Distances in the magnetosphere are often measured in Earth radii (R_E), with one Earth radius amounting to 6371 km or 3960 miles. In these units, the distance from the Earth's center to the "nose" of the magnetosphere is about 10.5 R_E and to the flanks abreast of the Earth about 15 R_E, while the radius of the distant tail is 25-30 R_E. By way of comparison, the moon's average distance is about 60 R_E.

About 2R_E ahead of the magnetopause is a standing shock front, like the one formed ahead of a supersonic bullet or airplane. As the near-earth solar wind passes through that front, it abruptly slows down and some of its kinetic energy is converted to heat. Later the wind speeds up again, and by the time it reaches 100-200 R_E downstream, not only has it regained its speed, but it has also infiltrated the magnetospheric tail--how and where is still the subject of active investigation.

Why is the Earth's field an obstacle to the solar wind?

As noted earlier (discussion of the solar wind), field lines of the interplanetary magnetic field (IMF) are carried along by the solar wind as if they were strings and the moving ions were beads strung on them. A "bead" strung on a solar field line will thus always be on a solar field line, and unless field lines from different sources somehow manage to interconnect, it will never be on a field line connected to Earth. The two plasmas, the Earth's and the solar wind's, then form two separate families, and the magnetopause is simply the boundary separating them.

Neutral Points and the Cusps

On a plot of magnetic field lines, such points stand out as the ones where field lines appear to intersect each other. Intersecting field lines seem to make no sense: how can the magnetic field point in two directions at once? If our plots nevertheless show such points, the intensity of the magnetic force at them must be zero, so that the direction of that force is irrelevant.

When spacecraft were actually sent to the cusp region--the European HEOS 1 and HEOS 2 (1968, 1972) and the University of Iowa's "Hawkeye" (1974)--they seemed to observe a disordered magnetic field, weak but not zero. The absence of a strong field to hold off the solar wind means that these are "weak spots" on the magnetopause, and solar wind plasma penetrates there to fill two funnel-shaped regions. Some of the plasma particles travel all the way to the top of the atmosphere, where their collisions produce a red auroral glow.

The Open Magnetosphere

Since the nose of the magnetosphere (on the average) maintains its position and is not "worn away" by the reconnection process, the magnetic field lines swept into the tail, and the plasma particles strung along them, must somehow flow back towards Earth, inside the magnetosphere. This (according to Dungey) explained the large-scale sunward flow of the tail, deduced from auroral motions and flows in the polar ionsphere, on field lines which extended into the tail.

Dungey's process of "magnetic reconnection" is expected to increase the flow of energy from the solar wind to the magnetosphere, by establishing a direct field-line link between the two. It does, however, require a southward slant of interplanetary magnetic field lines, a condition which only exists about half the time.

In 1966, soon after regular observations of the IMF began, a student of Dungey, Don Fairfield, did in fact note a strong correlation between such southward slants and the storminess of the magnetic field ("magnetic activity"). Nowadays "southward IMF" is recognized as the most important factor promoting storms and substorms in the magnetosphere, far more important than increased velocity or pressure of the solar wind, sunspot numbers, etc. This connection supports Dungey's idea of an "open magnetosphere," though direct evidence by observing interconnecting field lines is rather hard to obtain.

Reconnection with a Northward IMF?

Early ideas that the Sun sent out streams of electrons (which also produced the aurora) were given up when it was realized that the unbalanced negative electric charge of such electrons would completely disrupt the process.

The Chapman-Ferraro Cavity

The theory of the "Chapman-Ferraro Cavity" proved to be prophetic, except for one important detail: the flow of plasma from the Sun was not confined to isolated clouds, but went on all the time, in the form of the solar wind. Denser and faster clouds, such as arise from coronal mass ejections (see corona), were later identified as the real cause of sudden commencements.

The magnetopause itself was first observed in 1961 by NASA's Explorer 12. Many satellites have since then studied and probed it--notably, Europe's HEOS 1 and 2 (1975-6) above the poles, as well as NASA's ISEE-3 (1983) and Japan's Geotail (1992-4), extended missions which, during the marked years, probed the distant tail region, up to distances

Inner Magnetosphere

Other Regions and Particles

Finally, a large cloud of neutral hydrogen surrounds the Earth, the "geocorona" (click here for its picture). Since particles in space collide so rarely, these different populations can co-exist with relatively little interference.

