《科学》（20240830出版）一周论文导读

 
《科学》（20240830出版）一周论文导读  
 

 

编译｜冯维维

Science, 30 August 2024, Volume 385, Issue 6712

《科学》2024年8月30日，第385卷，6712期

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物理学Physics

In situ observations of large-amplitude Alfvén waves heating and accelerating the solar wind

大振幅阿尔芬波加热和加速太阳风的现场观测

▲ 作者：YEIMY J. RIVERA, SAMUEL T. BADMAN, MICHAEL L. STEVENS, JAYE L. VERNIERO, JULIA E. STAWARZ, CHEN SHI, JIM M. RAINES, KRISTOFF W. PAULSON, CHRISTOPHER J. OWEN, AND STUART D. BALE

▲链接：

https://www.science.org/doi/10.1126/science.adk6953

▲摘要：

离开日冕后，太阳风继续加速和冷却，但比预期的自由膨胀的绝热气体要慢得多。阿尔芬波是传输能量的行星际磁场的扰动。研究者使用帕克太阳探测器和太阳轨道飞行器的现场测量来调查太阳风流，因为它穿过内部日球层。

观测结果显示，日冕外缘和金星轨道附近的等离子体正在升温和加速，同时还存在着振幅较大的阿尔芬波。研究者计算出，阿尔芬波所产生的阻尼和机械功足以为内日球层中快速太阳风的加热和加速提供动力。

▲ Abstract：

After leaving the Sun’s corona, the solar wind continues to accelerate and cools, but more slowly than expected for a freely expanding adiabatic gas. Alfvén waves are perturbations of the interplanetary magnetic field that transport energy. We use in situ measurements from the Parker Solar Probe and Solar Orbiter spacecraft to investigate a stream of solar wind as it traverses the inner heliosphere. The observations show heating and acceleration of the plasma between the outer edge of the corona and near the orbit of Venus, along with the presence of large-amplitude Alfvén waves. We calculate that the damping and mechanical work performed by the Alfvén waves are sufficient to power the heating and acceleration of the fast solar wind in the inner heliosphere.

Grain boundaries are Brownian ratchets

晶界是布朗棘轮

▲ 作者：CAIHAO QIU, MAIK PUNKE, YUAN TIAN, YING HAN, SIQI WANG, YISHI SU, MARCO SALVALAGLIO, XIAOQING PAN, DAVID J. SROLOVITZ , AND JIAN HAN

▲链接：

https://www.science.org/doi/10.1126/science.adp1516

▲摘要：

多晶材料的晶粒尺寸是制约其性能的重要组成部分。与之前的假设相反，研究者通过建模表明，在非定向驱动力的作用下，晶界（GBs）可以向一个方向移动。然而，并不是所有的晶界类型都是这样，主要是那些几何上不对称的晶界。这一观察结果有助于更好地理解一些晶粒粗化行为，也可能是工程微观结构和改善材料性能的另一种工具。

研究者证明晶界表现为布朗棘轮，在振荡驱动力或循环热退火下表现出方向依赖的迁移和单向运动。他们在几乎所有非对称GB中观察到这些现象，但在对称GB中没有。这一观察建立在分子动力学和相场晶体模拟的基础上，模拟了广泛的GB类型以及双晶和多晶微观结构中的驱动力。

研究者通过现场实验观察证实了这些模拟结果，并用马尔可夫链模型分析了这些结果，并探讨了GB棘轮行为对材料加工和微观结构定制的影响。

▲ Abstract：

The grain size of a polycrystalline material is one important part of the microstructure that constrains the properties. In contrast to previous assumptions, Qiu et al. have shown through modeling that grain boundaries (GBs) can move in one direction in response to a nondirectional driving force. However, not all grain boundary types do so, mostly just the ones that are geometrically nonsymmetric. This observation helps us better understand some grain-coarsening behavior and may also be another tool for engineering microstructures and improving the material properties.

We demonstrate that grain boundariesbehave as Brownian ratchets, exhibiting direction-dependent mobilities and unidirectional motion under oscillatory driving forces or cyclic thermal annealing. We observed these phenomena for nearly all nonsymmetric GBs but not for symmetric ones. Our observations build on molecular dynamics and phase-field crystal simulations for a wide range of GB types and driving forces in both bicrystal and polycrystalline microstructures. We corroborate these simulation results through in situ experimental observations. We analyze these results with a Markov chain model and explore the implications of GB ratchet behavior for materials processing and microstructure tailoring.

地质与地理Geology & Geography

The marine biodiversity impact of the Late Miocene Mediterranean salinity crisis

晚中新世地中海盐度危机对海洋生物多样性的影响