What are the Van Allen belts?

The Van Allen belts are two rings of electrically charged particles that encircle our planet. The belts are shaped like fat doughnuts, widest above Earth's equator and curving downward toward Earth's surface near the polar regions. These charged particles usually come toward Earth from outer space—often from the Sun—and are trapped within these two regions of Earth's magnetosphere.

Since the particles are charged, they spiral around and along the magnetosphere's magnetic field lines. The lines lead away from Earth's equator, and the particles shuffle back and forth between the two magnetic poles. The closer belt is about 2,000 miles (3,000 kilometers) from Earth's surface, and the farther belt is about 10,000 miles (15,000 kilometers) away.

How were the Van Allen belts discovered?

In 1958 the United States launched its first satellite, Explorer 1, into orbit. Among the scientific instruments aboard Explorer 1 was a radiation detector designed by James Van Allen (1914-2006), a professor of physics at the University of Iowa. It was this detector that first discovered the two belt-shaped regions of the magnetosphere filled with highly charged particles. These regions were subsequently named the Van Allen belts.

Do other objects in the solar system have Van Allen belts?

Yes. All the gas giant planets are thought to have such belts, and in Jupiter's magnetic field such belts have been observationally confirmed.

An illustration depicting the two layers of Van Allen Belts around the Earth. (NASA)

An illustration depicting the two layers of Van Allen Belts around the Earth. (NASA)


What is a neutrino?

A neutrino is a tiny subatomic particle that is far smaller than an atomic nucleus; it has no electrical charge and a tiny mass. (Electrons are many thousands of times more massive than neutrinos, and protons and neutrons are many millions of times more massive.) Neutrinos are so tiny and ghostly that

Am I getting hit by neutrinos right now?

You—and every square inch of Earth's surface—are being continuously bombarded by neutrinos from space. Billions of neutrinos slice through your body every second.

Fortunately, neutrinos are so unlikely to interact with any matter— including the atoms and molecules in the human body—that the billions upon billions of neutrinos that hit you every second have no discernible effect at all. In fact, the odds that any neutrino striking Earth will interact with any atom in our planet at all is about one in a billion. Even when it does happen, the result is merely a tiny flash of harmless light.

they almost always pass through any substance in the universe without any interference or reaction.

How was the existence of neutrinos proven?

The existence of neutrinos was first suggested in 1930 by the Austrian physicist Wolfgang Pauli (1900-1958). He noticed that in a type of radioactive process called beta decay, the range of the total energy given off in observations was greater than theoretical predictions. He reasoned that there must be another type of particle present to account for, and carry away, some of this energy. Since the amounts of energy were so tiny, the hypothetical particle must be very tiny as well and have no electric charge. A few years later, the Italian physicist Enrico Fermi (1901-1954) coined the name "neutrino" for this enigmatic particle. The existence of neutrinos was not experimentally confirmed, however, until 1956, when American physicists Clyde L. Cowan, Jr. (1919-1974) and Frederick Reines (1918-1998) detected neutrinos at a special nuclear facility in Savannah River, South Carolina.

If neutrinos are so elusive, how do scientists observe them striking Earth?

It is possible to detect neutrinos from space by their very rare interactions with matter here on Earth, but not with conventional telescopes. The first effective neutrino detector was set up in 1967 deep underground in the Homestake Gold Mine near Lead, South Dakota. There, the American scientists Ray Davis, Jr. (1914-) and John Bahcall (1934-2005) set up a tank filled with 100,000 gallons of nearly pure perchlorate (used as dry-cleaning fluid), and monitored the liquid for very rare neutrino interaction events. Other experiments have since used other substances, such as pure water, for neutrino detections.

Where are the neutrinos coming from?

The vast majority of neutrinos striking our planet come from the Sun. The nuclear reactions at the core of the Sun create huge numbers of neutrinos; and unlike the

What was the "solar neutrino problem?"

From the very beginning of neutrino astronomy research, there was a discrepancy between the theory of nuclear fusion and the number of neutrinos detected from the Sun. Neutrino telescopes on Earth detected only about half as many neutrinos as they should have. This strange result was checked again and again and repeatedly confirmed. This became known as the solar neutrino problem. Was the Sun generating less energy at its core than expected? Was nuclear fusion theory wrong?

The problem was finally solved nearly four decades after it was first discovered. Neutrinos, as it turns out, can actually change their characteristics when they strike Earth's atmosphere. That meant that there were the right number of neutrinos leaving the Sun, but so many of them changed "flavor" upon reaching Earth that they escaped detection by the neutrino telescopes deep underground. This discovery was a major breakthrough in fundamental physics. It confirmed very important properties about neutrinos that have major implications on the basic nature of matter in the universe.

light that is produced, which takes thousands of years to flow their way out of the Sun's interior, the neutrinos come out of the Sun in less than three seconds, reaching Earth in just eight minutes.

Have neutrinos ever been shown to have hit Earth from somewhere other than the Sun?

In 1987, the first supernova visible to the unaided eye to occur in centuries appeared in the southern sky. At almost exactly that same moment, neutrino detectors around the world recorded a total of 19 more neutrino reactions than usual. This worldwide detection does not sound like much, but it was hugely significant because it was the first time neutrinos were confirmed to have reached Earth from a specific celestial object other than the Sun.

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