All these regions have been visited by satellites, and a fair amount is known about their average properties. However, their detailed structure and the way they vary with time are only poorly known, because their features (like weather) keep changing, and only a few isolated satellites are usually available to track such changes. Imagine studying the weather with only a few isolated observatories! A great deal of ingenuity has been applied in the past to exploring the Earth's magnetosphere, but the greatest need is now for many more simultaneous and coordinated observations in the various regions of earthspace.

For a fold-out paper model of the magnetosphere, link here.

Click here to link one of two images, in black and white or in color. The black and white image can be copied in an ordinary xerox machine and can be colored by you in whichever way you prefer. However, if you have a color printer, especially one that can handle construction paper, you might want the second version. Either version is printed out lengthwise, like an ordinary sheet of text.

Instructions

1. (If the black-and-white image is used; optional.) If you wish, you can color different regions, such as: Plasma Sheet (including area marked "Plasma Convection?"), Plasma Mantle and Low Latitude Boundary Layer (use same color), Tail Lobes and Inner Radiation Belt. The remaining area inside the magnetosphere (between plasma sheet and inner magnetosphere) can also be colored, and the region outside the magnetosphere (in the solar wind) may be left white or given a light color. You may link to the color image above and use it as a guide.

   Note that lines on the bottom of the model are boundary lines between regions, while lines on the long vertical section are magnetic field lines. However, those field lines that connect to boundary lines on the bottom piece also form part of the region boundaries.

2. Using a straightedge, score the paper by drawing hard lines along the two main crossed lines with a ballpoint pen (preferably black) or with a stylus.

3. Cut out the area marked "Cut Out" but leave tab intact.

4. Crease along the scores, then fold to produce a three-sided corner with the printed picture on the inside.

5. Use tape to attach the tab to the back side of the panel carrying the words "Tail Lobes" and "Plasma Sheet," to hold the paper in its folded position. If no tape is available, carefully cut a slot in the marked place to the right of the words "Plasma Sheet" and insert the end of the tab.

The best we can do is station-keeping: for instance, the motion of a satellite in a synchronous orbit around the equator is matched by the rotation of the Earth below it, allowing it to permanently stay above the same equatorial spot.

Station-keeping in orbits around the Sun

However, orbital laws require planets closer to the Sun to move faster, by a formula found in 1619 by Johannes Kepler. While the Earth goes around the Sun in 365 days, Venus which is closer only needs 225 days and Mercury, closer still, only 88. Thus any spacecraft going around the Sun in an orbit smaller than the Earth's will soon overtake it and move away, and will not keep a fixed station relative to Earth.

Spacecraft Observatories at L1

Such a spacecraft must have its own rocket engine. First, the position is unstable: if the spacecraft slips off it, it will slowly drift away, and sooner or later some correcting action is needed. In fact, the preferred position is actually some distance to the side of L1, for if the spacecraft is right on the Sun-Earth line, the antennas which track it from Earth are also aimed at the Sun, a source of interfering radio waves. Thus corrections are in fact needed quite regularly. Furthermore, the most economic way of getting to L1 is letting the spacecraft pass close to the Moon and using the moon's gravity to extract an extra boost from the Moon's orbital motion. Those maneuvers too require on-board propulsion, as does the final approach to L1.

Other Lagrangian Points

For those interested in space colonies at the Lagrangian points:

• Gerald K. O'Neill, "The Colonization of Space", Physics Today September 1974, p. 32.

• Gerard K. O'Neill, "The High Frontier", William Morrow and Co., NY, 1977; Anchor Books (Doubleday) 1982.

NASA's "Wind" spacecraft was launched 1 November 1994 by a Delta rocket from Cape Canaveral. It was designed to observe the solar wind approaching Earth, from a position near the Lagrangian point L1, and it is part of the International Solar-Terrestrial Physics (ISTP) Initiative.

That plan was later changed (see further below) when the solar observatory SOHO was placed in a similar orbit. "Wind" was then moved to a complicated orbit which allows it to sample different parts of space around Earth, while continuing to observe the solar wind. It still carries a large reserve of fuel for its rocket engine.

Encounters with the Moon

Note The "gravity-assist" maneuver, in which a spacecraft may gain or lose velocity through an encounter with a moving planets, is discussed in some detail (including calculations) in section #35 "Starships" of the sister-site From Stargazers to Starships. See also article "A Bus Between the Planets" by James Oberg and Buzz Aldrin, "p. 58-60, "Scientific American", March 2000

Instruments and Observations

Radio wave receivers monitor emissions from the Sun and from space plasmas, and a magnetometer samples the interplanetary magnetic field (IMF) up to 44 times a second. Because the IMF is very weak (about 1/10,000 the Earth's surface field), the magnetic fields produced by electric currents on the spacecraft are strong enough to disturb its observation, and the magnetometer is therefore placed (here and on most deep-space spacecraft) at the end of a long boom, away from the interference.

