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Birth of a Radiation Belt

(text of an article by D.P. Stern)

    The recent impacts on Jupiter by the fragments of comet Shoemaker-Levy are a sobering reminder of the violent forces that exists in space, released now and then in spectacular fashion. A violent event of a different type occurred in March 1991, when a powerful interplanetary shock wave hit the Earth's magnetic field and created a new radiation belt. Nothing like that had happened since July 1962, when the US Air Force exploded a hydrogen bomb above the atmosphere, creating a belt of trapped radiation which lasted five years and caused the demise of three spacecraft.

    At 03:42 Greenwich time on March 24, 1991, the population of high-energy electrons and protons trapped in the Earth's magnetic field suddenly received huge reinforcements. The new belt was so intense that it knocked out (within a few days) the MARECS-1 communication satellite, and NOAA's weather satellite GOES-7 was also seriously degraded.

The Solar Wind

    This time the culprit was no H-bomb but the old familiar Sun. The Sun sends out life-giving light and heat, but it also has its violent outbursts, especially around the peak of its 11-year sunspot cycle. In addition to sunlight, the Sun also sends out the solar wind, a continuous flow of very hot, very rarefied gas, blowing from the Sun's topmost atmosphere, the million-degree solar corona. That gas is so hot that its atoms are broken apart, turning it into a "soup" of free-floating negative electrons and positive ions (mainly protons). In scientific terms, the gas becomes an electrically conducting "plasma," and as it moves outward at some 250 miles/second, it is too rarefied and too hot for its electrically charged particles to recombine again.

    This solar wind fills the entire solar system, well beyond the outermost planets, but it does not reach the Earth, because we are shielded by our planet's magnetic field. Instead it flows around the magnetic obstacle, the way a creek will flow around a rock in its path. A shielded cavity is formed, the "magnetosphere" surrounding the Earth; the closest the solar wind penetrates towards the Earth is about 10-11 RE (Earth radii), or some 40,000 miles, on the side of the cavity facing the Sun.

Interplanetary Plasma Clouds

    But the flow of the solar wind is not always steady and peaceful: superposed on it now and then are violent outbursts on the Sun, causing huge expanding clouds preceded by sudden shock fronts. The outbursts seem to be associated with sunspots, strongly magnetic areas on the Sun whose number rises and falls in an 11 year cycle. Some of the magnetic energy associated with sunspots can apparently be released rather abruptly--how and why, we only dimly comprehend, and for that matter, we also have no good idea of what makes the corona so hot.

    Besides expanding clouds, the Sun's outbursts also create great numbers of fast ions, enough to fill the inner solar system, each with an energy that can reach several million times that of solar wind particles; such particles behave very much like intense nuclear radiation, and are a danger to the lives of any astronauts who happen to be on their way to the moon or to Mars, outside the shield of the Earth's magnetosphere. Intense X-rays and radio waves are also emitted, coming from high energy electrons which did not manage to escape the Sun.

    Until recently such radiation was credited to solar flares, bright spots that suddenly appeared in the Sun's high atmosphere near sunspots, supposedly signifying energy release in the corona. But in 1973 astronauts aboard the Skylab space station saw something new: huge bubbles of hot gas, expanding upwards much faster than the solar wind, fast enough to push shock waves ahead of them into interplanetary space. Such "coronal mass ejections" (CMEs) seemed closely related to the interplanetary shocks which now and then strike the magnetosphere. Each year a few such shocks are strong enough to push the magnetosphere's boundary past the synchronous orbit, at 6.6 RE, where communication satellites generally dwell. Recent opinion has been that CMEs, rather than flares, are more likely to be signs of sudden solar energy releases that affect the Earth.

The Event of 24 March, 1991

    On March 23, 1991, a fairly significant flare erupted on the Sun, and hours later, energetic protons appeared in the Earth's vicinity. It took a day for the shock to arrive (ordinary solar wind takes four or five days), and it struck the magnetosphere on the afternoon side; later the shock also passed the space probe Ulysses, 2.5 times more distant from the Sun and on its way to study the region above the Sun's poles. Spacecraft data afterwards suggested the boundary might have been pushed back to a record depth, within 4 RE of the Earth's center, and that the impact also created a second shock wave inside the cavity, spreading throughout the magnetosphere.

    The research spacecraft CRRES, operated by the US Air Force with NASA participation, was at that instant deep inside the radiation belt, at a distance of 2.55 RE. CRRES (pronounced "cress") stands for Combined Release and Radiation Effects Satellite, reflecting the spacecraft's multiple duties--to probe the radiation belt, as well as to release clouds of barium and lithium vapor, tracing motions of the magnetosphere the same way as a plume of smoke traces the motion of wind. CRRES was also a testbed for a variety of electronics circuits, to help engineers design electronics and microcomputers to perform reliably in space, even in the heart of the radiation belt.

    The first thing CRRES saw was a torrent of highly energetic protons and electrons. The protons had energies above 20 Mev, twenty million electron volts, some 20,000 times the energy of the average proton in the solar wind. The electrons had about 15 Mev, and the energy of either type was quite sufficient to penetrate a spaceship and cause damage. In the concluding words of a scientific study of this event, "it is fortunate that present-day space missions do not spend much time in this region of the Earth's magnetosphere." On that particular day and for a long time afterwards, that region was indeed a hot place to be in.

