#30.     Magnetic Storms

    The term "magnetic storm," meaning a world-wide magnetic disturbance, was coined by Alexander von Humboldt (1769-1859). A naturalist who gained attention by exploring the jungles of Venezuela, Humboldt devoted much of his life to the promotion of science. He produced five volumes of "Kosmos" (starting the modern usage of that term), an encyclopaedic account covering the broad spectrum of the sciences. It was "Kosmos" which brought to the world's attention the discovery of the sunspot cycle by Heinrich Schwabe.

    After journeying through Siberia, Humboldt convinced the Czar to set up a network of magnetic observatories across the Russian lands, and additional stations were established throughout the British Empire, from Toronto to Tasmania. This network clearly showed that magnetic storms were essentially identical all over the world: a steep decrease of the field over 6-24 hours, followed by a gradual recovery which lasted 1-4 days. The change in the magnetic field was small, in modern units some 50-300 nT (nanotesla) out of a total intensity of 30-60,000 nT, but its world-wide scale suggested that something quite big was happening out in space.

    Readings of magnetic observatories are available on the web. For a sample, the magnetogram below is from a 1998 storm, taken by an observatory in Japan.

    The main signature of a magnetic storm is a southward magnetic field, weakening the northward field usually observed in equatorial regions. It suggests thatits origin is a "ring current" circling the Earth, and we now know that such a current does exist, carried by the outer radiation belt. In magnetic storms the outer belt becomes much more intense, reinforced by protons coming from the tail, as well as by O+ ions from the ionosphere.

    The neutral hydrogen atoms of the geocorona move quite slowly and have much less energy than ring current ions (if they had more, the Earth's gravity would not hold them!). A collision often ends up transferring an electron from the hydrogen atom to the ring-current ion, without much change in the particles' energies.

    The hydrogen atom, having lost an electron, becomes an ion (proton), and because of its low energy, contributes very little to the ring current. On the other hand, the ring current ion which has gained an electron is now neutralized, becoming a fast neutral atom with a great deal of energy. Since the Earth's magnetic field can only trap charged particles, the fast atom usually disappears quickly into distant space. In this way the "charge exchange" process gradually removes newly added particles from the ring current. Only the more energetic ones remain, since their probability of undergoing charge exchange is much lower.

    This process has an unusual application, allowing the ring current to observed remotely, a bit like the way astronomers observe distant stars through their telescopes. Astronomers use light, which moves in straight lines. Similarly, if one could build a camera that used energetic neutral atoms (ENA) created in the ring current by charge exchange, it might be able to picture the ring current, too, since ENAs also move along straight lines.

    The "Image" mission using such a camera is in fact scheduled for launch in the year 2000. Technically this is a rather difficult feat, because the number of ENAs coming from the ring current, especially outside magnetic storms, is rather small. A pilot experiment of this type ran for five weeks aboard the Swedish satellite Astrid, launched in December 1994, and produced some very simple ENA images.

Storms and Substorms

    Substorms have now been studied for many years, from space and from the ground. Their details vary from one event to the next, just as thunderstorms in the atmosphere never seem alike, but many scientists have nevertheless concluded that they are a fundamental mode of energy release and of particle acceleration.

    Magnetic storms usually have a well-defined trigger--often the arrival of an interplanetary shock. Their main effect on the magnetosphere is the injection of many energetic ions and electrons from the tail, causing the ring current to grow significantly. Yet substorms also inject such particles, as was shown in 1971 by the instruments aboard the synchronous ATS-1, an experimental communication satellite with a "piggyback" scientific payload. Many other satellites have studied substorm injections since then, confirming that substorms also injected ions and electrons into the ring current--only, not as many, penetrating less far and with lower average energy.

    Apart from its sudden initiation, is a magnetic storm primarily a series of large and intense substorms? That was apparently the view of Sydney Chapman (1888-1970), distinguished researcher of magnetic storms, who introduced the term "substorm" to suggest precisely that idea. Chapman noted in 1963 that the same storms which at near-equator observatories, e.g. in Hawaii, followed simple curves of growth and decay, in Alaska seemed to consist of a number of distinct "sub-storms."

    Substorms however exist at other times as well (as S. Akasofu, Chapman's student, discovered soon afterwards). They do not need much of a stimulus: during times of southward interplanetary field, the tail seems to quickly reach the brink of instability, and small changes in the solar wind can then precipitate a substorm. Although magnetic storms seem to come from more powerful sources, such as the arrival of interplanetary shocks, they too seem to require (usually) a southward IMF. A strong shock arriving with a northward IMF may shake the magnetosphere, but not to the point of creating a storm. Thus much still remains to be clarified.

M-Regions and Coronal Holes

    The relation between sunspots and smaller magnetic storms, however, seemed less firm. In 1904 E.W. Maunder of the Royal Observatory in Greenwich, England, proposed that many such storms belonged to an entirely different class, tending to recur at intervals of 27 days, the Sun's rotation period. It was as if something on the rotating Sun was beaming those storms at us. However, attempts to identify those regions on the Sun suggested that they were bland and featureless, containing no sunspots. Astronomers named them "M regions" (M for magnetic storms), and for a long time no one had a clue about what set them apart.

    Space observations finally pointed the way. In 1962 the space probe Mariner 2, on its way to Venus, noted that the solar wind contained recurrent fast streams, whose sources appeared to rotate with the Sun. The arrival of such streams was found to trigger moderate storms of the kind studied by Maunder, but their cause was still unclear.

    A decade later, in 1973, astronauts aboard the space station Skylab observed the Sun in soft X-rays. Such pictures, like the one on the right which was taken by the Japanese satellite Yohkoh, highlight the hot spots of the corona:

Click here for a full size version of this image.

    Bright X-ray regions in the corona were often associated with sunspots, which (it seemed) pumped extra energy into the regions above them. In contrast, the elusive "M-regions" turned out to be the dark areas in-between, named "coronal holes." Apparently, the arches and loops of magnetic field lines produced in sunspots trapped and held back solar plasma, hindering it from flowing away as solar wind. In "coronal holes", on the other hand, the magnetic field was weaker and its field lines stuck out straight into space, providing an easy route along which solar wind could flow. Thus although such regions were cooler than their neighbors, they were better sources of solar wind. The polar caps of the Sun, away from the sunspot belts, form two very large "coronal holes", and the solar wind emanating from them was expected to be fast and steady, a prediction confirmed by Ulysses. The "holes" which produce fast solar wind streams at Earth are generally extensions of the polar ones.

        Further Exploration