#26.     The Polar Caps

Plasma phenomena occuring at one place in the magnetosphere also affect other locations This linkage is most pronounced between points that share the same field line, since field lines guide the motion of ions and electrons.

In both a dipole field and the actual magnetosphere, the field lines that extend to the greatest distances are those which begin or end near the magnetic poles. It follows that the places on Earth most sensitive to distant magnetic effects are the "polar caps", the regions around the magnetic poles. A good example is the polar aurora.

If we were to trace back the field lines on which aurora appears (especially in substorms), we would probably arrive at the thick plasma sheet extending down the tail of the magnetosphere. Although that is where the process originates, the final energization of auroral electrons (as will be explained elsewhere) often happens quite close to Earth.

The Auroral Oval

Cameras aboard satellites can look down at the aurora and snap its instantaneous picture at some given moment. What they see (click for image) is a roughly circular strip, centered a little nightward of the magnetic pole, known as the auroral oval. During large magnetic storms the oval grows in size and may even reach the population centers of Europe and America, giving people there a rare opportunity to watch auroras from their own backyards.

The narrow auroral oval gives the instantaneous shape of the aurora. The "auroral zone" plotted by Loomis and by Fritz is much more smeared out, because it is the result of long term statistical averaging of aurora observations, affected by the expansion of the oval during stormy times and other variations.

Inside the Oval -- the Polar Rain

Field lines starting from points of the dark region inside the auroral oval, which includes the magnetic pole, extend to even greater distances. Early researchers, who believed auroral electrons came from the Sun (see history) could not understand why the aurora was absent from the vicinity of the magnetic pole itself. From satellite data we now know that field lines inside the oval extend to the "tail lobes," the twin bundles of field lines that extend down the Earth's magnetic tail. Ultimately they probably lead into the solar wind, somewhere far on the nightside of the Earth. But the wind there is flowing rapidly away from Earth and its ions are not likely to reverse their direction and head upstream, back towards the Earth. Hence one expects very little plasma to come from that direction.

Yet something does flow earthwards on those field lines, a thin "polar rain" of fast electrons, with energies around 500 electron volts (ev). Solar wind protons have about 1000 ev each, but the electrons which move along with them, being about 2000 times lighter, also have a much smaller average energy. Electrons of 500 ev are a completely different population, easily able to outrace the solar wind and follow field lines in any direction. They are too few to produce a visible aurora, but instruments aboard satellites readily observe them. They provide the best evidence that the tail lobes are indeed connected to the solar wind.

All magnetic field lines have a direction, which is why they are labeled by arrows--out of the southern polar cap, and into the northern one. Similarly, interplanetary field lines also have their directions: for about half the time, typically a week at a stretch, they may point away from the Sun, and the rest of the time, for comparable periods, they point towards it (in addition they are twisted by the Sun's rotation and by other factors).


Schematic view of the
way polar field lines
are linked to the IMF
and to the Sun

When they point away, they can only connect to the northern polar cap (drawing), while the bundle of lines coming out of the southern cap by necessity heads into the outer solar system. When the interplanetary field points towards the Sun, the opposite holds and it is exclusively the southern polar cap which connects to the Sun.

In 1976 it was discovered that when the interplanetary field lines pointed away from the Sun, the polar rain was much more intense in the northern cap then in the southern one, while when they pointed towards the Sun, the southern cap received the bigger share. Clearly, those electrons must have come from the Sun, and favored the pole with the direct sunward connection. It was also evident that the interplanetary field lines were somehow linked through the tail lobes to the appropriate polar caps, although how and where that connection is made is still not known for sure.

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Last updated March 13, 1999