太阳紫外高分辨观测

对近年来的紫外空间观测仪器（包括在研项目）作了扼要介绍，并对一些关键问题如烃基污染致使仪器灵敏度迅速下降，镜面紫外反射率低下及改进，探测器换代的必要性和困难等作了评述，文中还介绍了目前取得的紫外观测结果对宁静太阳及太阳活动区物理中的一些基本问题如色球和日冕加热，太阳风的加速，色球和过渡区中的物质流以及耀斑触发和能量传输方面所提供的有价值的诊断信息。     

无缝光谱仪用紫外透射闪耀光栅的制作

无缝光谱巡天是如今国际天文学研究的重点方向, 相比较地基巡天任务, 空间巡天可获取更多紫外波段与红外波段的光谱信息, 其发展受到色散元件的制约, 针对紫外波段的核心色散元件---紫外透射闪耀光栅开展了一系列研究. 通过全息干涉光刻产生图形, 离子束垂直刻蚀将图形转移至基底形成光栅掩模, 利用光栅掩模对倾斜离子束的遮挡作用, 使得槽底不同部位受到离子束的轰击通量不同, 从而获得非对称的槽形结构. 实验分析了倾斜离子束刻蚀中沉积物的主要组成以及对槽形和闪耀角度的影响, 并在去除沉积物影响的情况下, 成功制作了线密度为333lines/mm, 闪耀角度分别为13.2°、10.5°的紫外透射闪耀光栅, 峰值衍射效率分别可达理论值的88%以及92%, 为制作高衍射效率紫外透射闪耀光栅奠定了基础.     

The fine structure of the solar atmosphere in the far ultraviolet

High resolution spectroheliograms in the ultraviolet emission lines He i, He ii, O iv, O v, and Ne vii have been photographed during a sounding rocket flight. Simultaneously, broad band filtergrams of the far ultraviolet solar corona were obtained from the same flight. This paper describes qualitatively the spatial distribution of the UV emission. A comparison with an H filtergram is made. The most significant results can be summarized as follows: We find most of the ultraviolet emission concentrated around spicules, with different degree of concentration, decreasing with higher temperatures. 4 different areas of ultraviolet emission can be distinguished. (1) The normal network, bright in all UV emission lines from the chromosphere into the corona. (2) The coronal holes, bright in all UV emission lines up to 600 000 K but depressed in coronal lines from 1 million degrees upward. (3) The coronal depressions near active centers, absence of all ultraviolet emissions and (4) Active regions, where ultraviolet emission comes from plages, sunspots and coronal loops. High non-thermal Doppler velocities can be found in certain plage kernels around 10⁵ to 2 × 10⁵ K. Sunspots are bright in the ultraviolet, but do not exhibit He i or He ii emission. The corona above sunspots is weak. Sunspots do not show high non-thermal Doppler velocities. The He i and He ii emission does not follow either chromospheric, transition zone or coronal pattern; one can recognize some typical behavior of each.     

Fast Segmentation of Solar Extreme Ultraviolet Images

A segmentation scheme for identifying large-scale structures (coronal holes, active regions, etc.) in solar extreme ultraviolet images, is presented. Unlike standard approaches, both the image intensity and the relative contribution of different wavelengths are used. Spectral information is important for compensating luminosity changes. The approach is illustrated with images taken in the extreme ultraviolet by the EIT telescope onboard SOHO. This supervised segmentation scheme, which incorporates a Bayesian classifier, is computationally simple, and can easily be used to track in near-real time structures, such as coronal holes.     

Habitable zones and UV habitable zones around host stars

Ultraviolet radiation is a double-edged sword to life. If it is too strong, the terrestrial biological systems will be damaged. And if it is too weak, the synthesis of many biochemical compounds cannot go along. We try to obtain the continuous ultraviolet habitable zones, and compare the ultraviolet habitable zones with the habitable zones of host stars. Using the boundary ultraviolet radiation of ultraviolet habitable zone, we calculate the ultraviolet habitable zones of host stars with masses from 0.08 to 4.00 M _⊙. For the host stars with effective temperatures lower than 4,600 K, the ultraviolet habitable zones are closer than the habitable zones. For the host stars with effective temperatures higher than 7,137 K, the ultraviolet habitable zones are farther than the habitable zones. For a hot subdwarf as a host star, the distance of the ultraviolet habitable zone is about ten times more than that of the habitable zone, which is not suitable for the existence of life.     

The application of the brewster angle data for the lunar surface mapping by the real part of the refractive index

To map the lunar surface in terms of mineralogy, we suggest to use the data on variations of the real part of the refractive index obtained from the measurements of Brewster’s angle in the far ultraviolet spectral range. Such data can be acquired with an ultraviolet polarimeter onboard a spacecraft launched into a circumlunar polar orbit.     

Variable blue objects in Kapteyn Selected Area 94

We present a list of ultraviolet excess objects in the Palomar Schmidt field centred on Kapteyn Selected Area 94. All these objects were selected on the basis of optical variability. They can serve as quasar candidates.     

Comparison of lunar ultraviolet reflectivity with that of terrestrial rock samples

The ultraviolet and visible albedos of a number of terrestrial basalts, gabbros and anorthosites have been investigated over the wavelength range 800 Å to 8000 Å and compared with previously reported measurements of the lunar albedo. For most of the terrestrial samples the albedo changed only slightly between visible and middle ultraviolet wavelengths in striking contrast to the Moon where the ultraviolet albedo is about a factor of five or ten less than it is in the visible. Some of the lighter coloured terrestrial anorthositic samples were however found to have albedo curves that fairly closely approximate the ultraviolet darkening of the Moon. The general shape of the lunar ultraviolet albedo may be caused by a layer of anorthositic fragments on the Moon such as have been found to be a very abundant component of the Apollo ‘coarse-fines’.     

Ultraviolet spectroscopy of Asteroid (4) Vesta

We report a comprehensive review of the UV–visible spectrum and rotational lightcurve of Vesta combining new observations by Hubble Space Telescope and Swift Gamma-ray Burst Observatory with archival International Ultraviolet Explorer observations. The geometric albedos of Vesta from 220 nm to 953 nm are derived by carefully comparing these observations from various instruments at different times and observing geometries. Vesta has a rotationally averaged geometric albedo of 0.09 at 250 nm, 0.14 at 300 nm, 0.26 at 373 nm, 0.38 at 673 nm, and 0.30 at 950 nm. The linear spectral slope as measured between 240 and 320 nm in the ultraviolet displays a sharp minimum near a sub-Earth longitude of 20°, and maximum in the eastern hemisphere. This is consistent with the longitudinal distribution of the spectral slope in the visible wavelength. The photometric uncertainty in the ultraviolet is ∼20%, and in the visible wavelengths it is better than 10%. The amplitude of Vesta’s rotational lightcurves is ∼10% throughout the range of wavelengths we observed, but is smaller at 950 nm (∼6%) near the 1-μm band center. Contrary to earlier reports, we found no evidence for any difference between the phasing of the ultraviolet and visible/near-infrared lightcurves with respect to sub-Earth longitude. Vesta’s average spectrum between 220 and 950 nm can well be described by measured reflectance spectra of fine particle howardite-like materials of basaltic achondrite meteorites. Combining this with the in-phase behavior of the ultraviolet, visible, and near-infrared lightcurves, and the spectral slopes with respect to the rotational phase, we conclude that there is no global ultraviolet/visible reversal on Vesta. Consequently, this implies a lack of global space weathering on Vesta, as previously inferred from visible–near-infrared data.     

Short-term effects of gamma ray bursts on oceanic photosynthesis

We continue our previous work on the potential short-term influence of a gamma ray bursts on Earth’s biosphere, focusing on the only important short-term effect on life: the ultraviolet flash which occurs as a result of the retransmission of the γ radiation through the atmosphere. Thus, in this work we calculate the ultraviolet irradiances penetrating the first hundred meters of the water column, for Jerlov’s ocean water types I, II and III. Then we estimate the UV flash potential for photosynthesis inhibition in the whole photic zone, showing that it can be important in the first tens of meters of the water column.     

