共查询到19条相似文献，搜索用时 140 毫秒
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激光驱动亥姆霍兹电容线圈靶的磁重联实验已经提出并进行了多年.当实验中的金属板被强激光照射时产生自由电子,这些自由电子的运动在连接两金属板的两个平行线圈中产生电流,由两个平行线圈内部电流产生的磁场之间随即发生重联.该实验不同于其他直接由Biermann电池效应所产生高β(等离子体热压与磁压的比值)环境下的磁重联实验.对该类实验进行了3维磁流体动力学数值模拟,首次展示了亥姆霍兹电容器线圈靶如何驱动磁重联的过程.数值模拟结果清楚地表明,磁重联的出流等离子体在线圈周围发生与实验结果相一致的堆积现象.线圈电流产生的磁场可高达100 T,使得磁重联区域周围的等离子体β值达到10^-2.与实验室结果进行比较,数值模拟重复了实验展示的大多数特征,可有助于深入认识和理解实验结果背后的物理学原理. 相似文献
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射电望远镜天线伺服控制系统中的非线性特性, 对系统动力学特性辨识有着显著的影响, 会提高辨识难度, 增加辨识模型的复杂程度. 系统非线性特性的测量与补偿也会增加系统辨识工作量. 针对上述问题, 提出了一种基于非线性采样数据的线性重构方法, 用于动力学特性建模. 通过提取原采样数据的相位与幅值, 对受到噪声与非线性畸变影响的系统采样数据进行线性重构, 降低待辨识模型的复杂度. 搭建了半实物实验平台, 以平台实际采样为基础, 重构线性数据, 利用奇异值法与自回归神经网络评估并辨识平台动力学模型. 实验结果表明, 建模数据奇异值拐点从100阶下降至40阶, 仅用10个神经网络节点200次训练即实现了模型辨识. 相似文献
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基于磁流体力学模拟的太阳高能粒子物理模式研究进展 总被引：1，自引：0，他引：1
太阳高能粒子(SEP)事件是一类重要的空间天气灾害性事件,其数值预报研究在空间天气预报研究中占有很重要的地位。SEP事件主要包括3种类型:与太阳耀斑爆发相关联的脉冲型事件,与日冕物质抛射驱动的激波相关联的缓变型事件,以及同时具有缓变型和脉冲型事件特征的混合型事件。其中,缓变型SEP事件持续时间较长并且高能粒子强度较大,对这类事件的模拟是当前研究的难点。目前针对缓变型SEP事件的模拟工作业已发展了多个理论和数值模型。每个模型都对SEP加速和传播的复杂过程作了基本的假设,这些模型的模拟结果能够部分重现观测到的SEP事件特征。而若要提高预报SEP事件的能力,则需要将描述三维日冕物质抛射驱动的激波模型与描述高能粒子在行星际空间中的加速和传输的模型耦合起来,建立基于接近真实的SEP加速和传播的三维太阳风背景模拟及以激波参数为输入的SEP模型。主要回顾了缓变型SEP事件中粒子的加速和传输方面的研究进展,以及可用于获取CME激波传播参数的磁流体力学太阳风模型研究现状;综述了缓变型SEP事件的激波一粒子模型(shock-and-particle model);最后对未来工作进行了讨论和展望。 相似文献
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针对BP (Back Propagation)神经网络模型预测卫星钟差中权值和阈值的最优化问题, 提出了基于遗传算法优化的BP神经网络卫星钟差短期预报模型, 给出了遗传算法优化BP神经网络的基本思想、具体方法和实施步骤. 为验证该优化模型的有效性和可行性, 利用北斗卫星导航系统(BeiDou navigation satellite system, BDS)卫星钟差数据进行钟差预报精度分析, 并将其与灰色模型(GM(1,1))和BP神经网络模型预报的结果比较分析. 结果表明: 该模型在短期钟差预报中具有较好的精度, 优于GM(1,1)模型和BP神经网络模型. 相似文献
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Heyvaerts和Priest最近提出了一个线性无力场演化的简化模型来定量计算磁拱脚点做缓慢的剪切运动所引起的日冕加热。由于他们在能量的计算中漏掉了一些二阶项,并且保留了导致位移无界的磁场的线性演化项,本文对他们的工作进行了修正,同时还对脚点运动引起的磁拱无力场演化进行了进一步探讨。本文主要结果如下:(1)得到的加热效率(即耗散能量在光球供给能量中所占比例)比Heyvaerts和Priest所得结果大。(2)磁拱无力场的无耗散线性演化是不可能的。(3)由磁场位形具体说明了非线性无力场发生磁力线重联的可能性,并指出最容易发生磁力线重联的高度大约为一个磁拱宽度。 相似文献
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《天文研究与技术》2021,(1)
双星星族光谱拟合的实质是在双星星族理论光谱库对应的参数空间进行搜索,通过最小二乘法找到与被拟合观测光谱最相近的理论光谱。快速、准确的拟合是有效利用星族合成方法处理海量星系光谱的难点和关键。为了提高双星星族光谱拟合的速度,针对双星星族光谱拟合计算量巨大、拟合效率不高等问题,在双星星族光谱拟合的优化模型基础上,采用策略改进的遗传算法对模型进行求解,并与双星拟合(Binary Star to Fit, BS2fit)算法和传统遗传算法进行比较。实验表明,通过策略改进的遗传算法对双星星族光谱拟合的速度平均可提高43.5%,一定程度上推动了演化星族合成方法在天文研究中的应用。 相似文献
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The paper presents a modified genetic algorithm called adapted genetic algorithm with adjusting population size (AGA-POP) for precise determination the orbital elements of binary stars. The proposed approach is a simple, robust way that can be considered to be a new member in the class of self organizing genetic algorithms. The proposed AGA-POP is applied on the star η Bootis of MK type G0 IV to find a set of optimal orbital elements. This leads to obtain the best fitting of Keplerian and phase curves. The modified method is compared with other different methods such as standard genetic algorithm, adapted genetic algorithm (AGA) and least square methods. Simulation results show the effectiveness of using AGA-POP compared with other different classic genetic algorithms in reducing the computation time. Also, better performances have been achieved when using the proposed technique. 相似文献
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Matteo Rosa Sentinella Lorenzo Casalino 《Celestial Mechanics and Dynamical Astronomy》2009,105(1-3):211-227
