物理学 双语译文 电子
2021-02-04 17:53:30

How do electrons close to Earth reach almost the speed of light?



by Helmholtz Association of German Research Centres



The contours in color show the intensities of the radiation belts. Grey lines show the trajectories of the relativistic electrons in the radiation belts. Concentric circular lines show the trajectory of scientific satellites traversing this dangerous region in space. Credit: Ingo Michaelis and Yuri Shprits, GFZ

颜色轮廓显示了辐射带的强度。灰线显示了辐射带中相对论电子的轨迹。同心圆线表示科学卫星在太空中穿越这一危险区域的轨迹。图片来源:GFZIngo MichaelisYuri Shprits


A new study found that electrons can reach ultra-relativistic energies for very special conditions in the magnetosphere when space is devoid of plasma.



Recent measurements from NASA's Van Allen Probes spacecraft showed that electrons can reach ultra-relativistic energies flying at almost the speed of light. Hayley Allison, Yuri Shprits and collaborators from the German Research Centre for Geosciences have revealed under which conditions such strong accelerations occur. They had already demonstrated in 2020 that during solar storm plasma waves play a crucial role for that. However, it was previously unclear why such high electron energies are not achieved in all solar storms. In the journal Science Advances, Allison, Shprits and colleagues now show that extreme depletions of the background plasma density are crucial.

NASAVan Allen Probes航天器最近进行的测量表明,电子可以达到以光速飞行的超相对论能量。 Hayley AllisonYuri Shprits和德国地球科学研究中心的合作者已经揭示出在这种情况下会发生如此强烈的加速。他们已经在2020年证明,在太阳风暴期间等离子波起着至关重要的作用。但是,以前尚不清楚为什么在所有太阳风暴中都无法获得如此高的电子能量。现在,在《科学进展》杂志上,AllisonShprits及其同事表明,极端耗尽背景血浆密度至关重要。


Ultra-relativistic electrons in space


At ultra-relativistic energies, electrons move at almost the speed of light. Then the laws of relativity become most important. The mass of the particles increases by a factor ten, time is slowing down, and distance decreases. With such high energies, charged particles become most dangerous to even the best protected satellites. As almost no shielding can stop them, their charge can destroy sensitive electronics. Predicting their occurrence—for example, as part of the observations of space weather practiced at the GFZ—is therefore very important for modern infrastructure.


To investigate the conditions for the enormous accelerations of the electrons, Allison and Shprits used data from a twin mission, the Van Allen Probes, which the US space agency NASA had launched in 2012. The aim was to make detailed measurements in the radiation belt, the so-called Van Allen belt, which surrounds the Earth in a donut shape in terrestrial space. Here—as in the rest of space—a mixture of positively and negatively charged particles forms a so-called plasma. Plasma waves can be understood as fluctuations of the electric and magnetic field, excited by solar storms. They are an important driving force for the acceleration of electrons.

为了研究电子巨大加速的条件,AllisonShprits使用了来自美国航天局NASA2012年发射的双任务Van Allen Probes的数据。目的是对辐射带进行详细测量,所谓的范艾伦带(Van Allen belt),它在陆地空间中以甜甜圈形状环绕地球。与空间的其余部分一样,此处带正电和带负电的粒子的混合物形成所谓的等离子体。等离子体波可以理解为由太阳风暴激发的电场和磁场的波动。它们是电子加速的重要驱动力。


Data analysis with machine learning



During the mission, both solar storms that produced ultra-relativistic electrons and storms without this effect were observed. The density of the background plasma turned out to be a decisive factor for the strong acceleration: electrons with the ultra-relativistic energies were only observed to increase when the plasma density dropped to very low values of only about ten particles per cubic centimeter, while normally such density is five to ten times higher.


Using a numerical model that incorporated such extreme plasma depletion, the authors showed that periods of low density create preferential conditions for the acceleration of electrons—from an initial few hundred thousand to more than seven million electron volts. To analyze the data from the Van Allen probes, the researchers used machine learning methods, the development of which was funded by the GEO.X network. They enabled the authors to infer the total plasma density from the measured fluctuations of electric and magnetic field.

使用包含这种极端等离子体耗竭的数值模型,作者表明,低密度周期为电子的加速创造了优先条件-从最初的几十万电子伏特到超过700万电子伏特。为了分析Van Allen探针的数据,研究人员使用了机器学习方法,该方法的开发由GEO.X网络资助。他们使作者能够从测得的电场和磁场波动中推断出总等离子体密度。

The crucial role of plasma


"This study shows that electrons in the Earth's radiation belt can be promptly accelerated locally to ultra-relativistic energies, if the conditions of the plasma environment—plasma waves and temporarily low plasma density—are right. The particles can be regarded as surfing on plasma waves. In regions of extremely low plasma density they can just take a lot of energy from plasma waves. Similar mechanisms may be at work in the magnetospheres of the outer planets such as Jupiter or Saturn and in other astrophysical objects," says Yuri Shprits, head of the GFZ section Space physics and space weather and Professor at University of Potsdam.

这项研究表明,如果等离子环境的条件(等离子波和暂时的低等离子密度)正确的话,地球辐射带中的电子可以立即被局部加速为超相对论能量。这些粒子可以看作是在等离子上冲浪在极低的等离子体密度区域中,它们仅能从等离子体波中吸收大量能量。类似的机制可能在诸如木星或土星等外行星的磁层中以及在其他天体物理物体中起作用,” Yuri Shprits说, GFZ部门空间物理学和太空天气负责人,波茨坦大学教授。

"Thus, to reach such extreme energies, a two-stage acceleration process is not needed, as long assumed—first from the outer region of the magnetosphere into the belt and then inside. This also supports our research results from last year," adds Hayley Allison, PostDoc in the Section Space physics and space weather.

因此,要想达到如此高的能量,长期以来就不需要两阶段的加速过程-首先是从磁层的外部区域进入皮带,然后是内部。这也支持了我们去年的研究成果,海莉·艾莉森(Hayley Allison),《太空物理学与太空天气》部分的PostDoc




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