"Wind" also carries two gamma ray detectors, to observe and time gamma ray bursts from distant space, probably beyond our galaxy (more on them in the section on high energy particles in the universe

Dateline December 1998

At the end of 1997, WIND rounded the L1 Lagrangian point and headed back to Earth. With the ACE spacecraft now positioned near L1, capable of routine monitoring of the solar wind, WIND with its unique capabilities can be positioned elsewhere, providing broader coverage of the solar wind as we approach the next sunspot maximum, around 2000-2001. The new set of orbits is achieved by flying near the moon and using its gravity to alter the spacecraft trajectory.

As of the end of 1998, the new mission is well under way, with "WIND" in its new "petal orbits" which explore the magnetosheath at points abreast of the magnetosphere but relatively distant from the ecliptic. For additional details and updates, see the home page of the "Wind" mission.

The magnetotail is also the main source of the polar aurora. Even before the space age observers noted that in the arctic winter, when the sky was dark much of the time, the brightest auroras were seen in the hours around midnight. It was widely believed then that auroral electrons came from the Sun, and the fact that aurora seemed concentrated on the side facing away from the Sun puzzled everyone. Those observations made much more sense after satellites discovered and mapped the magnetosphere's long tail.

The Plasma Sheet

The Diffuse Aurora

Plasma Convection

James Dungey's theory of reconnection suggested an answer of sorts. Recall (section on the magnetopause) that in an ideal plasma, ions and electrons that share a field line move together and continue sharing it at all times ("like beads on a wire"). Dungey pointed out an exception to this rule, that when the plasma flowed through a "neutral point" or "neutral line" at which the magnetic force was zero, the plasmas on both sides of that point could become separated and could "reconnect" to different field lines.

A while later, that line would have moved to position of the line right of "4", then to the position "5", and after that, perhaps half an hour later, the reconnection process would be reversed somewhere downstream of Earth, at a neutral point or line near the number "6". The interplanetary parts are then rejoined and flow away, and the terrestrial halves are reunited too.

Neglecting spill-over at boundary points like the sharp bend in line "3" (and glossing over some important, and as yet not completely understood, plasma physics), one realizes that the above process will transport near-noon plasma, originally earthward of the bend on line "3", to the distant tail. Dungey proposed that the plasma then flowed back earthward, through the plasma sheet.

If all particles on a field line move together, as tail plasma convects back earthward, the particles on the same field lines but just above the atmosphere must keep up with it. Flows of plasma in that region, consistent with Dungey's prediction, have indeed been observed by probing antennas and by "driftmeter" instruments aboard near-Earth satellites, in orbits that cross the polar regions at low altitudes. The electric field associated with them has also been measured, and for that reason most scientists now support the notion of circulating plasma.

However, in the tail itself the earthward flow has been harder to confirm and seems to be rather irregular, coming in fits and bursts, especially during magnetic substorms. The distant neutral point near "6" is hard to pinpoint using only isolated satellites, and other plasma processes may also play a help break the temporary links between terrestrial field lines (with their plasma) and interplanetary space. "Geotail" observations suggest that this separation occurs about 70-100 R_E away on the night side

Substorms and Magnetic Storms

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 are also involved--in particular, magnetic storms require external stimuli such as the arrival of a shock front or a fast stream in the solar wind.

Substorms at Earth and in Space

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, namely, the ring current which circles the Earth at distances of tens of thousands of kilometers. The electric currents associated with the substorm, on the other hand, come down to the ionosphere, only about 130 km above the ground.

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 R_E.

Substorm Energy

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.

The Release of Energy

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 Currents

Substorms in Perspective

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.

This limited extent suggested that the currents which disturbed the field flowed somewhere nearby, probably near the auroral arcs. The Norwegian Kristian Birkeland, who carefully observed auroral disturbances around the turn of the century, concluded that those currents flowed parallel to the ground, along the auroral formation.

Dynamos

In all those cases, some electrically conducting fluid appears to be moving through a magnetic field--plasma in space and on the Sun, molten iron (probably) in the Earth's core. It can then be shown from the principles of physics that if a closed electric circuit exists, parts of which are moving through a magnetic field while other parts are not, an electric current will arise (additional conditions must also be satisfied). The electric energy needed to drive the current is taken from the motion, which is slowed down.