A hybrid evolutionary algorithm which synergistically exploits differential evolution, genetic algorithms and particle swarm optimization, has been developed and applied to spacecraft trajectory optimization. The cooperative procedure runs the three basic algorithms in parallel, while letting the best individuals migrate to the other populations at prescribed intervals. Rendezvous problems and round-trip Earth–Mars missions have been considered. The results show that the hybrid algorithm has better performance compared to the basic algorithms that are employed. In particular, for the rendezvous problem, a 100% efficiency can be obtained both by differential evolution and the genetic algorithm only when particular strategies and parameter settings are adopted. On the other hand, the hybrid algorithm always attains the global optimum, even though nonoptimal strategies and parameter settings are adopted. Also the number of function evaluations, which must be performed to attain the optimum, is reduced when the hybrid algorithm is used. In the case of Earth–Mars missions, the hybrid algorithm is successfully employed to determine mission opportunities in a large search space. 相似文献
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Annibal Hetem Jr Jane Gregorio-Hetem 《Monthly notices of the Royal Astronomical Society》2007,382(4):1707-1718
The protoplanetary discs of T Tauri and Herbig Ae/Be stars have previously been studied using geometric disc models to fit their spectral energy distribution (SED). The simulations provide a means to reproduce the signatures of various circumstellar structures, which are related to different levels of infrared excess. With the aim of improving our previous model, which assumed a simple flat-disc configuration, we adopt here a reprocessing flared-disc model that assumes hydrostatic, radiative equilibrium. We have developed a method to optimize the parameter estimation based on genetic algorithms (GAs). This paper describes the implementation of the new code, which has been applied to Herbig stars from the Pico dos Dias Survey catalogue, in order to illustrate the quality of the fitting for a variety of SED shapes. The star AB Aur was used as a test of the GA parameter estimation, and demonstrates that the new code reproduces successfully a canonical example of the flared-disc model. The GA method gives a good quality of fit, but the range of input parameters must be chosen with caution, as unrealistic disc parameters can be derived. It is confirmed that the flared-disc model fits the flattened SEDs typical of Herbig stars; however, embedded objects (increasing SED slope) and debris discs (steeply decreasing SED slope) are not well fitted with this configuration. Even considering the limitation of the derived parameters, the automatic process of SED fitting provides an interesting tool for the statistical analysis of the circumstellar luminosity of large samples of young stars. 相似文献
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A new approach of the initial orbit determination for too-short-arc with angular measurements is implemented by building the probabilistic model in the solution space with the estimation of distribution algorithm. Without any assumption about distribution, the non-parametric kernel density estimation is employed in the model building. The method, unlike other evolutionary algorithms, such as the genetic algorithm and particle swarm optimization, considers the fitness as well as the characteristics of the solution space. Numerical experiments with real observations indicate that without any constraints, the proposed technique has a good performance for the observations of usual accuracy. 相似文献
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We present new analyses of variations in O-C diagrams of three Algol-type eclipsing binary stars:AD And,TW Cas and IV Cas.We have used all published minima times(including visual and photographic)as well as newly determined ones from our and Super Wasp observations.We determined orbital parameters of 3~(rd) bodies in the systems with statistically significant errors,using our code based on genetic algorithms and Markov chain Monte Carlo simulations.We confirmed the multiple nature of AD And and the triple-star model of TW Cas,and we proposed a quadruple-star model of IV Cas. 相似文献
