Diffusive fractionation in the chromosphere

A new mechanism for the FIP fractionation in the solar wind in the form of a stationary diffusion model is proposed. It is based on a weakly stratified chromospheric layer of constant density and temperature, permeated everywhere by ionizing photons and a homogeneous magnetic field. Our model does not invoke any particular geometry or special set up of the system and is founded solely on robust and well understood atomic collisonal physics. Technically, a boundary value problem of four coupled differential equations is solved for each chemical element, i.e. a continuity equation and a momentum equation for both atoms and singly ionized particles. For the main gas (hydrogen), an analytical solution can be found. This then serves as a background for the numerical integration of each trace gas system (several elements from He to Fe). We find that, after a few hydrogen diffusion lengths, each minor species asymptotically approaches a constant density. The ratios of these density values to some reference element reproduce the observed FIP fractionation pattern remarkably well.     

Magnetic fine structures in coronal loops

The formation of magnetic fine structures and associated electric currents is considered in the context of the coronal heating problem. The penetration of field-aligned electric currents into the lower atmosphere is discussed. It is argued that currents strong enough to heat the corona can persist only for short periods of time. The formation of thin current sheets is discussed. It is argued that photospheric magnetic structures (flux tubes) play an important role in the generation of coronal currents.     

The Structure of the Chromosphere Properties Pertaining to Element Fractionation

We review the structure and dynamics of the solar chromosphere with emphasis on the quiet Sun and properties that are relevant to element fractionation mechanisms. Attention is given to the chromospheric magnetic field, its connections to the photosphere, and to the dynamical evolution of the chromosphere. While some profound advances have been made in the “unmagnetized” chromosphere, our knowledge of the magnetically controlled chromosphere, more relevant for the discussion of element fractionation, is limited. Given the dynamic nature of the chromosphere and the poorly understood magnetic linkage to the corona, it is unlikely that we will soon know the detailed processes leading to FIP fractionation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Element Separation in the Chromosphere Ionization-Diffusion Models for the FIP-Effect

Ionization-diffusion mechanisms to understand the first ionization potential (FIP) fractionation as observed in the solar corona and the solar wind are reviewed. The enrichment of the low-FIP elements (<10 eV) compared to the high-FIP elements, seen in e.g. slow and fast wind or polar plumes, is explained. The behaviour of the heavy noble gases becomes understandable. The absolute fractionation, i.e. in relation to hydrogen, can be calculated and fits well to the measurements. The theoretical velocity-dependence of the fractionation will with used to determine the velocities of the solar wind in the chromosphere. This revised version was published online in June 2006 with corrections to the Cover Date.     

Jets and brightenings generated by energy deposition in the middle and upper solar chromosphere

Numerical simulations of energy depositions in the middle and upper solar chromosphere result in ejection of chromospheric material into the corona and heating of the chromospheric gas. These simulations may be capable of describing some of the features seen by the soft X-ray telescope on board theYohkoh satellite.     

Fine scale temporal structures in solar hard X-ray bursts observed by PHEBUS

We report here on preliminary results of a systematic study of fast temporal fluctuations in impulsive and extended solar X-ray bursts observed by PHEBUS at energies around 100 keV. Subsecond timescales are quite common in the impulsive events and are not observed in extended ones.     

Solar mass flow in fine-scale structures

Observations of transient and steady velocities at chromospheric, transition region and coronal temperatures in the quiet Sun and coronal holes are reviewed. The relevance of fine-scale structures in governing the mass balance of the solar atmosphere is stressed. At present, a coherent picture of these mass flows does not exist. However, the current observational base of transition region and coronal velocity information is limited but should greatly improve with measurements from the SOHO satellite.     

Shortcomings of the Standard Solar Model

The SSM, invented in early nineteen sixties, remains a useful construction. There are now much larger number of its predictions that may be compared with observations than when it was first introduced. Seismic sounding based on oscillations frequencies provides the best test of the physical input for modelling stellar evolution. The results of the test must be viewed as a support for the standard theory of stellar evolution. However, significant differences in the sound-speed, photospheric He abundance, and other parameters between the Sun and the current models remain. Shortcomings in the EOS and in treatment of convection have been revealed. The differences in the sound-speed in the radiative interior may be explained by small opacity errors but other explanations are possible. Results of seismic sounding support the idea that the element mixing in the outer part of the radiative interior occurred during a significant fraction of the Sun's life. Such mixing is considered as a possible explanation of the deficit of lithium. The shortcomings of SSM cannot explain the deficits of measured neutrino fluxes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Transition region line-shifts in the rebound shock spicule model

Spectral emission lines created in the solar chromosphere — corona transition region show net red-shifts. It has been proposed that this may be the result of the return of spicular material. We simulate a spicule numerically using the rebound shock model and find that the resulting hydrodynamic evolution leads to a perceived up-flow in transition region spectral lines even though the average velocity in the line forming region is directed downward. The explanation for this apparent paradox is found in the correlation between density and velocity in the waves generated by the rebound shock spicule.     