Long ago, a machine that generated electricity was named "dynamo" (today's preferred term is "generator"). In analogy, a set-up where a conducting fluid flows through a magnetic field and creates an electric current in the above fashion, is called a "fluid dynamo," or when a fluid process is clearly implied, simply "dynamo".

Faraday understood this very well. Knowing that water conducted electricity (because of dissolved salts), he tried in 1831 to observe such a dynamo experimentally. He strung a wire along Waterloo bridge in London and dipped its ends into the river at two separated points, so that its circuit could be completed through the flowing water. He then tried to measure the electric current generated by the river's flow through the magnetic field.

An interesting dynamo of this type exists between the planet Jupiter and its innermost big moon Io (click for details). The space shuttle can also generate electric power that way (at the expense of its orbital energy), using a long wire dangling from it into space. Such a space tether experiment was conducted aboard the space shuttle Columbia on 25 February, 1996, ending unexpectedly when the tether suddenly broke.

Sir Joseph Larmor in England first proposed in 1919 that dynamos consisting entirely of fluids might explain the creation of sunspot magnetic fields. His idea was that as solar plasma flowed through magnetic fields, dynamo effects produced the very same currents which also created those magnetic fields. Because this is a process which "lifts itself by its own bootstraps" (the magnetic fields needed by the dynamo are produced by the electric currents which are its output), it must build up gradually, starting from some "seed" magnetic field of a different source.

Such a "dynamo mechanism" was also believed to be the source of the magnetic field arising inside the Earth's molten core. It is impossible to observe that region directly, but in 1964 Stanislaw Braginsky, in Russia, mathematically demonstrated a possible class of such "dynamos." The slow decade-by-decade variation of the Earth's field probably comes from slow changes in the flow patterns of the core - not, as Halley once proposed, from magnetized spherical layers, one inside the other, all rotating differently.

Birkeland Currents

If the bundle of open lines maps to the region inside the auroral oval, it can be shown that the "dynamo currents" from the above process flow earthward on the morning side of the magnetic pole and spaceward on the evening side. One might guess that the circuit would be completed by connecting the two flows across the polar ionosphere, from the morning side, to the evening side (drawing below; the situation is more complicated, however, because the flow also deforms the field lines).

When in 1973 the navy satellite Triad (see history) flew through this region in a low-altitude orbit, its magnetometer indeed detected the signatures of two large sheets of electric current, one coming down on the morning side of the auroral zone, one going up on the evening side, as expected. Because Kristian Birkeland had proposed long before currents which linked Earth and space in this fashion, they were named Birkeland currents (by Schield, Dessler and Freeman, in a 1969 article predicting some of the features observed by Triad). Typically, each sheet carries a million amperes or more.

Their result is plotted below: the map is centered at the northern magnetic pole (though southern data were combined to produce this graph), midnight is at the bottom, and noon (where some minor additional currents exist) is at the top. Dark shading indicates currents flowing towards Earth, light shading, currents flowing outward into space.

In 1969 Schield, Dessler and Freeman, supporters of Birkeland's ideas, proposed a fairly detailed theory of how such currents might originate in space, and also predicted the secondary "region 2" currents. They even proposed a name for them--"Birkeland currents."

That same year Naoshi Fukushima in Japan pointed out that such "Birkeland currents" connected to the ionosphere would be almost "invisible" from the ground, producing there only a very weak magnetic disturbance, because magnetic fields contributed by various parts of their circuit tended to cancel out.

As Fukushima had foreseen, that circuit had only a small effect on magnetometers on the ground.Instead, the disturbances measured at the surface of the Earth come mainly from the electrojets, which are a by-product of the main circuit due to the unusual electric conductivity properties of the ionosphere. It is possible that they indeed close largely in the ionosphere.

Although signs of currents flowing from space along magnetic field lines were occasionally detected by earlier satellites, it was the US Navy's Triad, carrying the instrument of Alfred Zmuda and James Armstrong, that in 1973 traced their full pattern. Triad was an experimental navigation satellite, and it circled the Earth in a low altitude orbit which went from pole to pole, while the Earth rotated beneath it. Zmuda and Armstrong were allowed to place on it a magnetometer as an extra "piggyback experiment," and it continued to return scientific data for many years after the Navy's experiment had ended.

