問題詳情

C.  The aurora borealis (northern lights) and aurora australis (southern lights) are beautiful, dynamic luminous displays seenin the nighttime sky. The most common form of an aurora is curtain-like luminosity extending east to west. Auroras occur inthe upper atmosphere of both poles and are occasionally visible from middle latitudes as a dark red glow near the pole-wardhorizon. Auroral displays are the strongest at times of greatest sunspot activity, when magnetic storms disturb radiocommunications.  An extensive series of satellite, rocket, and ground-based observations has revealed that they result from large-scaleelectrical discharge processes surrounding the Earth. The discharges are powered by the electromotive force generated by theinteraction between the solar wind and the Earth’s magnetic field. The auroral luminosity comes from excited, or ionizedatoms and molecules: energetic electrons carrying the discharge current, are channeled toward the poles by the Earth’smagnetic field and collide with and excite, or ionize, upper atmospheric atoms and molecules. The most common source oflight is the green color emitted by energized oxygen atoms. The occasional crimson color results from the emission band ofexcited nitrogen molecules.  Auroral activity is controlled by solar activity through the magnetic field and the speed of the solar wind. A gusty solarwind generated by a solar flare often considerably enhances the auroral discharge, causing the auroral oval to expand to ageomagnetic latitude of 60 degrees or lower. During the declining sunspot period, a fairly intense beam of the solar wind canblow 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 ofabout 27 days, the beam hits the Earth at 27-day intervals, causing the so-called 27-day recurrence of auroral activity.
46. The main subject of the passage is ________.
(A) the auroras as bad omens
(B) the Earth’s magnetic field
(C) the visible spectacle of the Sun’s solar wind interaction with the Earth’s magnetic field
(D) the infra-red and ultra-violet light show in the Earth’s polar regions

參考答案

答案：C
難度：簡單0.857143
統計：A(1),B(2),C(18),D(0),E(0)

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