On typical field lines, attached to the Earth at both ends, such motion wouldsoon lead the particles into the atmosphere, where they would collide and losetheir energy. However, an additional feature of trapped motion usually preventsthis from happening: the sliding motionslows down as the particle moves intoregions 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 interestingcontrast with iron, which isattracted to where the magnetic field is strong.

    The magnetic force is much stronger near the Earth than far away, and onany field line it is greatest at the ends, where the line enters the atmosphere.Thus electrons and ions can remaintrapped 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 itsradiation belts.

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

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

More about trapped particles

    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 thattime artificial satellites, the first ever. Russia successfully orbited itsfirst Sputnik ("satellite") on October 4, 1957, but the official US entry,Vanguard, failed at launch. The US then quickly assembled an alternative rocketcarrying a different satellite, the small Explorer 1 built by James Van Allenand his team at the University of Iowa. It was launched on 31 January, 1958.

Launch of Explorer 1

Clickhere for a full size version of this image.

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

More about the discovery of the radiation belt

    Further out is the large region of thering current, containing ions andelectrons 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 heliumwhich have lost their two electrons, a type of ion that is plentiful in thesolar wind.In addition, a certain percentage are O+ oxygen ions, similar to those inthe ionosphere of the Earth, though much more energetic. This mixture of ionssuggests that ring current particles probably come from more than one source.

More about the Earth's radiation belts

    Space phenomena involve energy ontwo very different scales. One scaleinvolves the energy of individual ions and electrons, which often move at arespectable fraction of the velocity of light (an upper limit which they cannever reach). The faster the particle moves, the higher its energy and thegreater is the thickness of material needed to stop it. Energies like these aremeasured inelectron volts (eV): auroral electrons have 1000-15,000 eV, protonsin the inner belt perhaps 50 million eV, while the energy of cosmic ray ions mayreach many billions. In contrast, air molecules in the atmosphere only haveabout 0.03 eV, raising what could be the most fundamental question in spaceresearch--how come a few particles get so much?

    The other scale is that ofglobal space phenomena: magnetic storms,substorms, auroral displays and electric currents linking Earth and space. Whofoots 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 themagnetosphere are not yet completely clear.

More about energy

More about high-energy particles

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

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

More about synchronous satellites