铸型搅动细晶铸造与力学性能

对比研究了高温合金铸型搅动细晶铸造和普通铸造叶轮的力学性能。研究结果表明,细晶铸造显著提高了K418B合金叶轮的室温和高温拉伸以及中温持久性能,并大幅度改善了合金的低周疲劳性能。细晶铸造K418B整体叶轮650℃的低周疲劳寿命为普通铸造的3倍。     

高速飞行器翼舵缝隙激波风洞精细测热试验研究

高速飞行器的气动控制翼舵面,为了转动灵活,在弹体和翼舵面之间存在缝隙。缝隙的存在会导致高速热气流进入,在舵轴根部产生强分离再附区域,形成高热、高压、高剪切严酷热环境,对飞行器的热防护提出了很高要求。由于影响翼舵缝隙流动的因素十分复杂,缝隙内热环境的准确预测非常困难。目前传统的激波风洞缝隙测热试验受限于薄膜热流传感器2mm直径,只能在分离再附区布置有限测点,无法捕捉到热流峰值,导致计算与试验存在较大偏差。本文根据缝隙分离再附区热环境特点,针对精细测量的可行性,从传感器选取、测点布置方案、测量及数据后处理等方面进行了详细分析,提出了分布式热电偶精细测量方法,实现了采用点测热达到面测热的效果。针对简化的圆柱弹身加舵面的模型,完成翼舵缝隙精细测热试验,获得了翼舵干扰区峰值热流。试验研究了不同缝隙高度、舵偏角、迎角对翼舵干扰区热环境的影响规律,试验结果表明:翼舵缝隙对弹身干扰主要集中在舵轴干扰区。舵轴干扰区热环境随着缝隙高度的增加而增强,随着舵偏角和迎角的增大而增大。同时,试验结果与CFD计算结果对比表明,两者基本吻合。     

Effect of no-load rate on recast layer cutting by ultra fine wire-EDM

Via material erosion in wire electrical discharge machining (WEDM), recast layers form on the surfaces of workpiece. In addition, ultra fine Wire-EDM can be usually cut once. To reduce the thickness of the recast layer as much as possible, the wire electrical discharge-electrochemical machining (WEDCM) method was proposed, which is based on the micro conductivity of the dielectric and microelectrolytic characteristics by adjusting the no-load rate of the pulse in the machining process. Furthermore, a state discrimination and servo control system based on discharge current was designed. The experiment results of different no-load rates show that the electrolytic effects increase as the no-load rate increases, and the main machining process is spark discharge erosion with a no-load rate in the range of 10% to 80%. At 90% no-load rate, the amount of recast layer formation in the forward direction of the wire electrode is almost the same as that of electrolytic dissolution, and it can be practically processed without a recast layer. Compared with 10% no-load rate, the kerf width only increases by 7.5%.     

Determination of Sulfur Abundance in the Solar Wind

Solar chemical abundances are determined by comparing solar photospheric spectra with synthetic ones obtained for different sets of abundances and physical conditions. Although such inferred results are reliable, they are model dependent. Therefore, one compares them with the values for the local interstellar medium (LISM). The argument is that they must be similar, but even for LISM abundance determinations models play a fundamental role (i.e., temperature fluctuations, clumpiness, photon leaks). There are still two possible comparisons—one with the meteoritic values and the second with solar wind abundances. In this work we derive a first estimation of the solar wind element ratios of sulfur relative to calcium and magnesium, two neighboring low-FIP elements, using 10 years of CELIAS/MTOF data. We compare the sulfur abundance with the abundance determined from spectroscopic observations and from solar energetic particles. Sulfur is a moderately volatile element, hence, meteoritic sulfur may be depleted relative to non-volatile elements, if compared to its original solar system value.     

The Composition of the Solar Wind in Polar Coronal Holes

The solar wind charge state and elemental compositions have been measured with the Solar Wind Ion Composition Spectrometers (SWICS) on Ulysses and ACE for a combined period of about 25 years. This most extensive data set includes all varieties of solar wind flows and extends over more than one solar cycle. With SWICS the abundances of all charge states of He, C, N, O, Ne, Mg, Si, S, Ar and Fe can be reliably determined (when averaged over sufficiently long time periods) under any solar wind flow conditions. Here we report on results of our detailed analysis of the elemental composition and ionization states of the most unbiased solar wind from the polar coronal holes during solar minimum in 1994–1996, which includes new values for the abundance S, Ca and Ar and a more accurate determination of the ²⁰Ne abundance. We find that in the solar minimum polar coronal hole solar wind the average freezing-in temperature is ∼1.1×10⁶ K, increasing slightly with the mass of the ion. Using an extrapolation method we derive photospheric abundances from solar wind composition measurements. We suggest that our solar-wind-derived values should be used for the photospheric ratios of Ne/Fe=1.26±0.28 and Ar/Fe=0.030±0.007.     

