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61.
The full set of high-resolution observations from the Galileo Ultraviolet Spectrometer (UVS) is analyzed to look for spectral trends across the surface of Europa. We provide the first disk-resolved map of the 280 nm SO2 absorption feature and investigate its relationship with sulfur and electron flux distributions as well as with surface features and relative surface ages. Our results have implications for exogenic and endogenic sources. The large-scale pattern in SO2 absorption band depth is again shown to be similar to the pattern of sulfur ion implantation, but with strong variations in band depth based on terrain. In particular, the young chaos units show stronger SO2 absorption bands than expected from the average pattern of sulfur ion flux, suggesting a local source of SO2 in those regions, or diapiric heating that leads to a sulfur-rich lag deposit.While the SO2 absorption feature is confined to the trailing hemisphere, the near UV albedo (300-310 nm) has a global pattern with a minimum at the center of the trailing hemisphere and a maximum at the center of the leading hemisphere. The global nature of the albedo pattern is suggestive of an exogenic source, and several possibilities are discussed. Like the SO2 absorption, the near UV albedo also has local variations that depend on terrain type and age. 相似文献
62.
氢是太阳大气中最主要的元素。氢原子的赖曼(Lyman)谱线,尤其是赖曼阿尔法(Ly-α)谱线的辐射,是太阳色球和低过渡区能量损失的主要形式。在太阳的赖曼α像中,网络组织的辐射比较强,而辐射最强的地方是活动区。由于存在辐射转移效应,在宁静区,低阶赖曼谱线的谱形中央一般会形成一个凹陷,而在中央两侧则形成两个峰,两峰往往呈现出一定的不对称性。数值模拟和观测研究表明,赖曼谱线双峰的不对称性与高层大气中各种系统性流动有关。在太阳活动区,赖曼谱形在谱斑区与在宁静区类似;而在黑子区,赖曼谱形几乎没有中央凹陷。赖曼谱形也可用于诊断日珥、耀斑和日冕物质抛射等结构和现象的等离子体特性。该文回顾了赖曼谱线的观测历史,阐明了观测与模拟结果所揭示的物理过程,并结合笔者的认识进行了相应的评论。 相似文献
63.
Methane is, together with N2, the main precursor of Titan’s atmospheric chemistry. In our laboratory, we are currently developing a program of laboratory simulations of Titan’s atmosphere, where methane is intended to be dissociated by multiphotonic photolysis at 248 nm. A preliminary study has shown that multiphotonic absorption of methane at 248 nm is efficient and leads to the production of hydrocarbons such as C2H2 (Romanzin et al., 2008). Yet, at this wavelength, little is known about the branching ratios of the hydrocarbon radicals (CH3, CH2 and CH) and their following photochemistry. This paper thus aims at investigating methane photochemistry at 248 nm by comparing the chemical evolution observed after irradiation of CH4 at 248 and at 121.6 nm (Ly-α). It is indeed important to see if the chemistry is driven the same way at both wavelengths in particular because, on Titan, methane photolysis mainly involves Ly-α photons. An approach combining experiments and theoretical analysis by means of a specifically adapted 0-D model has thus been developed and is presented in this paper. The results obtained clearly indicate that the chemistry is different depending on the wavelength. They also suggest that at 248 nm, methane dissociation is in competition with ionisation, which could occur through a three-photon absorption process. As a consequence, 248 nm photolysis appears to be unsuitable to study methane neutral photochemistry alone. The implications of this result on our laboratory simulation program and new experimental developments are discussed. Additional information on methane photochemistry at 121.6 nm are also obtained. 相似文献
64.
陇中黄土高原夏季陆面辐射和热量特征研究 总被引:4,自引:1,他引:3
利用兰州大学半干旱气候与环境观测站(SACOL)的观测资料,分析了陇中黄土高原夏季陆面辐射和热量收支的特征.通过研究不同典型天气条件对陆面过程微气象特征的影响,发现地表反射率在晴天会出现早晨偏大的不对称结构;晴天与多云天气相比不平衡量较大,而阴天时的阵性降水会使局地能量收支出现不平衡.利用最小二乘法(OLS)线性回归得到的夏季平均不闭合度是19.6%.在半干旱区云和降水对辐射和能量收支的影响不容忽视,达到约25%的削弱程度,比极端干旱的敦煌荒漠区要大,又进一步证明了半干旱区夏季的平均气候特征与云量较多的多云天气(5≤Mean total cloud amount<8)接近.另外,7月日平均波恩比最大是4.1,平均是1.95,比极端干旱区的敦煌波恩比小1个数量级,说明榆中所处的黄土高原半干旱区比敦煌所处的极端干旱区在气候上要湿润很多. 相似文献
65.
