#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" (startng 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 12-24 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.

The image below is from the magnetic storm of May 5, 1998, as observed by the Kakioka observatory in Japan. The top trace is the one characterizing the storm and the drop is close to 3 divisions or about 130 nT, occuring over about 3 hours. The same observatory can also also provide you with today's magnetic record.

The disturbing field pointed southward, suggesting that it came from 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 Geocorona

Most of the trapped ions added in magnetic storms, especially those with lower energies, disappear again within a few days. They are usually removed by collisions with the outermost part of the Earth's atmosphere, a huge cloud of hydrogen known as the geocorona extending to a distance of 4-5 Earth radii. It was photographed in 1972 (picture below) by Apollo astronauts on the Moon, using an ultraviolet-light camera developed by George Carruthers and his team at the US Naval Observatory.

The loss process involves so-called charge exchange collisions.

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 may have an unusual application, by letting us observe the ring current from a distance, 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 uses 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.

An "Image " mission using such a camera has in fact been approved by NASA. 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

What creates a magnetic storm?

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 particle acceleration.

In contrast, it is not at all clear that magnetic storms are similarly fundamental. Their main distinguishing feature 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: it is just that the injected particles are fewer, their penetration less pronounced and their average energy lower.

Is then a magnetic storm merely 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 this 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."

Although substorms also exist at other times (as S. Akasofu, Chapman's student, discovered soon afterwards), there might be a difference. "Garden variety" substorms 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. Magnetic storms on the other hand seem to come from much more powerful sources, in general from the arrival of interplanetary shocks (though southward IMF is also a strong factor).

M-regions and Coronal Holes

The connection between magnetic storms and sunspots was well established around the turn of the century. When large active sunspots were visible, big magnetic storms were much more likely. In today's terminology one might say that the intense field of sunspots was likely to lead to sudden releases of magnetic energy, manifested by flares and coronal mass ejections, and those in their turn sent out fast interplanetary plasma clouds whose shock fronts caused magnetic storms.

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, making it easy for the solar wind to escape. Thus although such regions were cooler than their neighbors, they made better sources of the 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.

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Last updated March 13, 1999