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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 (outermost 3 red lines on the right side of the drawing) 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![]() 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 long term statistical average of many aurora observations. During some of them the oval is large, during others it is small, and it can also be displaced towards midnight and in other ways, all of which add up to produce a broad band. Inside the Oval -- the Polar RainField 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 (blue lines in the figure on top). 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.
Questions from Users: Magnetic connections between planets and the Sun |
Next Stop: #26H. Polar Cap--History
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: education("at" symbol)phy6.org
Co-author: Dr. Mauricio Peredo
Spanish translation by J. Méndez