Читать книгу Exploring the Solar System - Peter Bond - Страница 97

For centuries, explorers and navigators have relied on the presence of Earth's magnetic field – hence the value of the magnetic compass. In fact, this magnetic field is by far the strongest of all the terrestrial planets. Theory suggests that the field is generated by a dynamo effect created by currents circulating in the planet's liquid outer core.

The magnetic field resembles a dipole, with one North Pole and one South Pole, which is inclined about 10º to Earth's rotation axis. At these magnetic poles, which are fairly close to Earth's geographic poles, a compass needle will point straight down, or up, respectively.

However, the magnetic field is very dynamic. Studies of ancient magnetized rocks, both on the continents and the ocean floor, show that the magnetic poles have wandered across the surface throughout Earth's history. This motion is continuing today. During the 20^th century, Earth's magnetic north pole moved over 1,100 km across the Canadian Arctic. In recent decades its rate of migration has increased significantly, and it is now moving across the Arctic Ocean towards Siberia.

Every so often, the magnetic field also reverses, so that the magnetic poles switch polarity, causing a compass needle to point towards the south instead of north.^13* This sequence of reversals is recorded in the once‐molten rocks of the ocean floor, forming “stripes” that run parallel on either side of a mid‐ocean ridge. As such, they provide key evidence supporting sea floor spreading, continental drift, and plate tectonics.

Although the last magnetic reversal occurred almost 800,000 years ago, the rate of reversals during the last 10 million years has averaged 4 or 5 every million years. However, research published in 2018 indicated that the field can reverse in less than two centuries. On the other hand, the interval has often been much longer. For example, during the Cretaceous era, 70 million years ago, the time between magnetic reversals was about one million years.

The magnetic field may also experience an “excursion” – a large, but temporary, decrease in its overall strength – rather than a reversal. Today, the field strength is steadily declining, leading to suggestions that a reversal may be on the way. Even so, the current strength of the magnetic field is still as high as it has been in the last 50,000 years.

There is no evidence to suggest that a reversal has any major impact on the planet's atmosphere or biosphere. Even in the absence of a magnetic field, the atmosphere is still able to filter out most cosmic radiation.

The area of space dominated by Earth's magnetic field – the magnetosphere – takes the shape of a tadpole or windsock, which is shaped by the flow and pressure of the incoming solar wind. This invisible magnetic bubble extends, on average, some 60,000 km toward the Sun and trails more than 300,000 km away on the planet's night side, forming a long magnetotail which stretches far beyond the Moon's orbit (Figure 3.39). The outer edge of the magnetosphere is called the magnetopause.

A curved bow shock – a type of shock wave – typically occurs about 90,000 km from Earth, though its size and location vary according to the density and pressure of the solar wind. The bow shock is created where the solar wind is slowed suddenly when it nears the outer magnetosphere – rather like water piling up in front of the bow of a ship.