科學圖說

舞動的精靈(Dancing Spirits)

來自太陽的帶電粒子到達地球附近時，地球磁場會迫使部分帶電粒子沿著磁場運行。地球磁線南北兩極附近較集中，當大量帶電粒子進入地球極地高層大氣時，會碰撞激發高層大氣中的氮原子和氧原子，產生瞬間的原子能階躍遷，出現巨觀的紅色和綠色光譜，就是所謂的極光。北美洲的印地安人稱「舞動的精靈」，中世紀的歐洲人，視極光為來自神的召喚。綠色極光較常見，紅色極光只有在太陽活動強烈的情況下才能看見。

When charged particles from the sun come close to the earth, the earth’s magnetic field forces some of them to move along the earth’s field lines and accumulate at the poles. When large numbers of these charged particles enter the upper atmosphere, they collide and cause energy level transitions of nitrogen and oxygen, producing red and green light, known as auroras. In indigenous cultures of North America, they are considered “dancing spirits”. In medieval Europe, people believed that auroras were a calling from God. Green light is more common. Red auroras are visible only under more intense solar activity.