The Acceleration of Energetic Particles

    Faced with such phenomena, the engineer naturally worries about the safety of payloads and passengers. The scientist, however, stands amazed: how can particles gain such high energy, within no more than tens of seconds? In the laboratory particles can be energized by accelerators, cleverly designed machines which carefully channel their particles, but in nature conditions vary all the time and follow no precise rules.

    Yet we have abundant evidence that ions and electrons in space are indeed accelerated to high energies, all over the universe: in flares and CMEs near the Sun, in elusive "substorms" of the magnetosphere, in the radiation belts of Jupiter and other magnetized planets and in the unknown sources of cosmic radiation, the perpetual drizzle of extremely energetic ions which bombards the Earth. On March 24, 1991, it happened right in front of our eyes, as if by a conjuring trick. But how?

    Some evidence came from the particles themselves. Energetic ions and electrons trapped by the Earth's magnetic field drift around the equator--positive protons clockwise (viewed from north), negative electrons counter-clockwise. The sudden burst of electrons intercepted by CRRES quickly ebbed again, suggesting it was caused by a compact cloud of electrons which soon drifted away.

CRRES observations (left) and a computer simulation (right) of the sudden injection of high energy electrons on March 24, 1991. The horizontal axis measures time and the peaks are about 150 seconds apart.

    The cloud came back several times after circling the Earth, at intervals of about 150 seconds. The length of the drift period told researchers that the electrons had about 15 Mev of energy, and the fact that the radiation peak stayed well-defined over at least four returns suggested that their spread of energies was very small. The higher the energy, the faster the drift, so that a cloud of electrons with widely differing energies is quickly dispersed as fast electrons overtake slow ones. That is what happened to bomb-produced electrons in 1962; the initial pulse (revealed by its radio signal) was sharp and well-defined, but when it returned after one circuit it was already spread-out like a pile of sand. The protons observed on March 24, 1991 also displayed such periodic "drift echoes" which stayed together for several returns but since their drift was faster (protons are heavier), the separation of the return pulses was smaller.

Explaining the Sudden Acceleration

    Scientists then examined the shock wave itself, recorded by magnetic observatories around the globe through its associated magnetic pulse. At CRRES the shock was also accompanied by a strong electric field, a voltage spike. Wave phenomena in space generally combine both electric and magnetic fields, the former energize ions and electrons while the latter mainly steer them.

    Dr. Xinlin Li of Dartmouth College in New Hampshire (now at the U. of Colorado at Boulder) and his colleagues--Mary Hudson at Dartmouth, Ilan Roth, John Wygant and Mike Temerin at Berkeley, and Bernie Blake at the Aerospace Corporation--used a computer to model the path of the shock wave and to reconstruct the way it affected electrons already present in the magnetosphere. They selected a wide range of initial positions and energies, then calculated the tracks of more than 300,000 electrons, examining how each of them fared when the wave passed over it. The result resembled in many ways the way a surfer rides a wave. Electrons starting with unfavorable positions or energies gained little energy or even lost some; however a few lucky ones matched the speed of the advancing wave, rode the crest deep into the magnetosphere and gained much energy in the process. In their computer simulation Dr. Li and his colleagues managed to reproduce quite convincingly the initial pulse and two of the periodic "drift echoes."

    Did this explain the way ions and electrons are accelerated in nature? Not completely, because the favored electrons already needed a hefty amount of energy to start with, close to 2 Mev. The abundant low-energy electrons in the magnetosphere gained very little energy: like pieces of driftwood bobbing in the surf, as the shock passed them their energy rose briefly and then went down again. Electrons of around 2 Mev indeed exist in the magnetosphere, and they seem to be the ones from which the new radiation was formed. But their origin has always been something of a mystery: some scientists have even speculated that they may have escaped from the intense radiation belt of the distant planet Jupiter.

The New Belt

    The passage of the shock also left Earth with a new long-lived belt of high-energy protons. Prior to March 1991 the main radiation hazard in near-Earth space was the intense inner radiation belt, a by-product of cosmic radiation. Traverses by CRRES after that date showed a second peak of comparable intensity, apparently composed like the inner belt of protons above 20 Mev. It was more distant than the natural inner belt, centered in the region where CRRES had made its initial observations. In hindsight, the spacecraft was just about ideally placed for observing the event.

    (left) Before the event of March 24, 1991;
(right) Immediately afterwards. The horizontal axis measures distance from the Earth's center. The left edge is at the surface of the Earth (1 Earth radius = 1 RE), the peaks of the "old" inner belt (left panel) occur around 1.5 RE and the electron and ion peaks added by the new belt are at about 2.1-2.2 RE.
    CRRES continued to observe the new belt for seven months, until the spacecraft's untimely demise (due to battery failure) on 12 October, 1991, by which time the new belt had diminished somewhat in intensity. Since then scientists have only managed to sample the belt's far fringes, crossed by low-flying satellites. In past decades the US always had one or more scientific spacecraft in an elongated orbit, cutting through all levels of the magnetosphere; but none was left after CRRES fell silent. For all we know, some of the belt's remnants may still be orbiting above our heads.

Last updated 25 November 2001
Re-formatted 9-28-2004