When Triad crossed a current sheet, a characteristic "signature" was generally noted: the direction of the magnetic field rotated fairly abruptly, while its strength stayed practically the same. From such rotations a wealth of information was extracted about the structure and variation of the currents. Tragically, both experimenters had died by the time their article on "Birkeland currents" appeared in 1974.

If the orbit were entirely determined by the Earth's gravity, it would be easy to calculate and predict. At low altitudes, however, the air resistance from the fringes of the atmosphere is not entirely negligible, and neither it nor the effect of sunlight pressure can be accurately predicted. Triad had floating inside it a "proof mass," a gold-platinum sphere about an inch across, which, being completely enclosed, was affected neither by air resistance nor by sunlight. Even the feeble gravitational attraction of the rest of the satellite was taken into account, by moving the power supply, radio etc. (including the magnetometer) to the ends of two long booms, on opposite sides of the "proof mass".

It worked until the spacecraft ran out of fuel. However, it was tracked for many years afterwards because of the value of its magnetometer observations.

Of the large moons--comparable to our own moon or bigger--the outer three are icy spheres, but the innermost one, Io, is heated by tides, and as a result has volcanoes and an ionosphere which is a fair conductor of electricity. Jupiter itself like Earth is a magnet, but one that is 20,000 times stronger; as a result it has a large magnetosphere and a very intense radiation belt.

A dynamo is created in a magnetic field by an electric circuit, part of which is moving relative to the rest (additional conditions must also be met). The circuit may consist entirely of fluids (as in sunspots), but solid conductors can also be involved.

The path of the space probe Voyager 1 was designed to check out this dynamo, by flying close to where its currents were expected to flow. It did so on March 5, 1979, and its magnetometer very clearly detected the signature of a current of about a million amperes. Previous to that it was noted that unlike any other moon of Jupiter, Io had a strong influence on radio emissions from Jupiter's magnetosphere, which depended on its position: it could be that the moon's unique electric currents were involved in this.

Postscripts

See here for a picture of Io "in its true colors" and links to sites about it.

An earlier tether experiment ended prematurely when problems arose with the deploying mechanism, but the one on February 25, 1996, began as planned, unrolling mile after mile of tether while the observed dynamo current grew at the predicted rate. The deployment was almost complete when the unexpected happened: the tether suddenly broke and its end whipped away into space in great wavy wiggles. The satellite payload at the far end of the tether remained linked by radio and was tracked for a while, but the tether experiment itself was over.

It took a considerable amount of detective work to figure out what had happened. Back on Earth the frayed end of the tether aboard the space shuttle was examined, and pieces of the cable were tested in a vacuum chamber. The nature of the break suggested it was not caused by excessive tension, but rather that an electric current had melted the tether.

The electric conductor of the tether was a copper braid wound around a nylon string. It was encased in teflon-like insulation, with an outer cover of kevlar, a tough plastic also used in bullet-proof vests, all this inside a nylon sheath. The culprit turned out to be the innermost core, made of a porous material which, during its manufacture, trapped many bubbles of air, at atmospheric pressure.

Later vacuum-chamber experiments suggested that the unwinding of the reel uncovered pinholes in the insulation. That in itself would not have caused a major problem, because the ionosphere around the tether, under normal circumstance, was too rarefied to divert much of the current. However, the air trapped in the insulation changed that. As it bubbled out of the pinholes, the high voltage ("electric pressure") of the nearby tether, about 3500 volts, converted it into a plasma (in a way similar to the ignition of a fluorescent tube), a relatively dense one and therefore a much better conductor of electricity.

The instruments aboard the tether satelite showed that this plasma diverted through the pinhole about 1 ampere, a current comparable to that of a 100-watt bulb (but at 3500 volts!), to the metal of the shuttle and from there to the ionospheric return circuit. That current was enough to melt the cable.

As the broken end whipped away from the shuttle, the plasma established electric contact with the ionosphere directly. The satellite on the distant end monitored the current: after about half a minute it stopped, then it reignited and flowed again for about another half minute, stopping for good when (presumably) all the trapped air was gone.

Because of the unexpected break, the tether experiment at the time was widely viewed by the press as an expensive failure. True, the planned operation at full deployment, for several hours, could not take place, nor could the tether and its satellite be retrieved, which was to have demonstrated the feasibility of deployable tethers.

However, many of the scientific experiments had already begun during deployment and yielded good data. And the break itself, though unfortunate, added an unscheduled experiment to the mission, one which highlighted the risks and complexities of operating scientific equipment in space.