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Most of Earth's seas experience two high and two low tides each day. There is also a less noticeable tide in its solid crust, which causes a variation in height of about half a meter.

The tides are caused by the combined effects of the gravitational pull of the Moon and the Sun. Clearly, the pull of lunar gravity is greatest where Earth's surface is nearest the Moon. On the opposite side of Earth, the pull of lunar gravity is weakest. The overall result is that there are equal‐sized oceanic bulges on opposite sides of the planet. The tidal bulges are fixed with respect to the direction of the Moon. However, the solid surface rotates beneath the oceans much more rapidly than the Moon orbits Earth. This gives rise to two high tides each day.

Figure 3.38 The highest tides, known as spring tides, occur when the Moon and Sun are aligned so that their combined gravitational pull increases the size of the tidal bulges in the oceans. When the Sun and Moon are 90° apart in the sky, their gravity pulls in different directions and the tides are least noticeable. These are the neap tides.

(Windows to the Universe / Lisa Gardiner)

Precisely the same effects are created by the interaction of the Sun and Earth, although the tidal bulges are slightly less than half the size of those created by the Moon. About twice a month, the Sun and Moon line up and then their gravitational effects reinforce each other to produce the largest tides, the spring tides. When the Sun and Moon are 90° apart in the sky, the tidal bulges are at their minimum. These are known as the neap tides.

Although this simple pattern predominates in the deep ocean, the times of the tides can be delayed by several hours, even for places that are quite close together, in the complex waters of the continental shelf and river estuaries.

In reality, Earth's rotation carries the tidal bulges slightly ahead of the point directly beneath the Moon. This means that the force between the Earth and the Moon is not acting exactly along the line between their centers. This causes a net transfer of rotational energy from Earth to the Moon, slowing the planet's period of rotation by about 1.5 milliseconds per century. It also causes the Moon to raise its orbit by about 3.8 cm per year. Over a period of 100 million years, the Moon will recede from Earth by 3,800 km.

Figure 3.39 Earth's magnetic field creates a huge “bubble” – the magnetosphere – around the planet. This acts like a shield, diverting electrons and protons in the solar wind around the planet. The motion of the solar wind shapes the magnetosphere like a tadpole, with a blunt “head” and a very long “tail.” Where the solar wind slows suddenly, upstream of the magnetosphere, a bow shock is formed. Some solar particles enter the magnetosphere at the polar cusps, above Earth's magnetic poles. These particles collide with the upper atmosphere and cause auroras. Other particles are trapped in the Van Allen radiation belts. Also shown are ESA's four Cluster spacecraft, which were launched to explore the magnetosphere and its interaction with the solar wind.

(After ESA)

Passing through the shock, which ranges in thickness from roughly 100 km to 2 Earth radii (approx. 12,700 km), the electrically charged particles of the solar wind are slowed, compressed, and heated. The region downstream of the bow shock, between the shock and the magnetopause, which is occupied by the shocked solar wind plasma, is known as the magnetosheath.

The magnetosphere acts as a protective shield against solar radiation and high‐energy particles that flow from the Sun and other sources in the Galaxy. However, the magnetic shield is not impenetrable.

Its weakest points are the cusps above the magnetic poles, where some protons and electrons from the solar wind cross into the magnetosphere and spiral down the field lines into the upper atmosphere.

When the particles collide with atoms and molecules in the air, the gases, particularly oxygen and nitrogen, are excited and start to glow. Oxygen generates a green or brownish‐red glow; nitrogen a blue or red glow. The result is oval‐shaped auroras – commonly known as the northern and southern lights – that more or less continuously surround the magnetic poles (Figure 3.40).

Particles also build up in the magnetotail on Earth's night side, until gusts in the solar wind or coronal mass ejections from the Sun make the plasma unstable. This triggers a magnetic substorm, which causes plasma to flow away from the disturbance.

At such times, accelerated electrons spiral down towards the magnetic poles and bombard the thin upper atmosphere. Such magnetic storms often result in spectacular brightening of the auroral ovals. Meanwhile, electric currents in the ionosphere may cause major magnetic disturbances on the ground.

Figure 3.40 NASA's Dynamics Explorer spacecraft imaged both the aurora borealis (“northern lights”) and the aurora australis (“southern lights”). These glowing ovals, centered above the magnetic poles, are caused by high energy particles striking the upper atmosphere. Each oval is about 500 km wide and 4,500 km in diameter. Green lines show outlines of land areas. Australia is at lower left.

(NASA/University of Iowa)

Close to Earth are two donut‐shaped regions, one inside the other, where high energy electrons and protons have been trapped by the magnetic field. They are known as the Van Allen radiation belts, after their discoverer.¹⁴

The inner belt was discovered by a Geiger counter carried on the first U.S. satellite, Explorer 1. It is located between 1,000 and 5,000 km above the equator and contains particles which have been captured from the solar wind or which originate from collisions between cosmic rays and atoms in the upper atmosphere. Since Earth's magnetic field is offset from the axis of rotation, the inner belt dips down towards the surface over the South Atlantic Ocean, off the coast of Brazil. This South Atlantic Anomaly poses a threat to astronauts and satellites in low‐Earth orbit.

The outer belt lies between 15,000 and 25,000 km above the equator, though it curves closer to the surface towards the magnetic poles. This region mainly contains particles from the solar wind.

The magnetosphere is constantly changing in response to levels of solar activity. Coronal mass ejections from the Sun and solar flares can prompt a sudden intensification of the magnetic field at ground level together with a rapid shift in orientation. Ground‐level electrical currents induced by geomagnetic storms may affect power grids and electronics, causing blackouts and interference with radios and telephones.