首页
登录
职称英语
It has been known for many decades that the appearance of sunspots is roughly
It has been known for many decades that the appearance of sunspots is roughly
游客
2025-01-04
27
管理
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. [br] 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://tihaiku.com/zcyy/3898745.html
相关试题推荐
ThefirstdecadesoftheRomanEmpirewerethegoldenageofLatinliterature
ThefirstdecadesoftheRomanEmpirewerethegoldenageofLatinliterature
Ithasbeenknownformanydecadesthattheappearanceofsunspotsisroughly
Ithasbeenknownformanydecadesthattheappearanceofsunspotsisroughly
Ithasbeenknownformanydecadesthattheappearanceofsunspotsisroughly
Whichofthefollowingwordscontainsoneboundmorpheme?A、Disappearance.B、Unto
Ithasbeenknownformanydecadesthattheappearanceofsunspotsisroughly
Ithasbeenknownformanydecadesthattheappearanceofsunspotsisroughly
Therecentsurgeinoilpricestoroughly$55abarrelteachessomeusefulles
Therecentsurgeinoilpricestoroughly$55abarrelteachessomeusefulles
随机试题
Oneofthemostinterestingofallstudiesisthestudyofwordsandwordor
细木工板的检查评价分类中,不属于规格尺寸和偏差()。A.长度和宽度的偏差 B
虽然不同的操作系统可能装有不同的浏览器。但是这些浏览器都符合()协议。A.SN
阿西美辛分散片处方中填充剂为()。A:微粉硅胶 B:微晶纤维素 C:羧甲基
患者男,13岁,面部检查,无明显的异常,覆盖4mm,和开始,磨牙轻远中关系,牙弓
下列哪项不是条件致病菌致病的因素A、寄生部位改变 B、全身免疫功能低下 C、
用横道图表示的建设工程进度计划,一般包括( )基本部分。 A、两个 B、
患者男性,52岁。3年来经常出现左下肢行走后疼痛,间歇性跛行,近来自感疼痛加重,
以下不属于证券经纪业务基本要素的是()。A.证券经纪商 B.证券交易的标的物
某机械制造厂2020年拥有货车3辆,每辆货车的整备质量均为1.499吨;挂车1部
最新回复
(
0
)