The Earth's Magnetosphere

Only three planets in the Solar System have magnetospheres and this is because only Earth, Jupiter and Saturn generate their own magnetic fields. Since the existence of auroras on a planet requires a magnetic field, only Earth, Jupiter and Saturn can have this beautiful phenomenon.

The people of Earth did not know they had a magnetic field until they discovered lodestones and began to use them for navigation. The magnetic field is generated within the earth by electrical currents flowing deep inside it. (Every electric current generates its own magnetic field; a current in a straight wire generates a magnetic field pointed at right angles to the wire; the magnetic field becomes weaker the greater is the distance from the wire).

As far as is known, people do not directly sense the earth's magnetic field, although there is now strong evidence that the birds do it, bees do, and even sewage bacteria move in directions dictated by the local magnetic field. People have been able to map out the strength and direction of the magnetic field everywhere on the earth's surface. This mapping can be done simply by hanging a needle-shaped magnet by its center on a fine string. A human hair, a glass fiber or a gold thread make an excellent suspension strings.

Using the suspended magnet technique, both the strength and direction of the magnetic field can be measured at any point. At the equator the needle hangs horizontally and points north-south. Over Alaska and western Canada, the needle hangs nearly vertically; there it is found that the strength of the field is nearly twice that at the equator.

Using more sophisticated magnetometers that can be flown in aircraft, rockets and satellites, the earth's magnetic field also can be measured at all altitudes above the surface. The higher one goes the weaker becomes the magnetic field, but it maintains an orderly pattern of direction at all altitudes.

However, at very high altitude a boundary is reached where it is evident that the magnetic field being measured is no longer that of the earth, but is instead that of the interplanetary medium which really is a weak extension of the sun's magnetic field. In this distant region the magnetic field is very weak and of an irregular nature that depends upon the sun's level of sunspot activity. Here there also is a continuous flow of charged particles, mostly electrons and protons, moving at high speed in a direction away from the sun, the so-called solar wind.

The boundary between the earth's magnetic field and the solar wind is called the magnetopause and the region inside the magnetopause-everywhere down to the earth's surface--is named the magnetosphere. Actually, the magnetosphere is bounded below by the ionosphere, lying roughly 100 km above the earth's surface. This is the same altitude where most auroras occur.

The solar wind blowing against the magnetosphere compresses it on the side of the earth toward the sun. There the boundary is about 10 earth radii from the center of the earth. Away from the direction of the sun, the magnetosphere stretches far out behind the earth, even farther than the distance to the moon.

Most of the electrons and protons in the solar wind sweep past the magnetosphere and journey on out to the fringes of the Solar System but some find their way across the magnetopause and on down to the polar atmospheres where they hit the atmosphere. Their impact is what produces the auroras.