教甄◆英文科題庫下載題庫

 The aurora borealis (northern lights) and aurora australis (southern lights) are beautiful, dynamic luminous displays seen in the nighttime sky. The most common form of an aurora is curtain-like luminosity extending east to west. Auroras occur in the upper atmosphere of both poles and are occasionally visible from middle latitudes as a dark red glow near the pole-ward horizon. Auroral displays are the strongest at times of greatest sunspot activity, when magnetic storms disturb radio communications. 

 An extensive series of satellite, rocket, and ground-based observations has revealed that they result from large-scale electrical discharge processes surrounding the Earth. The discharges are powered by the electromotive force generated by the interaction between the solar wind and the Earth’s magnetic field. The auroral luminosity comes from excited, or ionized atoms and molecules: energetic electrons carrying the discharge current, are channeled toward the poles by the Earth’s magnetic field and collide with and excite, or ionize, upper atmospheric atoms and molecules. The most common source of light is the green color emitted by energized oxygen atoms. The occasional crimson color results from the emission band of excited nitrogen molecules. 

 Auroral activity is controlled by solar activity through the magnetic field and the speed of the solar wind. A gusty solar wind generated by a solar flare often considerably enhances the auroral discharge, causing the auroral oval to expand to a geomagnetic latitude of 60 degrees or lower. During the declining sunspot period, a fairly intense beam of the solar wind can blow out from a rather quiet region of the Sun and last for from a few months to two years. Since the Sun rotates in a period of about 27 days, the beam hits the Earth at 27-day intervals, causing the so-called 27-day recurrence of auroral activity.