3. The invisible buffer zone with space - atmospheres, magnetospheres and the solar wind


Earth's magnetic dipole

Fig. .. 

The Earth's magnetic field is described by magnetic field lines that emerge out of the south magnetic pole, loop through nearby space and re-enter at the north magnetic pole. The lines are close together near the magnetic poles where the magnetic force is strong, and spread out near the magnetic equator where the magnetism is weaker than at the poles. You cannot see the invisible magnetism, but instruments can be used to measure it.

The magnetic field strength at the Earth's magnetic equator is 0.0000305 tesla, or 0.305 x 10-4 T. Maps of the surface magnetic fields of Earth show stronger fields near the poles where the magnetic field lines congregate, at roughly twice the strength of the field at the equator.

Earth's protective magnetosphere

Fortunately for life on Earth, the terrestrial magnetic field deflects the Sun’s wind away from the Earth, like a rock in a stream or a windshield that deflects air around a car. It hollows out a protective cavity in the solar wind called the magnetosphere. The magnetosphere of the Earth, or any other planet, is that region surrounding the planet in which its magnetic field dominates the behavior of electrically-charged particles such as electrons, protons and other ions. It diverts most of the solar wind around our planet at a distance far above the atmosphere, thereby protecting humans on the ground from possibly lethal solar particles.

Fig. .. 

The dipolar (two poles) magnetic configuration applies near the surface of the Earth, but further out the magnetic field is distorted by the Sun’s perpetual wind. Although it is exceedingly tenuous, far less substantial than a terrestrial breeze or even a whisper, the solar wind is powerful enough to mold the outer edges of the Earth’s magnetosphere into a changing asymmetric shape, like a tear drop falling toward the Sun.

After passing through the bow shock, the solar wind encounters and flows around the magnetopause, the boundary between the solar wind and the magnetosphere. The magnetic field carried in the solar wind merges with that of the planet, and stretches it out into a long magnetotail on the night side of Earth. The magnetic field points roughly toward the Earth in the northern half of the tail and away in the southern. The field strength drops to nearly zero at the center of the tail where the opposite magnetic orientations lie next to each other and currents can flow.

Trapped particles

The Earth's protective magnetic cocoon is not perfect. Energetic charged particles flowing from the Sun can penetrate the magnetic defense and become trapped within the magnetosphere. This was realized in 1958 when James A. Van Allen (1914 - ) and his students used instruments aboard the Explorer 1 and 3 satellites to unexpectedly discover a large flux of high-energy electrons and protons that girdle the Earth far above the atmosphere, moving within two belts that encircle the Earth’s magnetic equator but do not touch it. They resemble a gigantic, invisible, torus-shaped doughnut. This was the first major discovery of the Space Age.

These regions are sometimes called the inner and outer Van Allen radiation belts. Van Allen used the term “radiation belt” because the charged particles were then known as corpuscular radiation; the nomenclature does not imply either electromagnetic radiation or radioactivity. The radiation belts lie within the inner magnetosphere at distances of 1.5 and 4.5 Earth radii from the center of the Earth, creating a veritable shooting gallery of high-speed electrons and protons in nearby space.

Fig. .. 

Thus, the electrons and protons bounce back and forth between the north and south magnetic poles. It takes about one minute for an energetic electron to make one trip between the two polar mirror points. The spiraling electrons also drift eastward, completing one trip around the Earth in about half an hour. There is a similar drift for protons, but in the westward direction. The bouncing can continue indefinitely for particles trapped in the Earth’s radiation belts, until the particles collide with each other or some external force distorts the magnetic fields.

Fig. .. 

The magnetic fields of the solar wind and the Earth's magnetotail are sometimes pointing in opposite directions, and when this happens the two fields become linked, just as the opposite poles of two toy magnets stick together. When the solar and terrestrial magnetic fields touch each other, the magnetotail can be punctured, providing a back door entry that funnels energy and particles into the magnetosphere. Once inside the magnetic trap, the charged particles can be additionally accelerated to higher energies.

Planets with magnetospheres

Magnetic fields are ubiquitous in the solar system. Earth, Mercury and all the giant planets have strong magnetic fields generated within the planet, and Jupiter and Saturn have extensive magnetospheres. A magnetic field has been found on Mars, but the field's patchy nature suggests that it is not the result of an active internal dynamo. Instead the magnetic field on Mars is probably a remnant of former times, frozen into expanses of solidifying lava. Venus is the only major planet to have no detectable magnetic field. A magnetic field has even been found on at least one satellite, Jupiter's Ganymede.

Fig. ..  Fig. .. 

The general form of the Jovian magnetosphere resembles that of the Earth. but its dimensions are at least 1,200 times greater. It is larger than the Sun in size, with a bow shock at more than 3 billion meters or at least 42 times the planet's radius. Pioneer 10 and Voyager 1 first encountered the Jovian bow shock at 95 and 86 planetary radii, but the shock moved in and out due to the variable solar wind. Jupiter's largest satellites are all embedded within its magnetosphere and interact with it.

Jupiter's magnetic field was first recognized in 1954-55 by Earth-based observations of the planet's intense radio emission, and then directly measured by visiting spacecraft. The radio signals are generated by high-speed electrons trapped within the planet's magnetic field. The synchrotron radio emission is beamed in a direction nearly parallel to the magnetic equator, and it is therefore swept past an Earth-based observer as the planet rotates and brings the magnetic equator in and out of alignment with the line of sight. Periodic variations in the strength of the radio emission indicate that the magnetic field rotates with a period of precisely 9 hours 55 minutes 41 seconds. Since the magnetic fields are generated deep within the planet, this is assumed to be Jupiter's rotation period; it differs from the rotation speed inferred from visible clouds that are blown in different directions and at various speeds by powerful winds.

Fig. .. 

The magnetism near most of the planets can be described by a central magnetic dipole, the equivalent of placing a small but powerful bar magnet near the center of the planet, and with a magnetic axis that is nearly aligned with the planet's rotational axis. But on Uranus and Neptune the dipolar magnet has to be displaced toward the surface, by about one-third the radius of Uranus and more than half Neptune's radius, resulting in a variable magnetic field strength. At the cloud tops of Uranus and Neptune, the magnetic fields are about ten times stronger in the hemisphere that is closest to the buried, offset magnet, and their magnetic equators weave across the clouds.

The large offsets of Uranus's and Neptune's magnetic dipoles suggest that the fields are generated at an intermediate depth rather than within a deep central core like the other planets that have dipolar magnetic fields. Convection and rotation apparently produce currents of electrically-conductive, ionized water within a spherical shell far removed from the center of these two planets. The magnetic axes of Uranus and Neptune are titled at a large angle with respect to their rotational axis, by 58.6 degrees for Uranus and 46.8 degrees for Neptune. They have fully developed magnetospheres that are well-represented by the offset, tilted dipoles. The rotation periods of the magnetic fields of Uranus and Neptune, inferred from their periodic radio emission, are 17.24 and 16.11 hours, respectively.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University