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The Earth's magnetosphere, like the Earth's atmosphere, is never at rest. Of its many dynamic features, perhaps the most important and basic is the so-called magnetospheric substorm, a time of increased energy release in the magnetospheric tail. During substorms, in the polar regions, aurora becomes widespread and intense, also much more agitated, and the Earth's magnetic field is disturbed. Out in space, ions and electrons flow in much greater numbers and at higher energies, and changes in the magnetic field are much more profound than those seen at Earth.
Substorms and Magnetic StormsIn the 19th century and the first half of the 20th, the magnetic disturbances which received the greatest attention were "magnetic storms", so named by Alexander Von Humboldt. These decreases in the magnetic field are world-wide and are readily observed at any place. Typically, a storm takes about half a day to develop, and it gradually decays over the next few days.Magnetic storms are relatively rare. On the other hand, smaller "substorms" observable mainly in polar regions (and in space!) present a clearer pattern and seem to be more fundamental. They are also much more frequent, often just hours apart. The two are of course related, and during magnetic storms intense substorms are generally observed in the polar regions. "Storms" distinguish themselves by injecting appreciable numbers of ions and electrons from the tail into the outer radiation belt, and their world-wide magnetic disturbance reflects a rapid growth of the ring current. Substorms usually do not inject as many particles. It might thus be that magnetic storms are merely sequences of very intense substorms, but additional factors could also be involved.
Substorms at Earth and in SpaceOn Earth the most visible sign of a substorm is a great increase of polar auroras in the midnight sector. At ordinary times, quiescent auroral arcs are often seen there, but following the onset of a substorm, they intensify, move rapidly (mostly poleward) and expand, until they may cover much of the sky. The activity may build up for half an hour and then decay, but as with atmospheric weather, patterns are quite variable.Large magnetic disturbances are also observed, up to 1000 nT (nanotesla) which is about 2% of the total field in the auroral zone. The world-wide disturbance observed in a magnetic storm of respectable size may only reach 100 nT, but then, its source is much more distant. For while the circuit of electric currents associated with the substorm includes the ionosphere, only about 130 km above ground, the ring current involved in magnetic storms circles the Earth at a distance of tens of thousands of km.
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(above) The record of electrons intercepted by the synchronous satellite ATS 6 on 20 July 1974. The jagged peaks mark the arrival of electrons in substorms, and they gradually drift away again. The lower energies which persist belong to the plasma sheet in which the satellite is immersed for about half of its orbit.
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Much more profound changes are observed in space. Satellites in synchronous orbit which find themselves near midnight when a substorm erupts may see the magnetic field drop by half, and their on-board detectors in general register the arrival of many ions and electrons with (typically) 5-50 keV. These particles can affect a spacecraft, and in particular, the electrons may charge it negatively to hundreds and even thousands of volts, which could interfere with normal operations. With over 200 communication satellites inhabiting the synchronous orbit, there is obviously a good reason for studying substorm effects there. Still further out, in the plasma sheet, very fast flows of plasma are often seen, typically at 100-1000 km/sec; the plasma particles also seem to have higher energies than normal, and magnetic fields change rapidly and erratically. It is not easy to piece together a pattern from such observations, since most of the evidence comes from isolated satellites. What seems to happen is that magnetic field lines of the tail are first stretched tailwards and are then released, in a way frequently compared to the stretching and rebounding of a slingshot. As the lines bounce back, they propel and energize ions and electrons in the midnight region, at typical distances of 6-15 RE.
Substorm EnergyMost natural phenomena require an input of energy, which is then changed to some other form. This also holds true for substorms. It seems no accident that they generally occur when the interplanetary magnetic field (IMF) has a southward slant, which as noted in the discusion of the open magnetosphere, is a time when interplanetary field lines might be more strongly linked to those of the Earth and more energy flows from the solar wind to the magnetosphere.
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| This is also (as noted there) a time of faster "reconnection" between interplanetary and terrestrial field lines, a time of more rapid "peeling away" of magnetic field lines from the day side (together with their attached plasma), as they become attached to interplanetary field lines and are dragged with them into the tail. Any over-all plot of magnetospheric field lines shows the lines parted like combed hair: one group closes on the day side, around noon, another group is pulled back into the tail lobes, and the cusp marks the groups' separation. Increased "peeling away" near noon shifts the balance: fewer lines go sunward, more into the tail, and the cusp shifts to a field line anchored closer to the Earth's equator. Such a shift leads to two effects.