利用2008年4月至2012年12月菏泽市紫外线观测资料以及地面常规气象观测资料和空气质量资料,分析了该地区太阳紫外线辐射的变化特征及其与各因子的相关关系,并建立逐月预报方程。结果表明:菏泽紫外线辐射年总量达到187.98 W/m2,春夏两季明显高于秋冬两季,5月达到全年的最大值,1月为全年的最小值;3~9月紫外线辐射极大值均可达到5级,其他月份均可达到4级。不同天气条件下紫外线辐射强度存在明显差异,其中晴天紫外线辐射强且稳定,呈抛物线变化;多云天紫外线辐射波动较大,时强时弱;阴天紫外线辐射相对较弱。紫外线辐射强度与风速、能见度、气温呈正相关,与总云量、低云量、相对湿度呈负相关,与SO2、PM10、NO2、PM2.5多呈负相关。基于多元线性回归分析向后剔除变量方法得出的逐月预报方程,经检验总体预报效果较好,对当地紫外线等级预报工作具有参考意义。 相似文献
66.
A. Bhadra 《Astrophysics and Space Science》2007,310(1-2):169-172
Scalar tensor (ST) theories of gravitation contain an attractor mechanism towards general relativity. The mechanism is supposed
to start back at time of inflation. Consequently the characteristic coupling function of the ST theories could attain a very
large value during the radiation epoch. Here ST cosmology in the radiation dominated universe is studied under such situation.
A general solution of the scale factor in the radiation dominated flat universe is obtained that is characterized by an additional
degree of freedom. An implication of this extra parameter is discussed.
相似文献
67.
We briefly review recent developments in black hole accretion disk theory, emphasizing the vital role played by magnetohydrodynamic
(MHD) stresses in transporting angular momentum. The apparent universality of accretion-related outflow phenomena is a strong
indicator that large-scale MHD torques facilitate vertical transport of angular momentum. This leads to an enhanced overall
rate of angular momentum transport and allows accretion of matter to proceed at an interesting rate. Furthermore, we argue
that when vertical transport is important, the radial structure of the accretion disk is modified at small radii and this
affects the disk emission spectrum. We present a simple model demonstrating how energetic, magnetically-driven outflows modify
the emergent disk emission spectrum with respect to that predicted by standard accretion disk theory. A comparison of the
predicted spectra against observations of quasar spectral energy distributions suggests that mass accretion rates inferred
using the standard disk model may be severely underestimated. 相似文献
68.
John D. Bunton 《Experimental Astronomy》2001,11(1):193-206
Future radiotelescopes need ever increasing sensitivity. With the limits of receiver sensitivity being reached this can only
be achieved by increasing collecting area. The increase in antenna numbers and the multi-resolution capabilities of these
telescopes, such as the SKA, means a significant departure from existing designs is needed. In this article, some fundamental
designs for antenna arrays that uniformly sample the UV plane are presented. The SKA is used to provide target specifications.
The designs presented allow asymmetric configurations providing more freedom for site selection. They also allow a direct
tradeoff between field of view and correlator complexity.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
69.
1IntroductionDuringthe past several years,concern hasincreasedover the potential pollution of watershed by estrogeniccompounds,including steroidal hormones fromhumanand ani mal sources.Effluents from wastewater treat-ment plants are sources of endocrine-d… 相似文献
70.
Motivated by detections of nitriles in Titan's atmosphere, cometary comae, and the interstellar medium, we report laboratory investigations of the low-temperature chemistry of acetonitrile, propionitrile, acrylonitrile, cyanoacetylene, and cyanogen (CH3CN, CH3CH2CN, CH2CHCN, HCCCN, and NCCN, respectively). A few experiments were also done on isobutyronitrile and trimethylacetonitrile ((CH3)2CHCN and (CH3)3CCN, respectively). Trends were sought, and found, in the photo- and radiation chemical products of these molecules at 12-25 K. In the absence of water, all of these molecules isomerized to isonitriles, and CH3CN, CH3CH2CN, and (CH3)2CHCN also formed ketenimines. In the presence of H2O, no isonitriles were detected but rather the cyanate ion (OCN−) was seen in all cases. Although isonitriles, ketenimines, and OCN− were the main focus of our work, we also describe cases of hydrogen loss, to make smaller nitriles, and hydrogen addition (reduction), to make larger nitriles. HCN formation also was seen in most experiments. The results are presented in terms of nitrile ice chemistry on Titan, in cometary ice, and in the interstellar medium. Possible connections to prebiotic chemistry are briefly discussed. 相似文献