分页标题#e#
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,
or merely a transient phenomenon.
17. The author focuses primarily on
(A) presenting two competing scientific theories concerning solar
activity and evaluating geological evidence often cited to support them
(B) giving a brief overview of some recent scientifrc developments
in s’olar physics and assessing their impact on future climatological research
(C) discussing the difficulties involved in linkinl: ter- restrial
phenomena with solar activity and indicating how resolving that issue
2011-09-28 16:39:00 来源：作文地带整理 双击单词自动翻译
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