17 極光的科學 Science Behind Auroras By Minnie Soo 蘇慧音 Watching the light show of nature – auroras – may be on the bucket list of many people. The vibrant, magical colors were known to be mesmerizing, with green and red hues flickering and flowing gracefully like a river in the sky. This majestic natural phenomenon is particularly treasured by those of us who reside far from the poles, but have you ever wondered why do auroras mainly occur in polar regions? What is the science behind auroras? Taking a step back from the poetic imagination about auroras, you will find the formation process hardly as graceful as you might think, and underneath this beauty, lies a cascade of energetically violent events. Solar Magnetic Field and Solar Wind The Sun, like most stars, is a hot, fiery ball of plasma. Plasma is the fourth state of matter similar to gas, but with most of the particles ionized and moving at spectacular speeds [1]. Due to the extremely high temperature of the Sun, some particles possess sufficient kinetic energy to escape the Sun’s gravity [2]. This stream of charged ions and electrons ejected from the Sun’s surface forms what is known as the solar wind. The flow of these fast-moving charged particles are electric currents by definition, leading to the formation of the strong, chaotic magnetic field of the Sun (Footnote 1). The solar wind also filled the solar system with a magnetic field known as the heliospheric magnetic field or interplanetary magnetic field [3]. As visualized by imaginary magnetic field lines, some of the lines form closed loops with the Sun, while others extend far out into the solar system (Figure 1) [4]. The Earth originally possesses a toroidal (donutshaped) magnetic field. However, due to the constant bombardment of the solar wind, the magnetic field lines on the side facing the Sun was squeezed and pushed, while the magnetic field lines on the opposite side was stretched into a long, tail-like shape (Figure 2) [3]. This dynamic interaction gives Earth’s overall magnetic field a comet-like appearance. When the solar wind reaches the Earth, the charged particles are deflected by the Earth’s magnetic field, so we are shielded from the bombardment of those harmful particles [3]. Figure 1 The solar magnetic field. White magnetic field lines are considered “closed”, while magenta and green lines are “open” field lines extending far out into the solar system with opposite magnetic polarities [4]. Photo credit: NASA's Scientific Visualization Studio
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