GRE经典题库：GRE试题5

会计考友

　studies, however, failed to link unequivocally terrestrial(30)
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2 y- V# C) V5 ^　　climate and the solar-activity cycle, or even to contirm
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' p, c" s: i0 i( R' O8 e. e　　the cycle's past existenue.
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9 K; S7 t$ ]# x: A! E% b) W' g　　If consistPn! and re!iab!e geo!sgigal~-arek-xologieal
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+ {$ _! z) e$ v; ?! R: d8 ]　　evidence tracing the solar-activity cycle in the distant
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* l, P  @1 _) J- ]' H& Y& V　　past could be found, it might also resolve an important(35)

* H6 m% T3 _' Z+ @7 Z. B- e　　issue in solar physics: how to model solar activity. Cur-

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

1 O# V' @. U8 Y+ ^　　supposes that the Sun's internal motions (caused by

" X; [, v) T- V- S+ h　　rotation and convection) interact with its large-scale
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2 ?. ~1 Y. ?4 W* f1 |8 A* x　　magnetic field to produce a dynamo. a device in which(40)
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　　mechanical energy is converted into the energy of a mag-

3 G. N; v" [9 ]- ?$ n　　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
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9 k0 b/ k7 V: Q& ~; Q7 m+ H　　changc for perhaps billions of years. The alternative(45)
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& B+ X6 M% M" F  n　　exp)anarion supposes that the Sun's large-sca)e magnetic
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4 |, A2 p4 N; t5 m% B　　field is a remnant of the field the Sun acquired when it
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$ `+ _. I' o4 l　　formed, and is not sustained against decay. In this
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　　model. the solar mechanism dependent on the Sun's

& ?; W% K" {% L; `　　magnetiC field runs down more quickly. Thus, the char-(50)
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/ Z1 J! A1 Q4 U　　acteristics of the solar-activity cycle uvuld be expected to

& O. N( o: W$ Z. a; [$ n　　change over a long period of time. Modern solar obser-
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　　vations span too short a time to reveal whether present

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

8 x, S' y  h9 E* E" g- w5 ^　　or merely a transient phenomenon.