 to April 2004. In addition to a bulk sample of the solar wind, separate samples of coronal hole (CH), interstream (IS), and coronal mass ejection material were obtained. Although many substrates were broken upon landing due to the failure to deploy the parachute, a number of results have been obtained, and most of the primary science objectives will likely be met. These objectives include He, Ne, Ar, Kr, and Xe isotope ratios in the bulk solar wind and in different solar-wind regimes, and ¹⁵N/¹⁴N and ¹⁸O/¹⁷O/¹⁶O to high precision. The greatest successes to date have been with the noble gases. Light noble gases from bulk solar wind and separate solar-wind regime samples have now been analyzed. Helium results show clear evidence of isotopic fractionation between CH and IS samples, consistent with simplistic Coulomb drag theory predictions of fractionation between the photosphere and different solar-wind regimes, though fractionation by wave heating is also a possible explanation. Neon results from closed system stepped etching of bulk metallic glass have revealed the nature of isotopic fractionation as a function of depth, which in lunar samples have for years deceptively suggested the presence of an additional, energetic component in solar wind trapped in lunar grains and meteorites. Isotope ratios of the heavy noble gases, nitrogen, and oxygen are in the process of being measured.     

弯曲网格上的间断有限元湍流数值解法研究

采用间断有限元方法对雷诺平均Navier-Stokes(RANS)方程进行了数值求解,对Spalart-Allmaras单方程湍流模型进行了部分修正,使得求解器更加鲁棒。构造了分段高次多项式来逼近真实物面,同时物面附近采用多层弯曲网格来避免网格交叉,此外提出了一种快速计算积分点的曲面物面距的方法。采用混合网格对NACA0012翼型以及RAE翼型进行了数值计算,并与实验数据以及前人数据进行了对比。计算结果表明,采用物面弯曲网格结合修正的湍流模型方法在相对稀疏的网格上就能得到比较好的数值解。     

三旋流器加装外套环对燃烧性能的影响

对带有两种不同头部结构（基准型为方案1,头部加装外套环为方案2）的三旋流单头部燃烧室进行了研究,对有两种头部结构的燃烧室进行了流场和燃烧场的数值模拟,在相同的进口试验条件下,用燃气分析法和15点测温耙分别测量了两种头部的燃烧效率、出口冒烟数和出口温度场.结果表明:三旋流器的燃烧效率超过了99.7%,加装外套环后其流场、温度场分布均较基准型有所改善,回流区更加饱满,油气混合和燃油雾化效果增强,出口温度分布系数（OTDF）和出口冒烟数分别降低了36%和25%.      

喷口尺寸对高温升燃烧室副喷嘴性能影响

为了获得某高温升燃烧室副喷嘴性能及其与喷口尺寸的相互关系,通过试验研究了供油压力和离心喷嘴的关键结构参 数对燃油流量、喷雾锥角和雾化性能的影响。研究的关键参数有喷口长度与喷口直径,采用3维相位多普勒粒子分析仪测量了某 一直线上各点的数密度分布,以及R-R（Rosin-Rammler）分布的特征直径D 和均匀度指数N。结果表明：在供油压力为164~3350 kPa的条件下,燃油流量和喷雾锥角随喷口长径比（旋流室与喷口直径比不变）和旋流室与喷口直径比（喷口长径比不变）的增大 而减小,增大喷口长径比和减小旋流室与喷口直径比可改善雾化效果。研究获得的规律为三旋流高温升燃烧室喷嘴优化设计提 供了重要的理论依据。     

基于数字孪生的设备加工过程监测技术研究

针对典型航空产品装配生产线的生产过程中缺乏有效的人机交互手段,设备实时运行状态可视化监控程度低等问题,提出了一种基于数字孪生的加工设备运行状态监测方法,包括模型构建与优化处理、NC程序解析与加工仿真、软硬件通信与数据传输和实时仿真与数据分析.最后在某航空发动机外涵机匣装配生产线验证了该方案,可以实现NC程序加工仿真分析...