Radiation Belts

On typical field lines, attached to the Earth at both ends, such motion would soon lead the particles into the atmosphere, where they would collide and lose their energy. However, an additional feature of trapped motion usually prevents this from happening: the sliding motion slows down as the particle moves into regions where the magnetic field is strong, and it may even stop and reverse. It is as if the particles were repelled from such regions, an interesting contrast with iron, which is attracted to where the magnetic field is strong.

The magnetic force is much stronger near the Earth than far away, and on any field line it is greatest at the ends, where the line enters the atmosphere. Thus electrons and ions can remain trapped for a long time, bouncing back and forth from one hemisphere to the other (see picture above, not to scale--the actual spiral gets much smaller near Earth). In this way the Earth holds on to its radiation belts.

In addition to spiraling and bouncing, the trapped particles also slowly drift from one field line to another one like it, gradually going all the way around Earth. Ions drift one way (clockwise, viewed from north), electrons the other, and in either drift, the motion of electric charges is equivalent to an electric current circling the Earth clockwise.

That is the so-called ring current, whose magnetic field slightly weakens the field observed over most of the Earth's surface. During magnetic storms the ring current receives many additional ions and electrons from the nightside "tail" of the magnetosphere and its effect increases, though at the Earth's surface it is always very small, only rarely exceeding 1% of the total magnetic field intensity.

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Discovery of the Radiation Belt

Prior to 1958 scientists were quite aware that ions and electrons could be trapped by the Earth's magnetic field, but not that such trapped particles actually existed. At most it was proposed that during magnetic storms a temporary trapped population created a ring current, decaying again as the storm ebbed.

The years 1957-8 were designated as the "International Geophysical Year" (IGY), and both the USA and the Soviet Union (Russia) prepared to launch at that time artificial satellites, the first ever. Russia successfully orbited its first Sputnik ("satellite") on October 4, 1957, but the official US entry, Vanguard, failed at launch. The US then quickly assembled an alternative rocket carrying a different satellite, the small Explorer 1 built by James Van Allen and his team at the University of Iowa. It was launched on 31 January, 1958.

Launch of Explorer 1

Click here for a full size version of this image.

Explorer 1 carried only one instrument, a small detector of energetic particles, a Geiger counter designed to observe cosmic rays (ions of very high energy and unknown origin, arriving at Earth from distant space--see later section). The experiment worked quite well at low altitudes, but at the top of the orbit no particles at all were counted. Explorer 3, which followed two months later, collected on tape a continuous record of data, which revealed that the zero counts actually represented a very high level of radiation. So many energetic particles hit the counter at the higher altitudes, that its mode of operation was overwhelmed and it fell silent. Not only was a radiation belt present at all times, it was remarkably intense.

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The Earth's Radiation Belts

The Earth has two regions of trapped fast particles. The inner radiation belt discovered by Van Allen is relatively compact, extending perhaps one Earth radius above the equator (1 RE = 6371 km or about 4000 miles). It consists of very energetic protons, a by-product of collisions by cosmic ray ions with atoms of the atmosphere. The number of such ions is relatively small, and the inner belt therefore accumulates slowly, but because trapping near Earth is very stable, rather high intensities are reached, even though their build-up may take years.

Further out is the large region of the ring current, containing ions and electrons of much lower energy (the most energetic among them also known as the "outer radiation belt"). Unlike the inner belt, this population fluctuates widely, rising when magnetic storms inject fresh particles from the tail of the magnetosphere, then gradually falling off again. The ring current energy is mainly carried by the ions, most of which are protons.

However, one also sees in the ring current "alpha particles," atoms of helium which have lost their two electrons, a type of ion that is plentiful in the solar wind.In addition, a certain percentage are O+ oxygen ions, similar to those in the ionosphere of the Earth, though much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source.

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Energy and Energetic Particles

Energy is the currency in which natural processes must be paid for: to speed up motions, to turn a machine, to make the sun shine or drive an electric current through a wire, energy is needed. A fundamental law of nature states that energy never disappears, just changes its form: e.g., the energy of sunlight can be converted to electricity by a solar cell, or the energy of the moving wind is converted by a windmill, but the total amount stays the same.

Space phenomena involve energy on two very different scales. One scale involves the energy of individual ions and electrons, which often move at a respectable fraction of the velocity of light (an upper limit which they can never reach). The faster the particle moves, the higher its energy and the greater is the thickness of material needed to stop it. Energies like these are measured in electron volts (eV): auroral electrons have 1000-15,000 eV, protons in the inner belt perhaps 50 million eV, while the energy of cosmic ray ions may reach many billions. In contrast, air molecules in the atmosphere only have about 0.03 eV, raising what could be the most fundamental question in space research--how come a few particles get so much?

The other scale is that of global space phenomena: magnetic storms, substorms, auroral displays and electric currents linking Earth and space. Who foots their energy bill? The main source of energy seems to be the solar wind, but the pathways by which this energy is transported and distributed in the magnetosphere are not yet completely clear.

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Synchronous Orbit

Probably the greatest number of operating satellites, more than 200, inhabit the so-called synchronous orbit, a circular orbit above the Earth's equator with a radius of 6.6 RE (Earth radii), approximately 42,000 km or 26,000 miles.

The orbital speed of any satellite depends on its distance from Earth. In a circular orbit just outside the dense atmosphere, a satellite needs only 90 minutes for one full circuit, but more distant satellites move more slowly, and at a radius of 6.6 RE the period is close to 24 hours, matching the rotation period of the Earth. A satellite at this distance, above the equator, always stays above the same spot on Earth, and when viewed from Earth (say, by a TV dish antenna) it is always in the same direction in the sky.

This makes the synchronous orbit the perfect place for satellites devoted to communications and to broadcasting, and it is also used for world-wide weather monitoring, e.g. by the GOES series of satellites of NOAA (National Oceanic and Atmospheric Administration). The synchronous orbit is also useful for scientific work, because on the nightside of the Earth it lies quite close to the transition from the ring current to the magnetospheric "tail".

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