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机器学习在当今诸多领域已经取得了巨大的成功,但是机器学习的预测效果往往依赖于具体问题.集成学习通过综合多个基分类器来预测结果,因此,其适应各种场景的能力较强,分类准确率较高.基于斯隆数字巡天(Sloan Digital Sky Survey,SDSS)计划恒星/星系中最暗源星等集分类正确率低的问题,提出一种基于Stacking集成学习的恒星/星系分类算法.从SDSS-DR7(SDSS Data Release 7)中获取完整的测光数据集,并根据星等值划分为亮源星等集、暗源星等集和最暗源星等集.仅针对分类较为复杂且困难的最暗源星等集展开分类研究.首先,对最暗源星等集使用10折嵌套交叉验证,然后使用支持向量机(Support Vector Machine,SVM)、随机森林(Random Forest,RF)、XGBoost(eXtreme Gradient Boosting)等算法建立基分类器模型;使用梯度提升树(Gradient Boosting Decision Tree,GBDT)作为元分类器模型.最后,使用基于星系的分类正确率等指标,与功能树(Function Tree,FT)、SVM、RF、GBDT、XGBoost、堆叠降噪自编码(Stacked Denoising AutoEncoders,SDAE)、深度置信网络(Deep Belief Network,DBN)、深度感知决策树(Deep Perception Decision Tree,DPDT)等模型进行分类结果对比分析.实验结果表明,Stacking集成学习模型在最暗源星等集分类中要比FT算法的星系分类正确率提高了将近10%.同其他传统的机器学习算法、较强的提升算法、深度学习算法相比,Stacking集成学习模型也有较大的提升. 相似文献
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We studied the evolution of two small flares (GOES class C2 and C1) that developed in the same active region with different
morphological characteristics: one is extended and the other is compact. We analyzed the accuracy and the consistency of different
algorithms implemented in Reuven Ramaty High-Energy Spectroscopic Imager (RHESSI) software to reconstruct the image of the
emitting sources, for energies between 3 and 12 keV. We found that all tested algorithms give consistent results for the peak
position, while the other parameters can differ at most by a factor 2. Pixon and Forward-fit generally converge to similar results but Pixon is more reliable for reconstructing a complex source. We investigated the spectral characteristics of the two flares during
their evolution in the 3–25 keV energy band. We found that a single thermal model of the photon spectrum is inadequate to
fit the observations and we needed to add either a non-thermal model or a hot thermal one. The non-thermal and the double
thermal fits are comparable. If we assume a non-thermal model, the non-thermal energy is always higher than the thermal one.
Only during the very final decay phase a single thermal model fits the observed spectrum fairly well. 相似文献
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Valerio Iafolla David M. Lucchesi Sergio Nozzoli Francesco Santoli 《Celestial Mechanics and Dynamical Astronomy》2007,97(3):165-187
We have estimated a preliminary error budget for the Italian Spring Accelerometer (ISA) that will be allocated onboard the
Mercury Planetary Orbiter (MPO) of the European Space Agency (ESA) space mission to Mercury named BepiColombo. The role of
the accelerometer is to remove from the list of unknowns the non-gravitational accelerations that perturb the gravitational
trajectory followed by the MPO in the strong radiation environment that characterises the orbit of Mercury around the Sun.
Such a role is of fundamental importance in the context of the very ambitious goals of the Radio Science Experiments (RSE)
of the BepiColombo mission. We have subdivided the errors on the accelerometer measurements into two main families: (i) the
pseudo-sinusoidal errors and (ii) the random errors. The former are characterised by a periodic behaviour with the frequency
of the satellite mean anomaly and its higher order harmonic components, i.e., they are deterministic errors. The latter are
characterised by an unknown frequency distribution and we assumed for them a noise-like spectrum, i.e., they are stochastic
errors. Among the pseudo-sinusoidal errors, the main contribution is due to the effects of the gravity gradients and the inertial forces, while among the random-like errors the main disturbing effect is due to the MPO centre-of-mass displacements produced by the onboard High Gain Antenna (HGA) movements and by the fuel consumption and sloshing. Very subtle to be considered
are also the random errors produced by the MPO attitude corrections necessary to guarantee the nadir pointing of the spacecraft.
We have therefore formulated the ISA error budget and the requirements for the satellite in order to guarantee an orbit reconstruction
for the MPO spacecraft with an along-track accuracy of about 1 m over the orbital period of the satellite around Mercury in
such a way to satisfy the RSE requirements. 相似文献