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Review of Aurora borealis spectacular manifestations of solar wind and atmosphere.
Authors: Dr (Ms) Swaroopa Rani N. Gupta
Number of views: 221
Auroras frequently appear either as a diffuse glow or as curtains that extend approximately in the east–west direction. At some times, they form quiet arcs; at others, they evolve and change constantly. These are called active aurora. Red: At the highest altitudes, excited atomic oxygen emits at 630 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this color visible only under more intense solar activity. Green: At lower altitudes, the more frequent collisions suppress the 630-nm (red) mode: rather the 557.7 nm emission (green) dominates. Blue: At yet lower altitudes, atomic oxygen is uncommon, and molecular nitrogen and ionized molecular nitrogen take over in producing visible light emission, radiating at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant. Ultraviolet: Ultraviolet radiation from auroras (within the optical window but not visible to virtually all humans) has been observed with the requisite equipment. Infrared: Infrared radiation, in wavelengths that are within the optical window, is also part of many auroras. Yellow and pink: Yellow and pink are a mix of red and green or blue.
In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation. X-ray emissions, originating from the particles associated with auroras, have also been detected. Aurora noise, similar to a hissing, or crackling noise, begins about 70 m (230 ft) above the Earth's surface and is caused by charged particles in an inversion layer of the
atmosphere formed during a cold night.
The basic cause of auroras involves the interaction of the solar wind with the Earth's magnetosphere. The varying intensity of the solar wind produces effects of different magnitudes. The prime source of auroral particles is the solar wind feeding the magnetosphere, the reservoir containing the radiation zones, and temporarily magnetically trapped, particles confined by the geomagnetic field, coupled with particle acceleration processes. Auroral arcs and other bright forms are due to electrons that have been accelerated during the final few 10,000 km or so of their plunge into the atmosphere. Protons are also associated with auroras, both discrete and diffuse. Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen atoms and nitrogen based molecules returning from an excited state to ground state. They are ionized or excited by the collision of particles precipitated into the atmosphere. Both incoming electrons and protons may be involved. Excitation energy is lost within the atmosphere by the emission of a photon, or by collision with another atom or molecule: Oxygen emissions green or orange-red, depending on the amount of energy absorbed. Nitrogen emissions blue or red; blue if the atom regains an electron after it has been ionized, red if returning to ground state from an excited state.
Bright auroras are generally associated with Birkeland currents, which flow down into the ionosphere on one side of the pole and out on the other. The solar wind and the magnetosphere, being two electrically conducting fluids in relative motion, should be able in principle to generate electric currents by dynamo action and impart energy from the flow of the solar wind.
Earth's magnetosphere is shaped by the impact of the solar wind on the Earth's magnetic field. This forms an obstacle to the flow, diverting it, at an average distance of about 70,000 km (11 Earth radii or Re), producing a bow shock 12,000 km to 15,000 km (1.9 to 2.4 Re) further upstream. The width of the magnetosphere abreast of Earth, is typically 190,000 km (30 Re), and on
the night side a long "magnetotail" of stretched field lines extends to great distances (> 200 Re). The high latitude magnetosphere is filled with plasma as the solar wind passes the Earth. The flow of plasma into the magnetosphere increases with additional turbulence, density and speed in the solar wind. This flow is favoured by a southward component of the IMF, which can then directly connect to the high latitude geomagnetic field lines. Geomagnetic storms may vary with Earth's seasons. Two factors to consider are the tilt of both the solar and Earth's axis to the ecliptic plane.
The electrons responsible for the brightest forms of aurora are well accounted for by their acceleration in the dynamic electric fields of plasma turbulence encountered during precipitation from the magnetosphere into the auroral atmosphere. In contrast, static electric fields are unable to transfer energy to the electrons due to their conservative nature. One early theory proposed for the acceleration of auroral electrons is based on an assumed static, or quasi-static, electric field creating a uni-directional potential drop. Another theory is based on acceleration by Landau resonance in the turbulent electric fields of the acceleration region. Auroral electrons created by large geomagnetic storms often seem to have energies below 1 keV, and are stopped higher up, near 200 km. