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Какие изменения претерпевает магнетизм Земли, и известны ли ученым причины этих изменений?

THE EARTH'S MAGNETISM

The Earth possesses a magnetic field, the strength of which varies with time and locality. The field is similar to that which would be produced by a powerful magnet situated at the centre of the Earth and pointing approxi­mately north and south. *A magnetized needle suspended to swing freely in all planes will set itself pointing to the Earth's magnetic North and South poles, at an angle to the horizontal.

The vertical plane through the axis of such a needle is termed the magnetic meridian defined as the vertical plane which contains the direction of the Earth's mag­netic field. The cause of the Earth's magnetism is not definitely known. The variations of the Earth's magnetic field with time are of two types, the "secular" (вековой, секулярный) and the "diurnal" [dar^/a:nl] (суточный). The secular variations are slow changes in the same sense but at different rates, as a result of which the

VIA

Earth's magnetic field has decreased by some 5% over the last hundred years. The cause of these variations is Unknown. *The diurnal variations are much smaller and more rapid (быстрый) variations which have been shown to be associated with changes in the ionosphere related to sunspot activity.

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