Dynamo coefficients from local simulations of the turbulent ISM     

O. Gressel  U. Ziegler  D. Elstner  G. Rüdiger 《Astronomische Nachrichten》2008,329(6):619-624

Observations in polarized emission reveal the existence of large‐scale coherent magnetic fields in a wide range of spiral galaxies. Radio‐polarization data show that these fields are strongly inclined towards the radial direction, with pitch angles up to 35° and thus cannot be explained by differential rotation alone. Global dynamo models describe the generation of the radial magnetic field from the underlying turbulence via the so called α ‐effect. However, these global models still rely on crude assumptions about the small‐scale turbulence. To overcome these restrictions we perform fully dynamical MHD simulations of interstellar turbulence driven by supernova explosions. From our simulations we extract profiles of the contributing diagonal elements of the dynamo α ‐tensor as functions of galactic height. We also measure the coefficients describing vertical pumping and find that the ratio between these two effects has been overestimated in earlier analytical work, where dynamo action seemed impossible. In contradiction to these models based on isolated remnants we always find the pumping to be directed inward. In addition we observe that depends on whether clustering in terms of superbubbles is taken into account. Finally, we apply a test field method to derive a quantitative measure of the turbulent magnetic diffusivity which we determine to be ∼2 kpckms^–1. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Hopf bifurcations with fluctuating gain     

S.A. Triana  D.H. Kelley  D. Zimmerman  D. Sisan  D.P. Lathrop 《Astronomische Nachrichten》2008,329(7):701-705

A Hopf bifurcation describes a transition from a steady state to an oscillatory one. This report describes how the presence of white Gaussian noise in the term driving the instability (the gain) affects the character of such a transition. We find that for small noise intensities the effective critical value of the bifurcation parameter increases linearly with the noise intensity. This result is used to quantitatively describe the onset of the magneto‐rotational instability in a laboratory experiment using liquid metal with turbulent background flow. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Nonhelical mean‐field dynamos in a sheared turbulence     

I. Rogachevskii  N. Kleeorin 《Astronomische Nachrichten》2008,329(7):732-736

Mechanisms of nonhelical large‐scale dynamos (shear‐current dynamo and effect of homogeneous kinetic helicity fluctuations with zero mean) in a homogeneous turbulence with large‐scale shear are discussed. We have found that the shearcurrent dynamo can act even in random flows with small Reynolds numbers. However, in this case mean‐field dynamo requires small magnetic Prandtl numbers (i.e., when Pm < Pm^cr < 1). The threshold in the magnetic Prandtl number, Pm^cr = 0.24, is determined using second order correlation approximation (or first‐order smoothing approximation) for a background random flow with a scale‐dependent viscous correlation time τ_c = (νk 2)^–1 (where ν is the kinematic viscosity of the fluid and k is the wave number). For turbulent flows with large Reynolds numbers shear‐current dynamo occurs for arbitrary magnetic Prandtl numbers. This dynamo effect represents a very generic mechanism for generating large‐scale magnetic fields in a broad class of astrophysical turbulent systems with large‐scale shear. On the other hand, mean‐field dynamo due to homogeneous kinetic helicity fluctuations alone in a sheared turbulence is not realistic for a broad class of astrophysical systems because it requires a very specific random forcing of kinetic helicity fluctuations that contains, e.g., low‐frequency oscillations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

公共电话交换网时间认证系统的研制     

和康元  吴贵臣  冯平 《时间频率学报》2008,31(1)

研制了一种通过公共电话交换网进行时间认证的系统设备,利用电话网为数字时间戳服务机构提供标准时间服务.经实测,时间同步精度可以在±0.3 ms范围内,能满足数字时间戳服务机构为银行、证券、股票和期货交易等电子商务以及电子政务活动提供时间认证服务方面的要求.  相似文献   

Formation of Transient Coronal Holes during the Eruption of a Quiescent Filament and its Overlying Sigmoid     

Li-Heng Yang  Yun-Chun Jiang  Dong-Bai Ren 《中国天文和天体物理学报》2008,8(3):329-336

By using Hα, He I 10830, EUV and soft X-ray (SXR) data, we examined a filament eruption that occurred on a quiet-sun region near the center of the solar disk on 2006 January 12, which disturbed a sigmoid overlying the filament channel observed by the GOES-12 SXR Imager (SXI), and led to the eruption of the sigmoid. The event was associated with a partial halo coronal mass ejection (CME) observed by the Large Angle and Spectrometric Coronagraphs (LASCO) on board the Solar and Heliospheric Observatory (SOHO), and resulted in the formation of two flare-like ribbons, post-eruption coronal loops, and two transient coronal holes (TCHs), but there were no significantly recorded GOES or Hα flares corresponding to the eruption. The two TCHs were dominated by opposite magnetic polarities and were located on the two ends of the eruptive sigmoid. They showed similar locations and shapes in He Ⅰ 10830, EUV and SXR observations. During the early eruption phase, brightenings first appeared on the locations of the two subsequent TCHs, which could be clearly identified on He Ⅰ 10830, EUV and SXR images. This eruption could be explained by the magnetic flux rope model, and the two TCHs were likely to be the feet of the flux rope.  相似文献   

Circumnuclear Star Forming Activity in NGC 3982     

Shui-Nai Zhang Qiu-Sheng Gu Yi-Peng Wang 《中国天文和天体物理学报》2008,8(5)

