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题目材料：
Fluctuations in the level of the Sun's magnetic activity over time can be deduced from the presence on Earth of varying amounts of certain radioactive elements, called radioisotopes, that form as a result of cosmic rays hitting Earth's upper atmosphere. Cosmic rays can be deflected by the Sun's magnetic field, which is extended to Earth and beyond by the solar wind, and when high solar activity
produces a relatively strong magnetic field, more cosmic rays are deflected and the formation of these radioisotopes is reduced. By measuring levels of radioisotopes in tree rings and core samples of polar ice, researchers can compile a history of solar magnetism that also gives clues to the effects of solar activity on Earth's climate. For example, radioisotope records reveal a period of very low solar activity in the seventeenth century, a time when glaciers expanded and winters lengthened in northern Europe, whereas the eleventh and twelfth centuries, when vineyards grew in areas of Britain that cannot support them today, appear to have been a period of unusually intense solar activity.
以上解析由 考满分老师提供。
