Storing Invisible Particles Within Earths Magnetosphere

Charged particles flowing from the Sun can enter the Earth's magnetic domain and become trapped within it. ttey can be stored along the stretched dipolar field lines, earthward of the tail magnetic connection site, in a region called the plasma sheet. It acts as a holding tank of electrons and ions, suddenly releasing them when stimulated by the ever-changing Sun. Nearer the Earth particles are stored in the radiation belts, regions of an unexpectedly high flux of high-energy electrons and protons, which girdle the Earth far above the atmosphere in the equatorial regions.

Inspired by Birkeland's experiments with electrons and a dipolar magnetic sphere, Carl St0rmer (1874-1957), a young theoretical physicist in Oslo, Norway, studied mathematically the motion of charged particles in the magnetic field of a dipole. Using tedious numerical calculations before the computer age, he showed in 1907 that electrically charged particles could be confined and suspended in space by the Earth's dipolar magnetism, spiraling around the magnetic field lines and bouncing back and forth between the Earth's magnetic poles for long periods if time (Fig. 8.5).

tte magnetic field exerts a force on the charged particles that guides them in a helical path; they can move freely along the magnetic field line, but are constrained to a circular motion around it. tte spiraling trajectory becomes more tightly coiled in the stronger magnetic fields close to a magnetic pole, until the charged particles are deflected at a mirror point, tte stronger magnetism pulls a particle's motion into smaller and smaller circles, slowing the downward motion, stopping it, and pushing the particle back, like throwing a ball against a spring. Most scientists were nevertheless completely surprised by the discovery of donut-shaped belts of electrons and protons trapped within the terrestrial magnetosphere.

tte first American satellite Explorer 1, launched on 31 January 1958 in response to the Soviet Union's successful Sputnik satellites in October and November 1957, had a Geiger counter aboard to measure the intensity of cosmic rays, tte instrument recorded the expected cosmic rays near the Earth, but counts rose more than expected at higher altitudes and then disappeared altogether. Explorer 3 confirmed the effect two months later. (Explorer 2 went into the ocean.)

It turned out that the satellites had entered a dense region of energetic particles that saturated the Geiger tube, causing the counter to read zero, ttere were so many high-energy particles coming into the detector that it shut down, as if space itself was radioactive, tte instrument showed that space is filled with "radiation"

FIG. 8.5 Magnetic trap Charged particles can be trapped by Earth's magnetic field. They bounce back and forth between polar mirror points in either hemisphere at intervals of seconds to minutes, and they also drift around the planet on time-scales of hours. As shown by the Norwegian scientist Carl St0rmer (1874-1957) in 1907, with the trajectories shown here, the motion is turned around by the stronger magnetic fields near the Earth's magnetic poles. Because of their positive and negative charge, the protons and electrons drift in opposite directions.

Magnetic Field Line

Trajectory of Trapped Particle

FIG. 8.5 Magnetic trap Charged particles can be trapped by Earth's magnetic field. They bounce back and forth between polar mirror points in either hemisphere at intervals of seconds to minutes, and they also drift around the planet on time-scales of hours. As shown by the Norwegian scientist Carl St0rmer (1874-1957) in 1907, with the trajectories shown here, the motion is turned around by the stronger magnetic fields near the Earth's magnetic poles. Because of their positive and negative charge, the protons and electrons drift in opposite directions.

of intensities a thousand times greater than expected, suggesting that energetic, charged particles encircle the Earth within donut-shaped regions near the magnetic equator (Fig. 8.6).

ttese regions are sometimes called the inner and outer Van Allen radiation belts, named after James A. Van Allen (1914- ), whose instruments first observed them; they have been dubbed radiation belts since the charged particles that they contain were known as corpuscular radiation at the time of their discovery, tte nomenclature is still used today, but it does not imply either electromagnetic radiation or radioactivity.

Van Allen and his colleagues at the University of Iowa were fully aware of St0rmer's prior work, as well as related studies by the American physicist Sam Bard Trieman (1925-1999) of cosmic rays interacting with the Earth's atmosphere and magnetic fields, reporting in 1959 that:

"tte existence of a high intensity of corpuscular radiation in the vicinity of the Earth was discovered by apparatus carried by satellite 1958 alpha [Explorer 1], ...It was proposed in our May 1,1958 report that the radiation was corpuscular in nature, was presumably trapped in Stormer-Treiman lunes about the Earth, and was likely intimately related to that responsible for aurorae. On the basis of these tentative beliefs it was thought likely that the observed trapped radiation had originally come from the Sun in the form of ionized gas.32

FIG. 8.6 Radiation belts Electrons and protons encircle the Earth within two donut-shaped, or torus-shaped, regions near the equator, trapped by the terrestrial magnetic field. These particles can damage the microcircuits, solar arrays and other materials of spacecraft that pass through the Van Allen radiation belts. A torus of high-energy particles creates a third radiation belt locatedjust inside of the inner Van Allen belt, but not shown here; it contains heavy nuclear ions that originated outside our Solar System and once drifted between the stars.

tte radiation belts mainly consist of high-energy electrons and protons, with lesser amounts of heavier ions. And since these charged particles are trapped in the Earth's magnetic cage, as St0rmer showed, one wonders how they got into the radiation belts in the first place. As we have already shown, some of them can enter through the back door, by magnetic reconnection of the solar and terrestrial magnetic fields in the magnetotail, and once in they can be additionally accelerated to high energies. Ms coupling can become especially effective when a coronal mass ejection encounters the Earth with the right orientation, supplying more electrons and protons to the magnetosphere and pumping those that are already there up to higher energies.

Particles within the inner magnetosphere close to the Earth can come from the upper terrestrial atmosphere below the radiation belts. Solar extreme ultraviolet and X-ray radiation create the ionosphere in the upper atmosphere, which can vary dramatically with solar activity. Although most of the ionosphere is gravitationallybound to the Earth, some of its particles have sufficient energy to escape and become trapped by the magnetic fields higher up.

In addition, very energetic cosmic rays, entering the atmosphere from outer space, can collide with atoms in our air and eject neutrons from the atomic nuclei, ttese neutrons travel in all directions, unimpeded by magnetic fields since they have no electrical charge. But once it is librated from an atomic nucleus, a neutron cannot stand being left alone. A free neutron lasts only 10.25 minutes on average before it decays into an electron and proton. Some of the neutrons produced by cosmic rays in our atmosphere move out into the inner radiation belt before they disintegrate, producing

FIG. 8.6 Radiation belts Electrons and protons encircle the Earth within two donut-shaped, or torus-shaped, regions near the equator, trapped by the terrestrial magnetic field. These

particles can damage the microcircuits, solar arrays and other materials of spacecraft that pass through the Van Allen radiation belts. A torus of high-energy particles creates a third radiation belt locatedjust inside of the inner Van Allen belt, but not shown here; it contains heavy nuclear ions that originated outside our Solar System and once drifted between the stars.

regions are now called the inner and outer Van Allen radiation belts, named after the American scientist James A. Van Allen (1914- ) who first observed them with the Explorer 1 and 3 satellites in 1958. The inner belt's charged particles tend to have higher energies than those in the outer belt. The trapped electrons and protons that are immediately snared by the magnetic fields and remain stored within them.

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