首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
It has been known for many decades that the appearance of sunspots is roughly periodic, with an average cycle of eleven years. M
It has been known for many decades that the appearance of sunspots is roughly periodic, with an average cycle of eleven years. M
admin
2010-06-18
57
问题
It has been known for many decades that the appearance of sunspots is roughly periodic, with an average cycle of eleven years. Moreover, the incidence of solar flares and the flux of solar cosmic rays, ultraviolet radiation, and X-radiation all vary directly with the sunspot cycle. But after more than a century of investigation, the relation of these and other phenomena, known collectively as the solar-activity cycle, to terrestrial weather and climate remains unclear. For example, the sunspot cycle and the allied magnetic-polarity cycle have been linked to periodicities discerned in records of such variables as rainfall, temperature, and winds. Invariably, however, the relation is weak, and commonly of dubious statistical significance.
Effects of solar variability over longer terms have also been sought. The absence of recorded sunspot activity in the notes kept by European observers in the late seventeenth and early eighteenth centuries has led some scholars to postulate a brief cessation of sunspot activity at that time (a period called the Maunder minimum). The 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 minimum has yet to be established, however, especially since the records that Chinese naked-eye observers of solar activity made at that time appear to contradict it. Scientists have also sought evidence of long-term solar periodicities by examining indirect climatological data, such as fossil records of the thickness of ancient tree rings. These studies, however, failed to link unequivocally terrestrial climate and the solar-activity cycle, or even to confirm the cycle’s past existence.
If consistent and reliable geological or archaeological evidence tracing the solar-activity cycle in the distant past could be found, it might also resolve an important issue in solar physics: how to model solar activity. Currently, there are two models of solar activity. The first 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 mechanical energy is converted into the, energy of a magnetic field. In 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 change for perhaps billions of years. The alternative explanation supposes that the Sun’s large-scale 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 characteristics of the solar-activity cycle could be expected to change over a long period of time. Modern solar observations 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.
The author focuses primarily on ______.
选项
A、two competing scientific models concerning the sun’s magnetic field
B、an overview of some recent scientific developments in solar physics
C、the reasons why a problem in solar physics has not yet been solved
D、the difficulties involved in linking terrestrial climate with solar activity
答案
D
解析
转载请注明原文地址:https://kaotiyun.com/show/QWlO777K
0
专业英语八级
相关试题推荐
A、Becauseitavoidsthepossibilityofadditionaltuitioncosts.B、Becauseithelpscollegestudentssecuremorescholarships.C
Inourlooks-obsessedsociety,manypeoplethinkthatbeingoverweightisanappearanceissue.Butbeingoverweightisactually
Accordingtothepassage,thedeclineofgoodmannersismoreworryingbecauseDr.BruceCharltonwouldprobablyprefertosee
Accordingtothepassage,thedeclineofgoodmannersismoreworryingbecauseAccordingtoCarolineMoore,themediahasproj
TheattitudeheldbytheassuredtowardslanguageisTheanxiousareconsideredalessfortunategroupbecause
TessoftheD’Urbervilles,oneof______bestknownnovels,portraysmanashavingnocontroloverhisownfate.
A、USambassadortotheUNB、theheadofCentralIntelligenceAgencyC、thearmscontrolchiefoftheStateDepartmentD、amember
Toseehowbigcarrierscouldcontroltheonlineworld,youmustunderstanditsstructures.EarthlinkgivesJenniferaccesst
随机试题
琥珀胆碱禁用于
某女,少腹隐痛不休,带下增多,大便时结时溏。时有低热,舌苔薄,脉虚弦。妇科检查:双附件增厚、压痛。其证型是
氧化与磷酸化偶联的部位是
此时辨证属于呃逆哪一型()下列治法何者最宜()
一齿轮箱在稳定状态下工作,输入端接受功率为100kW,输出功率为95kW,每分钟内齿轮箱传向环境的热量Q为()kJ。
下面程序段的运行结果是( )。A=2.56A=A+0.05I=AA=A/10
运用掌握的“应付职工薪酬”知识,选择正确答案。某电器生产企业,共有职工200名,其中企业行政管理人员30名,一线工人150名,另外15名为车间管理人员,5名为企业在建工程工作人员。2008年12月份该公司职工薪酬发放方面情况如下:(1)将自产电器作为福
固定制造费用差异是根据预算产量下的标准工时与实际产量下的标准工时的差额,乘以固定制造费用标准分配率计算得出的。()
在SQL中,ALTER命令有两个选择项,【】子命令用于修改字段名,【】子命令用于增加新的字段。
【B1】【B10】
最新回复
(
0
)