We present a study of the nearby Seyfert galaxy NGC 3982 using optical,infrared and X-ray data acquired by SDSS,Spitzer and Chandra.Our main results are as follows:(1) A simple stellar population synthesis on the nuclear and circumnuclear SDSS spectra gives unambiguous evidence of young stellar components in both the nuclear and circumnuclear regions.(2) The Spitzer Infrared Spectrograph (IRS) spectrum of the central region (～3") shows a power-law continuum,a silicate emission feature at 9.7 μm,and significant PAH emission features at 7.7,8.6,11.3 and 12.7/zm,suggesting the coexistence of AGN and starburst activities in the central region of NGC 3982.(3) We estimate the star formation rate (SFR) of the circumnuclear (～5"-20") region from the Ha luminosity to be for the active nucleus of NGC 3982 from radio to X-ray,and obtain a bolometric luminosity of Lbol=4.5×1042 erg s-1,corresponding to an Eddington ratio (Lbol/LEdd) of 0.014.The HST image of NGC 3982 shows a nuclear mini-spiral between the circumnuclear starforming region and the nucleus,which could be the channel through which gas is transported to the supermassive black hole from the circumnuclear star-forming region.  相似文献   

基于伪随机信号的磁电法渗漏模型试验     

张化鹏  钱卫  刘瑾  武立林  宋泽卓 《物探与化探》2022,(1):198-205

磁电法是近些年在国内兴起的一种电磁勘探方法,该方法在探测地下渗流等长走向的良导地质体方面具有技术优势,但是其抗干扰能力差,易受外部噪声影响,一直未能推广.近年来,伪随机信号在物探领域开始广泛应用,该技术通过对冲激响应及阶跃响应进行卷积运算来达到降噪目的 ,可极大地提升抗干扰能力,因此,本文结合磁电法的勘探原理及伪随机辨...  相似文献   

1901~2013年GPCC和CRU降水资料在中国大陆的适用性评估   总被引：2，自引：0，他引：2  

王丹  王爱慧 《气候与环境研究》2017,22(4):446-462

利用1901~2013年中国大陆地区的气象台站实测降水资料,对东英吉利（East Anglia）大学气候研究中心（Climatic Research Unit,CRU）和全球降水气候中心（Global Precipitation Climatology Centre,GPCC）的降水资料分别从季节、年际和年代际尺度上进行了评估。结果表明：1961~2013年CRU与GPCC降水资料均能较准确地描述中国大陆地区的降水特征,且在东部较西部地区、夏季较冬季与站点实测降水情况更为一致。将中国大陆划分为不同区域并在其季节、年际和年代际时间尺度上通过比较降水偏差绝对值的百分比、均方根误差和相关系数等统计量后发现：CRU在青藏高原和其它较大的山脉附近与站点实测降水的差别较大,且年均降水趋势在西北一带的阿尔金山脉、黄土高原、东南地区和长江下游地区,比实测降水的年均趋势小、甚至出现趋势相反的情况。此外,CRU降水的年代际变化趋势也偏小。而GPCC数据不论是降水量还是降水趋势都更接近实际情况。在1901~1961年,通过与65个长期气象观测站点的降水时间序列比较发现,CRU在110°E以西地区与站点观测的降水资料间的差别较大,而GPCC与站点观测资料的吻合较好。最后,利用1961~2013年两套降水资料和站点实测资料分别计算了标准化降水指数（SPI）,简单分析了中国大陆地区的干旱变化,发现GPCC对旱涝的时空变化特征的描述比CRU更接近站点实际观测;并且CRU也没有反映出1997年夏季中国地区出现的严重干旱情况,而GPCC较为准确地反映出了这一干旱事件特征。因此,本文的研究结果认为,就中国大陆地区长时期降水资料而言,GPCC的适用性优于CRU。  相似文献   

青藏高原大气甲烷浓度时空分布变化特征     

冯冬霞  高晓清  周亚  杨丽薇  惠小英 《气候与环境研究》2017,22(3):346-354

利用瓦里关大气本底站甲烷观测数据对美国Aqua卫星的AIRS观测结果进行对比分析,并分析研究了2003~2012年青藏高原对流层大气甲烷的时空分布特征,结果表明:1)AIRS观测结果与近地面观测资料变化趋势一致,存在显著的正相关关系,突变时间比较一致,可以用于青藏高原区域的甲烷浓度特征分析。2)青藏高原对流层甲烷浓度在空间分布上存在显著的西北—东南走向的低值带及其南北侧存在4个固定的高值中心,分别位于阿里、那曲、山南和玉树。3)青藏高原甲烷浓度呈现显著随高度而降低的趋势,年平均甲烷浓度分别为1.810ppm(1 ppm=10-6)、1.797 ppm和1.781 ppm。在对流层中层和中上层,甲烷浓度基本呈现低值带最低、南北侧均高的山谷型分布特征。在对流层层顶,以低值带为分界线,呈现明显的南高北低特征。4)青藏高原甲烷浓度随时间呈缓慢上升趋势,平均速度为0.0018 ppm/a,夏季上升最快,秋季上升最慢。5)青藏高原甲烷存在明显的单峰型季节变化特征,夏秋季高,冬春季低,与东部地区冬、夏双峰型特征不同,随着高度上升季节变化更为明显。  相似文献   

UV-B辐射对青岛大扁藻生长及其某些生理特性的影响   总被引：3，自引：0，他引：3  

蔡恒江  唐学玺  张培玉  于娟 《海洋科学进展》2005,23(4):460-465

通过实验生态学和生物化学的方法,研究了UV-B辐射对青岛大扁藻生长及其叶绿素a（Chl-a）含量、可溶性蛋白含量、过氧化氢（H2O2）含量、丙二醛（MDA）含量和超氧化物歧化酶（SOD）活力的影响。结果表明：UV-B辐射会抑制青岛大扁藻的生长并对其生理特性产生明显的影响。经过UVB辐射后,Chl-a含量和可溶性蛋白含量在初始阶段（0～3d）有所增加,而后却出现极显著的下降;过氧化氢含量、丙二醛含量和超氧化物歧化酶活力则呈现升高的趋势。  相似文献