为什么太阳风比预期的要热

新的研究有助于解释为什么太阳风比预期的要热

When a fire extinguisher is opened, the compressed carbon dioxide forms ice crystals around the nozzle, providing a visual example of the physics principle that gases and plasmas cool as they expand. When our sun expels plasma in the form of solar wind, the wind also cools as it expands through space—but not nearly as much as the laws of physics would predict.

当灭火器打开时，压缩的二氧化碳在喷嘴周围形成冰晶，这为气体和等离子体膨胀时冷却的物理原理提供了一个直观的例子。 当我们的太阳以太阳风的形式排出等离子体时，风也会随着它在空间中的膨胀而冷却下来——但冷却程度远不及物理定律所预测的那样。

In a study published April 14 in the Proceedings of the National Academy of Sciences, University of Wisconsin-Madison physicists provide an explanation for the discrepancy in solar wind temperature. Their findings suggest ways to study solar wind phenomena in research labs and learn about solar wind properties in other star systems.

在4月14日发表在《美国国家科学院院刊》的一项研究中，威斯康星大学麦迪逊分校的物理学家为太阳风温度的差异提供了一个解释。 他们的发现为在研究实验室研究太阳风现象，和了解其他恒星系统的太阳风特性提供了方法。

"People have been studying the solar wind since its discovery in 1959, but there are many important properties of this plasma which are still not well understood," says Stas Boldyrev, professor of physics and lead author of the study. "Initially, researchers thought the solar wind has to cool down very rapidly as it expands from the sun, but satellite measurements show that as it reaches the Earth, its temperature is 10 times larger than expected. So, a fundamental question is: Why doesn't it cool down?"

“自1959年发现太阳风以来，人们一直在研究它，但是这种等离子体的许多重要性质仍然没有得到很好的理解，”物理学教授兼该研究的主要作者 Stas Boldyrev 说。 “最初，研究人员认为太阳风从太阳扩散时必须迅速降温，但卫星测量显示，当太阳风到达地球时，它的温度是预期的10倍。所以，一个基本的问题是:为什么它没有冷却下来?”

Solar plasma is a molten mix of negatively charged electrons and positively charged ions. Because of this charge, solar plasma is influenced by magnetic fields that extend into space, generated underneath the solar surface. As the hot plasma escapes from the sun's outermost atmosphere, its corona, it flows through space as solar wind. The electrons in the plasma are much lighter particles than the ions, so they move about 40 times faster.

太阳等离子体是带负电荷的电子和带正电荷的离子的熔融混合物。 由于这种电荷，太阳等离子体受到延伸到太空的磁场的影响，磁场产生于太阳表面之下。 当炽热的等离子体从太阳最外层的大气层---- 日冕中逃逸时，它会以太阳风的形式在太空中流动。等离子体中的电子比离子轻得多，所以它们的移动速度是离子的40倍。

With more negatively charged electrons streaming away, the sun takes on a positive charge. This makes it harder for the electrons to escape the sun's pull. Some electrons have a lot of energy and keep traveling for infinite distances. Those with less energy can't escape the sun's positive charge and are attracted back to the sun. As they do, some of those electrons can be knocked off their tracks ever-so-slightly by collisions with surrounding plasma.

随着更多带负电荷的电子流离开，太阳带上了正电荷。 这使得电子更难逃离太阳的引力。 有些电子具有很大的能量，可以无限地运动。那些能量少的不能逃脱太阳的正电荷，被吸引回太阳。当它们这样做的时候，其中的一些电子会因为与周围等离子体的碰撞而稍稍偏离轨道。

In an effort to explain the temperature observations in the solar wind, Boldyrev and his colleagues, UW-Madison physics professors Cary Forest and Jan Egedal looked to a related, but distinct, field of plasma physics for a possible explanation.

为了解释太阳风中的温度观测结果，博尔迪列夫和他的同事，威斯康辛大学麦迪逊分校的物理学教授卡里 · 福瑞斯特和简 · 艾格达尔，从等离子体物理学的一个相关但不同的领域中寻找一个可能的解释。

Around the time scientists discovered solar wind, plasma fusion researchers were thinking of ways to confine plasma. They developed "mirror machines," or plasma-filled magnetic field lines shaped as tubes with pinched ends, like bottles with open necks on either end.

大约在科学家发现太阳风的时候，等离子体聚变研究人员正在想方设法限制等离子体。 他们发明了“镜子机器” ，即等离子体填充磁场线，形状像管子，两端捏紧，就像两端都有开口的瓶子。

As charged particles in the plasma travel along the field lines, they reach the bottleneck and the magnetic field lines are pinched. The pinch acts as a mirror, reflecting particles back into the machine.

当等离子体中的带电粒子沿着磁力线运动时，它们到达瓶颈，磁力线被压缩。 压力就像一面镜子，把粒子反射回机器。

The accuracy with which mirror machine theory predicts solar wind temperature opens the door for using them to study solar wind in laboratory settings.

镜像机理论预测太阳风温度的准确性，为在实验室环境中使用它们来研究太阳风打开了大门。