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
65
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 solarphysics: 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. Modem 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/ALlO777K
0
专业英语八级
相关试题推荐
我觉得我的条件正符合你们的工作描述。从我的简历中你可以看到我有一份证实我在解决问题、进行多重任务工作和处理顾客问题方面能力的证明资料。除了我们刚才讨论过的技能和经历之外,我是一个工作非常努力的人;我工作认真,而且很有方法;能与大家相处得非常好,而且我学得很
很多人说,美国也许在反恐战争中取得了有形的胜利,但却在信息战中吃了败仗。你同意这种说法吗?
Thecars,SUVsandpickupspeoplewillbuyintheyearsaheadarelikelytouselessfuel,andmanywillrelyonethanolorhous
Inourlooks-obsessedsociety,manypeoplethinkthatbeingoverweightisanappearanceissue.Butbeingoverweightisactually
MeaninginLiteratureInreadingliteraryworks,weareconcernedwiththe"meaning"ofoneliterarypieceoranother.Howev
Inordertoavoidcertaincharge,theanalysisinvolves
Nowweareengagedinagreatcivilwar,testingwhetherthatnation,oranynationsoconceivedandsodedicated,canlongendu
Thereisaperceptionabroadinthelandthat,becausesomesegmentsofthepublicaredissatisfiedwiththequalityofsomepar
Thedestructionofournaturalresourcesandcontaminationofourfoodsupplycontinuetooccur,largelybecauseoftheextreme
随机试题
患儿,5岁半。右下后牙肿疼两日。检查右下V冠大部分破坏,龋洞较深,叩(+),松动Ⅱ度,颊侧牙龈处有0.5mm×0.5mm脓肿。X线片示根吸收1/3,根分歧有低密度影。右下V的初诊治疗方案是
甲涉嫌盗窃罪被逮捕。甲父为其申请取保候审,公安机关要求甲父交纳10万元保证金。甲父请求减少保证金的数额。公安机关在确定保证金数额时应当考虑下列哪些情况?(2009—卷二—72,多)
设f(u,v)具有一阶连续导数,等于()。
根据《工程网络计划技术规程》(JGJ/T 121—99),在双代号时标网络计划中是( )。
施工过程中,建设单位必须按照国家有关规定办理申请批准手续的情形有()
2015年3月,某贸易公司进口一批红酒,成交价格为200万元人民币,关税税率为14%,货物运费率为2%,进口货物的保险费无法确定,该贸易公司应缴纳关税()万元。
我国一居民总公司在B国设有一分公司,某纳税年度,总公司在我国取得所得600万元,在B国分公司取得所得100万元,分公司适用30%税率,但因处在B国税收减半优惠期而向B国政府实际缴纳所得税15万元,我国所得税税率为25%,按照限额饶让抵免法,则我国应对总公司
罗森塔尔等人对教师的________进行了研究。
[*]
若有定义:doublea=22;inti=0,k=18;则不符合C语言规定的赋值语句是
最新回复
(
0
)