Aurora – AEAS精读 (Y10-12)

The spectacular aurora light displays that appear in Earth’s atmosphere around the north and south magnetic poles were once mysterious phenomena. Now, scientists have data from satellites and ground-based observations from which we know that the aurora brilliance is an immense electrical discharge similar to that occurring in a neon sign. To understand the cause of auroras, first picture the Earth enclosed by its magnetosphere, a huge region created by the Earth’s magnetic field. Outside the magnetosphere, blasting toward the earth is the solar wind, a swiftly moving plasma of ionized gases with its own magnetic filed. Charged particles in this solar wind speed earthward along the solar wind’s magnetic lines of force with a spiraling motion. The Earth’s magnetosphere is a barrier to the solar winds, and forces the charged particles of the solar wind to flow around the magnetosphere itself. But in the polar regions, the magnetic lines of force of the Earth and of the solar wind bunch together. Here many of the solar wind’s charged particles break through the magnetosphere and enter Earth’s magnetic field. They then spiral back and forth between the Earth’s magnetic poles very rapidly. In the polar regions, electrons from the solar wind ionize and excite the atoms and molecules of the upper atmosphere, causing them to emit aurora radiations of visible light. The colors of an aurora depend on the atoms emitting them. The dominant greenish white light comes from low energy excitation of oxygen atoms. During huge magnetic storms oxygen atoms also undergo high energy excitation and emit crimson light. Excited nitrogen atoms contribute bands of color varying from blue to violet. Viewed from outer space, auroras can be seen as dimly glowing belts wrapped around each of the Earth’s magnetic poles. Each aurora hangs like a curtain of light stretching over the polar regions and into the higher latitudes. When the solar flares that result in magnetic storms and aurora activity are very intense, aurora displays may extend as far as the southern regions of the United States. Studies of auroras have given physicists new information about the behavior of plasmas, which has helped to explain the nature of outer space and is being applied in attempts to harness energy from the fusion of atoms.

 

地球大气中围绕南北磁极出现的极光奇观曾经是一种神秘的现象。现在,科学家们从卫星和地面观测中获得数据,从中我们知道极光的光辉是一种巨大的放电现象,类似于霓虹灯中的放电现象。

要了解极光的成因,首先要了解地球被磁层包围的情况,磁层是由地球磁场形成的一个巨大区域。在磁层之外,向地球爆炸的是太阳风,这是一种带有磁场的快速移动的电离气体等离子体。太阳风中的带电粒子沿着太阳风的磁力线以螺旋运动向地球加速。地球的磁层是太阳风的屏障,迫使太阳风的带电粒子绕着磁层流动。但是在极地地区,地球和太阳风的磁力线聚在一起。在这里,太阳风的许多带电粒子冲破磁层进入地球磁场。然后它们在地球磁极之间快速地来回旋转。在极地地区,太阳风中的电子电离并激发上层大气中的原子和分子,使它们发射出可见光的极光辐射。

极光的颜色取决于发出极光的原子。主要的绿色白光来自于氧原子的低能量激发。在巨大的磁暴中,氧原子也会受到高能激发并发出红光。受激发的氮原子形成从蓝色到紫色不等的色带。从外太空看,极光可以被看作是环绕在地球磁极周围的黯淡发光带。每一次极光都像一幅光幕一样悬挂在两极地区,延伸到高纬度地区。当导致磁暴和极光活动的太阳耀斑非常强烈时,极光显示可能会延伸到美国南部地区。

对极光的研究为物理学家提供了有关等离子体行为的新信息,这有助于解释外层空间的性质,并正被应用于试图利用原子聚变产生的能量。