[原来欧若拉(Aurora)就是北极光，也算终于搞懂了当年张邵涵同名歌曲歌词的意思。]

 一张典型的北极光照片(A Photo of   Aurora  )

Aurora  is also known as the Northern Lights, an Aurora is a beautiful natural phenomenon that often occurs in the polar regions of Earth. It appears as colorful clouds and rays of green and red (and sometimes blue) light that dance across the sky. The aurora borealis and aurora australis (Latin for “northern” and “southern” dawn, respectively) occur in symmetric ovals centered on the northern and southern magnetic poles of Earth. The aurora is formed when charged particles (electrons and protons) are guided by the Earth’s magnetic field into the atmosphere near the poles. When these particles collide with atoms and molecules of the upper atmosphere, primarily oxygen and nitrogen, some of the energy in these collisions is transformed into the visible light that characterizes the aurora.

What causes the Aurora?

The energy source for the aurora is 149 million kilometers (km) (93 million miles) from Earth at the sun. The sun continuously emits charged particles (mostly protons and electrons), which are the byproducts of thermonuclear reactions occurring inside the sun. These charged particles make up the solar wind, which travels away from the sun through space at speeds ranging from 300 to 1,000 km/sec.—about a million miles per hour. Traveling at this high speed, the solar particles can reach the Earth in two to three days.

At Earth, the steady solar wind is deflected by Earth’s magnetic field, or magnetosphere. The solar wind flows around the magnetosphere much like a river flows around a stone. It also pushes on the magnetosphere and distorts it so that instead of a symmetric set of magnetic field lines—like one might have around a bar magnet—the magnetosphere is stretched and elongated into a comet shape with a long tail trailing away from Earth on the side away from the sun.

When there is a disturbance on the sun, such as a solar flare or coronal mass ejection, it can produce a disturbance in the solar wind. This in turn will cause a disturbance in the balance between the solar wind and Earth’s magnetic field. As a result, electrons and protons are accelerated within the magnetosphere. These charged particles are constrained to the magnetic field lines much like beads on a wire. The accelerated particles will travel down the magnetic field lines of Earth and collide with the atoms and molecules of the upper atmosphere where the magnetic field lines reach down to surface of the Earth near the north and south magnetic poles.

When the particles from the magnetosphere collide with the atoms and molecules of the atmosphere, the particle’s energy can be transferred to the atoms and molecules (typically O, N, and N₂) of the atmosphere forming excited states of O, N and N₂. When these finally release their energy and return to their normal ground state, they give up energy in the form of light. This is the light that we see from the ground as an aurora.

The physics of Auroral light formation

The high-energy electrons and protons traveling down Earth’s magnetic field lines collide with the atmosphere (i.e., oxygen and nitrogen atoms and molecules). The collisions can excite the atmospheric atom or molecule or they can strip the atmospheric species of its own electron and create an ion. The result is that the atmospheric atoms and molecules are excited to higher energy states. They relinquish this energy in the form of light upon returning to their initial, lower energy state. The particular colors we see in an auroral display depend on the specific atmospheric gas struck by energetic particles, and the energy level to which it is excited. The two main atmospheric gases involved in the production of auroral lights are oxygen and nitrogen:

• Oxygen is responsible for two primary auroral colors: green-yellow wavelength of 557.7 nanometers (nm) is most common, while the deep red 630.0 nm light is seen less frequently.
• Nitrogen in an ionized state will produce blue light, while neutral nitrogen molecules create purplish-red auroral colors. For example, nitrogen is often responsible for the purplish-red lower borders and rippled edges of the aurora.

The process is similar to the lights that illuminate a neon light or computer and TV screens. In a neon light, neon gas is excited by electrical currents. Likewise, in a picture or computer screen, a beam of electrons controlled by electric and magnetic fields strike the screen, making it glow in different colors, according to the type of chemicals (phosphors) that coat the screen.

Auroras typically occur between 95 and 1,000 km. Auroras stay above 95 km because at that altitude the atmosphere is so dense (and the auroral particles collide so often) that they finally come to rest at this altitude. On the other hand, auroras typically do not reach higher than 500-1,000 km because at that altitude the atmosphere is too thin to cause a significant number of collisions with the incoming particles.

资料来源： Environment in Focus Archives

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