Single-binary star separation by ultraviolet color index diagrams

Detection of a composite flux in photometry can serve as an indication of a photometrically unresolved binarity and can contribute to the parameterization of the components of binary systems. A main goal of the present study is to develop a method of automatic photometric detection of binaries, based on multi-color photometry, theoretical stellar spectral energy distributions and general understanding of binary evolution. In particular, we consider an ultraviolet photometry where, in combination with optical and infrared photometry, interstellar reddening can be easier distinguished from temperature reddening.     

The remarkable rapid X-ray, ultraviolet, optical and infrared variability in the black hole XTE J1118+480

The transient black-hole binary XTE J1118+480 exhibited dramatic rapid variability at all wavelengths which were suitably observed during its 2000 April–July outburst. We examine time-resolved X-ray, ultraviolet, optical and infrared data spanning the plateau phase of the outburst. We find that both X-ray and infrared bands show large amplitude variability. The ultraviolet and optical variability is more subdued, but clearly correlated with that seen in the X-rays. The ultraviolet, at least, appears to be dominated by the continuum, although the lines are also variable. Using the X-ray variations as a reference point, we find that the ultraviolet (UV) variability at long wavelengths occurs later than that at short wavelengths. Uncertainty in the Hubble Space Telescope timing prohibits a determination of the absolute lag with respect to the X-rays, however. The transfer function is clearly not a delta-function, exhibiting significant repeatable structure. For the main signal we can rule out an origin in reprocessing on the companion star – the lack of variation in the lags is not consistent with this, given a relatively high orbital inclination. Weak reprocessing from the disc and/or companion star may be present, but is not required, and another component must dominate the variability. This could be variable synchrotron emission correlated with X-ray variability, consistent with our earlier interpretation of the infrared (IR) flux as due to synchrotron emission rather than thermal disc emission. In fact, the broad-band energy distribution of the variability from IR to X-rays is consistent with expectations of optically thin synchrotron emission. We also follow the evolution of the low-frequency quasi-periodic oscillation in X-rays, UV, and optical. Its properties at all wavelengths are similar, indicating a common origin.     

Complex refractive index of Martian dust: Mariner 9 ultraviolet observations

Mariner 9 ultraviolet spectra of the 1971 Mars dust storm were studied to determine the cloud particle size distribution and complex index of refraction. The method consisted of matching the observed single particle scattering albedo and phase function with Mie scattering calculations for size distributions of spheres of homogeneous and isotropic material. Preliminary results indicate that the effective particle radius is 1 μm with an effective variance (a measure of distribution width) $\text{?0.2}$. The real component of the index of refraction is $\text{?1.8}$ at both 268 and 305 nm. For the imaginary index, a value of 0.02 was found at 268 nm and 0.01 at 305nm. These ultraviolet refractive indices are compatible with measurements at visible wavelengths which indicate that the real part of the refractive index is 1.75 with a negligible imaginary term. The rapid increase of refractive index and absorption coefficient with decreasing wavelength are indicative of an ultraviolet absorption band.An ultraviolet absorption band is not only diagnostic of the composition of the Mars material, but may have important implications for the development and evolution of life on Mars. A 30 μm layer of material that absorbs uv but transmits visible light can shield organisms from harmful irradiation while providing for photosynthesis.Comparison of the Mars ultraviolet refractive indices with laboratory measurements indicates that none of the terrestrial analog samples of limonite, basalt, andesite, or montmorrillonite have the required ultraviolet properties.     

Current sheets as the source of heating for solar active regions

Observational data and theoretical arguments suggest that the heating source for an active region is the quasi-steady dissipation of magnetic field in current sheets. Effects in the solar atmosphere which are due to the presence of current sheets are considered. The most important of them is the heating of the chromosphere by the strong ultraviolet radiation of the current sheet. This can give rise to the brightening of an active region in optical emission. The energy flux from the current sheet in different ranges of the ultraviolet spectrum and the depths (column densities) into the chromosphere where this energy is absorbed are estimated.     

