#30.     Magnetic Storms

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 Geocorona

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.

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

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 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.

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: