#23.     The Tail of the Magnetosphere
In contrast to the dayside magnetosphere, compressed and confined by the solar wind, the nightside is stretched out into a long "magnetotail". This part of the magnetosphere is quite dynamic, large changes can take place there and ions and electrons are often energized.

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 Tail Lobes

Most of the volume of the tail is taken up by two large bundles of nearly parallel magnetic field lines (see drawing). The bundle north of the equator points earthwards and leads to a roughly circular region including the northern magnetic pole, while the southern bundle points away from Earth and is linked to the southern polar region.

These two bundles, known as the "tail lobes", extend far from Earth: ISEE-3 and Geotail found them well-defined even at 200-220 RE (Earth radii) from Earth. At those distances the lobes are already penetrated by some solar wind plasma, but near Earth they are almost empty. One may compare typical plasma densities:

Solar wind near Earth 6 ions/cubic centimeter
Dayside outer magnetosphere 1 ion/cubic centimeter
"Plasma sheet" separating
tail lobes 		0.3 -- 0.5 ions/cubic centimeter
Tail lobes 0.01 ion/cubic centimeter

This extremely low density suggests that field lines of the lobe ultimately connect to the solar wind, somewhere far downstream from Earth. Ions and electrons then can easily flow away along lobe field lines, until they are swept up by the solar wind; but very, very few solar wind ions can oppose the wind's general flow and head upstream, towards Earth. With such a one-way traffic, rather little plasma remains in the lobes.

The Plasma Sheet

Separating the two tail lobes is the "plasma sheet", a layer of weaker magnetic field and denser plasma, centered on the equator and typically 2-6 Earth radii thick. Unlike field lines of the tail lobes, those of the plasma sheet do cross the equator, though they are quite stretched out. A weak magnetic field means that the plasma is less restrained here than nearer to Earth, and on occasion it sloshes or flaps around.

We have already met two systems of electric currents in the magnetosphere--the ring current carried by trapped plasma, and the magnetopause current confining the magnetosphere to the inside of a cavity in the solar wind, a current that flows on the surface of that cavity. A third system is the cross-tail current flowing across the plasma sheet from dawn to dusk (drawing below).

It is easy to see that the tail must contain additional currents, for the stretching-out of the tail lobes amounts to adding a magnetic field to the magnetosphere. Any magnetic field in space requires some electric current to produce it, and the cross-tail current can be viewed as the source of the tail lobes. Like every steady electric current, it too must flow in a closed circuit, and the closing occurs in two branches that follow the magnetopause around either tail.

The Diffuse Aurora

Because of the weak field in the plasma sheet, the ions and electrons of the plasma sheet are constantly stirred up, and some of them--especially electrons--continually leak out of the ends of their magnetic field lines. As such electrons approach the Earth, most bounce back thanks to the action of converging field lines (see the section on trapped particles), but some reach the atmosphere and are lost, producing in the process a diffuse aurora. The eye usually cannot see this spread-out glow, but satellite cameras do so quite well, showing a "ring of fire" surrounding the Earth's polar caps at most time, like the one picturedbelow.

The diffuse aurora was discovered by the Canadian spacecraft ISIS 2 in 1972, and it expands and contracts as the tail lobes swell and shrink due to variations in the solar wind and its magnetic field. It was extensively observed by (among others) the US Dynamics Explorer mission (1981-7), more recently by the Swedish satellites Viking (1986) and Freja (1992), and currently by the ISTP observatory on "Polar" .

Plasma Convection

If tail plasma continually leaks out of the plasma sheet, new ions and electrons must arrive to take its place, or else the plasma sheet would soon be drained and the extended tail field would quickly collapse. How is fresh plasma supplied?

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.

Dungey suggested that such a neutral point existed near the front of the magnetopause (marked N on the drawing). He proposed that interplanetary field lines (with the plasma riding on them) linked up there with terrestrial ones, forming compound lines like the one to the right of "3" in the drawing.
That line contains a sharp bend: most of the plasma on the section beyond the bend is interplanetary, most of it on the section closer to Earth is terrestrial. However, both plasmas move together, continue to share the same line, and slowly intermix.

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.

This would create a steady circulation of plasma in the magnetosphere and would also bring fresh ions and electrons into the plasma sheet, from the vicinity of "6". The process is often named "convection", a name used for circulating flows produced by heat, for instance the flow of water in a heated pot (drawing).

At this point one should look again at the line-sharing property.

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 his separation occurs about 70-100 RE away on the night side

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Last updated March 13, 1999