19 圖一 太陽磁場。白色是「閉合」的磁場線；而洋紅色和綠色則是各 具相反磁極的「開放」磁場線，它們延伸到太陽系的遠處 [4]。 圖片來源: NASA's Scientific Visualization Studio second-highest layer of Earth’s atmosphere known as the thermosphere [3, 14]. While the lower part of the thermosphere is primarily composed of molecular nitrogen (N2) and molecular oxygen (O2), the upper part is dominated by monoatomic oxygen (O). Nitrogen and oxygen are crucial to the generation of auroras [3, 15]. Before delving into the mechanism, it is important to note that electrons in an atom have discrete energy levels, much like the steps on a staircase. You can walk up integer steps such as 1, 2 or 5 steps, but you cannot walk 2.3 steps or anything in between with decimal parts. Electrons can only gain certain fixed amounts of energy to be excited and jump up to specific energy levels (or release some fixed amounts of energy and return to a lower energy state), and not anywhere in between. Each atom has its own unique set of energy levels, just as fingerprints are unique to each of us. As the plasma particles collide with the gas particles, they excite electrons within the atom. When the electron later drops back to a lower energy level, it releases energy in the form of light whose wavelength is decided by the amount of energy released. The relationship between the released energy E and the wavelength λ of emitted light is represented by the equation , where h is the constant known as Planck’s constant (6.63 × 10-34 J s) and c represents the speed of light (3.00 × 108 m s-1). We can see that the energy released is inversely proportional to the wavelength, which determines the color of light we perceive. Atomic oxygen (O) can emit green and red lights at the wavelengths of 558 nm and 630 nm respectively [3, 16]. Molecular nitrogen (N2), on the other hand, can emit blue and red lights at multiple wavelengths which combine to form a hue of magenta at lower altitudes [3, 17]. Now you see why it matters that the energy levels are discrete: Instead of creating a continuum of all 1. Editor’s note: By Ampere’s law, an electric current always generates a magnetic field with a strength proportional to the current. 不少人的願望清單裡大概都有一項是觀賞極光 ─ 大自 然的光影表演。那艷麗的幻彩令許多人為之著迷，當中紅、 綠色兩種色調在空中搖曳，宛如河流般優雅地流淌。這種 壯麗的自然現象尤其受我們這些居住在遠離兩極的人所 珍視，但你有沒有想過為甚麼極光主要在極地出現？極光 背後的科學原理又是甚麼呢？讓我們從對極光的詩意幻想 中抽離，你會發現極光的形成過程並不如想像中婉約，美 麗背後竟隱藏著一連串高能量的猛烈反應！ colors, only certain signature colors of auroras exist. They produce the mesmerizing display of the aurora borealis (northern lights) and aurora australis (southern lights).
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