Solving the mysteries of the diffuse interstellar medium with high-resolution UV spectroscopy

Understanding the complex structure, dynamics, and ionization of gas in the nearby interstellar medium is required before one can realistically model interstellar gas in other galaxies. High-resolution ultraviolet spectra provide the essential data for such studies because the resonance lines of most important atoms, ions, and molecules are located in the ultraviolet, and high spectral resolution is needed to resolve line profiles and determine the velocity structure along a line of sight. I list ten important physical questions concerning interstellar gas that require a more sensitive spectrometer than STIS and the desired spectral resolution to answer these questions.     

Features of the interstellar extinction curve

The extinction curves for spherical particles are subject to the errors of the particle material's refractive index. Their sensitivity to these errors has been investigated and is found to be dependent upon wavelength. For graphite, significant errors are produced in the far ultraviolet part of the extinction curve; for silicates, in the near ultraviolet; while for iron the error is relatively small. The wavelength dependence of the 10 μm and 20 μm absorption bands of small silicate spheroids upon their shape and alignment has been studied. It is found that the bands can be displaced by ～1 μm towards longer wavelengths from their positions for corresponding spheres: and that a further, though small, displacement can be superimposed upon this by their subsequent alignment.     

Quantifying dust and the ultraviolet radiation density in the local Universe

A sample of local galaxies for which far-infrared and ultraviolet fluxes are available is used to estimate the characteristic dust extinction in galaxies and to test whether standard dust properties are plausible. Assuming galaxies can be characterized by a single dust optical depth (certainly not valid for galaxies with a dominant starburst component), the infrared excess and ultraviolet colours of local galaxies are found to be consistent with normal Milky Way dust, with a mean value for   E ( B − V )  of 0.16. A significant fraction of the dust heating is caused by older, lower-mass stars, and this fraction increases towards earlier galaxy types.
Analysis of  ( F _FIR/ F _UV)  versus ultraviolet colour diagrams for starburst galaxies in terms of a simple screen dust model does not support a Calzetti (1997) rather than a Milky Way extinction law, though the absence of the expected strong 2200- Å feature in several galaxies with IUE spectra does show that more detailed radiative transfer models are needed – probably with nonspherical geometry.
A simple treatment in which the  100/60-μm  flux ratio is used to subtract the optically thick starburst contribution to the far-infrared radiation results in lower extinction estimates for the optically thin cirrus component, with a mean   E ( B − V )  of 0.10.
The ultraviolet luminosity density, corrected for dust extinction, is derived and a value for the local mean star formation rate inferred. This is consistent with previous estimates from ultraviolet surveys and from  Hα  surveys.     

Stellar spectroscopy in ground-based ultraviolet. I: Observational technique

We present a historical note on the main spectroscopy instruments close to the short-wave boundary of the optical atmospheric window. Ground-based ultraviolet observations imposemore stringent requirements to the optics of telescopes and spectrographs, as well as to the calibration means. We have to bear in mind the seasonal variations of the optical properties of the Earth’s atmosphere. We consider the capabilities of ultraviolet observations with high spectral resolution on the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences.     

Studies related to satellite thermal control: Measurements of earth-reflected sunlight and stability of thermal-control coatings

As part of a study of satellite thermal control, data were obtained on earth-reflected solar radiation and on the stability of thermal-control coatings. The intensity distribution of incoming solar radiation to earth was found to be shifted toward the ultraviolet region upon reflection by the atmosphere. As a result of the shift, the intensity of reflected radiation reaches a maximum in the near ultraviolet. At shorter wavelengths in the ultraviolet region, the intensity drops sharply to zero as a result of absorption of incoming radiation by ozone in the upper atmosphere. Comparisons of calculated intensity distributions with measured distributions for two different atmospheric conditions gave good agreement, except at the shorter ultraviolet wavelengths where the calculations did not adequately include the effect of ozone absorption. Measurements of stability of thermal-control coatings showed a lower degradation rate of white paints than was obtained on other flight experiments flown outside of the protective influence of earth's magnetosphere. The differences in degradation rate were much larger than expected, indicating that further study is required in the development of white coatings for spacecraft thermal control.