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First, it weakens the Earth's magnetic field near noon, where plasma and field lines have been peeled off, allowing the solar wind to push its way closer to Earth. As a result, when the interplanetary magnetic field (IMF) is "southward", the "nose" of the magnetosphere is seen to be (on the average) about 1 RE further earthwards than with "northward" IMF, out of a mean distance of 10-11 RE. Secondly, more of the magnetic field is drawn into the tail, and the tail lobes expand, storing additional magnetic energy in them. It is widely believed that the expanded lobes are the main storehouse of energy which powers the substorm. Sometimes, in "clean" substorms when the IMF suddenly "turns southward" after a long quiet period, one can observe this reservoir of energy charging up, as the tail field intensifies and magnetic field lines in synchronous orbit become increasingly stretched tailwards, slingshot-like. This "growth phase" typically lasts 40 minutes. |
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The exact way in which this energy is released, and the "trigger" which starts the process, are still subjects of debate and controversy. But it is widely held that the critical event is the formation of an X-shaped neutral point, or more likely, a neutral line extending some distance across the tail. That is not the distant neutral point of Dungey's theory, but an additional one formed quite close to Earth, at a distance of 15 to 30 RE (drawing on right).
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Magnetic reconnection then begins between oppositely directed field lines north and south of the middle of the plasma sheet (the "equatorial plane"), as explained in connection with the open magnetosphere. Each line on the northern side is broken in two at the neutral line, and the parts are spliced to corresponding parts of a line from the southern side, which is similarly divided in two. The broken and reconnected halves of lobe field lines form two new lines. The one on the earthward side, essentially a stretched terrestrial line, rebounds earthward, just like a released slingshot. The other one is connected tailwards, and because it no longer has any connection to Earth, it is expelled down the tail. Together with the plasma riding on it (and other plasma, originally further tailwards), it forms a sort of a plasma bubble known as a "plasmoid" (see drawing above). The passage of such plasmoids further down the tail has been deduced from observations by ISEE-3 and Geotail. Initially the newly-reconnected lines are those of the plasma sheet, but as the process sucks in magnetic field lines from both sides towards the neutral line, the tail lobes are soon reached. The effect on the magnetic field piled up in the lobes, and on the energy stored in them, is somewhat like that of a pin on a well-inflated balloon. Just as the pinhole allows air to escape and releases the energy stored in the balloon, so the neutral line allows field lines (with their plasma) to leave the lobe, reducing both the intensity and energy of the magnetic field there. Energy in nature is conserved. If it disappears in one form, it reappears in another: electric energy consumed by a motor is converted to kinetic energy of motion, and when motion is stopped by friction, its kinetic energy turns to heat. The magnetic energy taken from the tail lobes also reappears in different forms. Some is turned to heat, that is, it raises the velocity and hence the energy of plasma ions and electrons (heat being the kinetic energy of individual particles moving in disordered fashion--in both a gas and a plasma). The plasma most likely to be heated in this process is the one attached to the reconnected field lines: since those lines come from the tail lobes, whose plasma is extremely rarefied (see magnetotail), rather few particles share this energy and therefore the amount each of them receives may be quite big.
Electric CurrentsSome of the converted energy ends up driving electric currents, in a circuit linking the plasma sheet and Earth. The connecting links are magnetic field lines, which can conduct electric currents quite well, since ions and electrons attached to field lines slide rather easily along them.
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As noted (section on magnetotail, also drawing on right), a large electric current flows at all times across the plasma sheet, from the dawn edge to the evening edge (and then closes along the magnetospheric boundary). In a substorm some of this current, it seems, is diverted earthwards along magnetic field lines.
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The diversion starts in the morning-side half of the plasma sheet, where currents are withdrawn to flow earthward along field lines. They then continue (mostly) in the ionosphere, and finally return to space along other field lines, to the evening-side of the tail. The by-passed section in the middle of the plasma sheet, where the cross-tail current is weakened, seems also to be the region actively involved in the substorm, but how the substorm disrupts there the orderly flow of the cross-tail current is still a matter of controversy. The flow of electric currents along field lines may also be the key to the production of the substorm aurora, as will be discussed later.
Substorms in PerspectiveThe preceding account is explicit and tidy, suggesting a rather clear picture of what goes on in a substorm. Actually, much is guesswork: we have some pretty reasonable hunches, but nature may yet surprise us. For instance, the details of reconnection in the tail are hard to confirm, and its location and even existence are still being disputed.Even though the physics is quite different, one can compare a substorm to a thunderstorm. Meteorology experts have a clear and orderly view of this phenomenon: its energy is supplied by the moisture contained in warm, humid air, and a rising flow forms an updraft (like the rising central column in the pot shown in the section on convection), extending to great heights. One can describe the processes controlling the flow of air in that central "updraft" and the formation of rain (and even of lightning, a somewhat peripheral phenomenon). But a look at an actual thunderstorm reveals no tidy structure: flows are obscured by clouds, patterns are deformed, neighboring thunderstorms affect each other, and each storm is in fact different. An observer watching from the ground may find it hard to draw any conclusions. Launching balloons with instruments into the storm could help, but if only a few are available, their evidence may be contradictory, since those that miss the rising updraft move unpredictably. Substorms are like that, too, only more difficult--because of their greater distance and size, the small number of satellites available and perhaps the greater intricacy of plasma phenomena. Almost all we know about them comes from ground observations or from isolated satellite passes, which cannot be readily combined, since each storm behaves differently.
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