The ‘Standard’ Sun Modelling and Helioseismology

The ‘standard’ solar model is based on a number of simplifying assumptions and depends on knowledge of the physical properties of matter in the Sun. Given these assumptions, the constraint that the model have the observed surface luminosity provides an estimate of the initial solar helium abundance. From helioseismic analyses further information can be obtained about the present composition, including a fairly precise measure of the envelope helium abundance and an estimate of the hydrogen profile in the radiative interior. It must be emphasized, however, that these inferences may suffer from systematic error arising from incomplete knowledge about the equation of state and opacity of the solar interior. This revised version was published online in June 2006 with corrections to the Cover Date.     

SOHO contribution to the understanding of mass supply and flows in the solar corona

We expect a variety of dynamic phenomena in the quiescent non-flaring corona. Plasma flows, such as siphon flows or convective flows of chromospheric material evaporating into the corona, are expected whenever a pressure differences is established either between the footpoints or between the coronal and chromospheric segments of a coronal loop. Such flows can induce phenomena of spatial and temporal brightness variability of the corona. In particular, evaporation induces a net mass input into the corona and consequently coronal density enhancements. Flows are also expected in the regions where energy is released during magnetic reconnection. From the observational point of view the dynamics of the solar atmosphere has been investigated in great detail mostly in the lower transition region with the HRTS, and during flares with theSolar Maximum Mission andYohkoh. The high spectral, temporal and spatial resolution of theSOHO ultraviolet spectrometers should enable us in the near future to fill the gap providing a continuous coverage from the chromosphere to the corona, in the 10⁴–10⁶ K domain, and therefore to best study the dynamics throughout the solar atmosphere.     

Nickel Isotopic Composition and Nickel/Iron Ratio in the Solar Wind: Results from SOHO/CELIAS/MTOF

Using the Mass Time-of-Flight Spectrometer (MTOF)—part of the Charge, Elements, Isotope Analysis System (CELIAS)—onboard the Solar Heliospheric Observatory (SOHO) spacecraft, we derive the nickel isotopic composition for the isotopes with mass 58, 60 and 62 in the solar wind. In addition we measure the elemental abundance ratio of nickel to iron. We use data accumulated during ten years of SOHO operation to get sufficiently high counting statistics and compare periods of different solar wind velocities. We compare our values with the meteoritic ratios, which are believed to be a reliable reference for the solar system and also for the solar outer convective zone, since neither element is volatile and no isotopic fractionation is expected in meteorites. Meteoritic isotopic abundances agree with the terrestrial values and can thus be considered to be a reliable reference for the solar isotopic composition. The measurements show that the solar wind elemental Ni/Fe-ratio and the isotopic composition of solar wind nickel are consistent with the meteoritic values. This supports the concept that low-FIP elements are fed without relative fractionation into the solar wind. Our result also confirms the absence of substantial isotopic fractionation processes for medium and heavy ions acting in the solar wind.     

On the Differences in Composition between Solar Energetic Particles and Solar Wind

Although the average composition of solar energetic particles (SEPs) and the bulk solar wind are similar in a number of ways, there are key differences which imply that solar wind is not the principal seed population for SEPs accelerated by coronal mass ejection (CME) driven shocks. This paper reviews these composition differences and considers the composition of other possible seed populations, including coronal material, impulsive flare material, and interplanetary CME material.     

Modeling of dynamic evolution of coronal loops

Parameters of expanding magnetic loops and arches and of mass flows generated by them in the corona have been computed in a 1D two-fluid approximation. Two possible trigger mechanisms of the coronal transients have been considered: (i) sudden increase of the background magnetic field strength, and (ii) heating and compression plasma inside these magnetic structures. We discuss the formation of shock waves and their dependence on dynamics and geometry of the magnetic structures.     

细晶铸造K403合金热等静压及热处理工艺研究

普通铸造K403合金具有很好的高温力学性能,但塑性和中温力学性能差.为了提高K403合金的塑性和中温力学性能,采用细晶铸造工艺进行铸造,并对铸件进行热等静压和热处理.研究结果表明,细晶铸造K403合金经1190～1200℃/120～160Mpa/3～4h热等静压和1180℃/4h+980℃/6h热处理后,合金在中温下的拉伸、持久和低周疲劳性能得到大幅度的提高,且高温持久性能与普通铸造K403合金相当.