(Files in red–history)
25. Auroral Currents
25H. Birkeland Currents
25b. Io Dynamo
25c. Space tether
26. Polar Caps
26H. Birkeland, 1895
27. Aurora from Space
28. Aurora Origin
28a. Plus and Minus
29. Low Polar Orbit
30. Magnetic Storms
30a. Chicago Aurora
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 OvalCameras aboard satellites can look down at the aurora and snap its instantaneous picture at some given moment. What they see 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 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.
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).
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.
Questions from Users: Magnetic connections between planets and the Sun
Next Stop: #26H. Polar Cap--History