8. Energizing Space
The Earth's magnetic influence
Invisible magnetic fields emanate from the Earth, as well as the Sun. As early as 1600, William Gilbert, physician to Queen Elizabeth I of England, demonstrated that our planet is itself a great magnet, which explains the orientation of compass needles. It is as if there was a colossal bar magnet at the center of the Earth, with magnetic fields that emerge out of the south geographic polar regions, loop through nearby space, and re-enter at the north polar regions (Fig. 8.1). Since the geographic poles are located near the magnetic ones, a compass needle always points north or south. The magnetic fields are produced by electrically conducting currents in the Earth’s molten core, so the acts like it has a magnet buried at its center.
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. The energy-laden, electrically-charged solar wind blows out from the Sun in all directions and never stops, carrying with it a magnetic field rooted in the star. Although it is exceedingly thin, 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 (Fig. 8.2), like a tear drop falling toward the Sun.
The solar wind pushes the magnetic field toward the Earth on the day side that faces the Sun, compressing the outer magnetic boundary and forming a shock wave. It is called a bow shock because it is shaped like waves that pile up ahead of the bow of a moving ship. The Sun’s wind drags and stretches the terrestrial magnetic field 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 (Fig. 8.2).
Thus, the Earth’s magnetosphere is not precisely spherical. It has a bow shock facing the Sun and a magnetotail in the opposite direction. The term magnetosphere therefore does not refer to form or shape, but instead implies a sphere of influence. The magnetosphere of the Earth, or any other planet, is that region surrounding the planet in which its magnetic field dominates the motions of energetic charged particles such as electrons, protons and other ions. It is also the volume of space from which the main thrust of the solar wind is excluded.Yet, some of the energetic particles in space do manage to penetrate the Earth’s magnetic defense.
The merging between the magnetic fields of the solar wind and the Earth is most effective if they are pointing in opposite directions. With this orientation, the two fields become linked, just as the opposite poles of two toy magnets stick together, and the solar wind particles can enter the magnetosphere.
The wind’s magnetic field will be dragged by the flow of the wind behind the Earth into its magnetotail, wrapping and clinging around the magnetosphere like saran wrap (Fig. 8.3). The magnetosphere can then be punctured in the tail, providing a back door entry that funnels some of the wind into the magnetosphere. The passing solar wind is slowed down by the connected fields and decelerates in the vicinity of the tail. Energy is extracted from the solar wind and drives a large-scale circulation, or convection, of charged particles within the magnetosphere (Fig. 8.3).
When solar wind electrons and protons enter the Earth’s domain, they also become trapped within it and cannot easily get out. In fact, the inner magnetosphere is always filled with a veritable shooting gallery of electrons and protons, trapped within two torus-shaped belts that encircle the Earth’s equator but do not touch it (Fig. 8.4). These regions are often called the inner and outer Van Allen radiation belts, named after James A. Van Allen who discovered them in 1958. Van Allen used the term “radiation belt” because the charged particles were then known as corpuscular radiation; the nomenclature is still used today, but it does not imply either electromagnetic radiation or radioactivity.
More than half a century before the discovery of the radiation belts, Carl Størmer showed how electrons and protons can be trapped and suspended in space by the Earth’s dipolar magnetic field. An energetic charged particle moves around the magnetic fields in a spiral path that becomes more tightly coiled in the stronger magnetic fields close to a magnetic pole. The intense polar fields act like a magnetic mirror, turning the particle around so it moves back toward the other pole.
Thus, the electrons and protons bounce back and forth between the north and south magnetic pole (Fig. 8.5). 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.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University