美国GRE模拟试题:美国研究生入学考试试题（一）(3)_Essay

　　cycle and the allied rnagnetic-polarity cycle have been (10)

　　linked to periodicities discerned in records of such vari-

　　ables as rainhll. temperature， and winds. lnvariably，

　　however， the relation is weak. and commonly ofdubious

　　statistical significance.

　　Effects of solar variability over longer terms have also (15)

　　been sought. The absence of recorded sunspot activity in

　　the notes kept by European observers in the late seven-

　　teenth and early eighteenth centuries has led some schol-

　　ars to postulate a brief cessation of sunspot activity at

　　that time (a period called the Maunder minimum)。 The (20)

　　Maunder minimum has been linked to a span of unusual

　　cold in Europe extending from the sixteenth to the early

　　nineteenth centuries. The reality of the Maunder mini-

　　mum has yet to be established， however， especially since

　　the records that Chinese naked-eye observers of solar (25)

　　activity made at that time appear to contradict it. Scien-

　　tists have also sought evidence of long-term solar period-

　　icities by examining indirect climatological data， such as

　　fossil recoras of the thickness of ancient tree rings. These

　　studies， however， failed to link unequivocally terrestrial(30)

　　climate and the solar-activity cycle， or even to contirm

　　the cycle‘s past existenue.

　　If consistPn! and re!iab!e geo!sgigal~-arek-xologieal

　　evidence tracing the solar-activity cycle in the distant

　　past could be found， it might also resolve an important(35)

　　issue in solar physics： how to model solar activity. Cur-

　　rently， chere are two models of solar activity. The tirst

　　supposes that the Sun‘s internal motions (caused by

　　rotation and convection)interact with its large-scale

　　magnetic field to produce a dynamo. a device in which(40)

　　mechanical energy is converted into the energy of a mag-

　　netic field. ln short. the Sun‘s large-scale magnetic field

　　is taken to be self-sustaining， so that the solar-activity

　　cycle it drives would be maintained with little overall

　　changc for perhaps billions of years. The alternative(45)

　　exp)anarion supposes that the Sun‘s large-sca)e magnetic

　　field is a remnant of the field the Sun acquired when it

　　formed， and is not sustained against decay. In this

　　model. the solar mechanism dependent on the Sun‘s

　　magnetiC field runs down more quickly. Thus， the char-(50)

　　acteristics of the solar-activity cycle uvuld be expected to

　　change over a long period of time. Modern solar obser-

　　vations span too short a time to reveal whether present

　　cyclical solar aCtivity is a long-lived feature of the Sun，