The solar wind which travels in interplanetary space at a supersonic speed (400-770 km per second) as a fully ionized and magnetized gas under the magnetic field of the sun, on approaching the earth, interacts with the earth's magnetic field while at a distance of several earth-radii, producing a comet-shaped cavity called a magnetosphere around the earth, while the lines of force of both the magnetic fields interconnect. The charged particles of the solar wind moving across the interconnected magnetic lines of force generate electric power of as much as 109 kW
near the boundary of the magnetosphere. A part of the electric current, so generated, flows down the interconnected magnetic lines of force towards the polar regions of the earth with sufficient energy to bombard the atoms and molecules of the gases present there till it is stopped by the atmosphere at an altitude of about 100 km. The gases affected by the process are mostly oxygen and nitrogen. The atoms and molecules of these gases absorb the radiation in the extreme ultraviolet and X-ray wavelengths and in the process undergo chemical dissociation or excitation and ionization. The excited and ionized atoms and molecules then give out their own characteristic radiation in the form of brilliant colourful lights known as polar auroras.
Auroral displays take varieties of forms, such as arcs, rays, curtains, crowns (corona), draperies, bands or even diffuse luminous surfaces. They may appear over latitudes poleward of about 45°, but the frequency of their occurrence is maximum around the magnetic poles. People have observed auroras over the Polar Regions, one in each hemisphere, for ages. They are known as 'aurora borealis' in the northern hemisphere and 'aurora australis' in the southern hemisphere. Recently, the NASA's Dynamic Explorer 1 satellite, flying over the earth's polar-regions, found that the auroras formed around the magnetic poles as glowing ovals or rings, about 500 km wide, 4500 km in diameter, and centered on the magnetic poles. Most auroral displays occur in the layer between 100 and 250 km above the earth's surface and are dominated by the green light emitted by the oxygen atoms at wave-length 5577 A (1 Angstrom = 10~8cm) and ruby-red light emitted by the oxygen ions at wave-lengths 6300 and 6364 A. Atomic and ionized molecular nitrogen also add emissions in several bands of wavelength. The presence of these spectral lines provides unmistakable evidence of the presence of oxygen and nitrogen in both molecular and atomic form in the upper atmosphere. In the case of oxygen, atomic oxygen is produced by the strong absorption of ultraviolet radiation in the Runge-Schumann band (wave-lengths 1751-1200A) by the oxygen molecules:
It is likely that similar absorption of ultraviolet radiation by the nitrogen molecules leads to their dissociation into nitrogen atoms. However, the exact process in the case of atomic nitrogen is not yet clearly established.
The auroral activity undergoes sudden changes at times, for example during solar flares, when the solar wind becomes gusty and the orientation of the solar wind magnetic field with respect to the earth's magnetic field changes. The disturbed magnetic fields then cause what are known as geomagnetic storms. During such storms, sometimes cracks occur in the earth's magnetic shield and the dangerous solar wind leaks through them. Some recent studies with space probes such as NASA's IMAGE craft and four European Cluster satellites flying at different altitudes to obtain simultaneous measurements of one of the cracks in the shield found evidence of large volumes of the solar wind flowing through it towards the earth. The data indicated that the opening was twice the size of the earth at an altitude of about 38,000 miles, narrowing to about the size of California at the height of the earth's upper atmosphere.
Such storms have been found to interfere with the earth's radio communication.
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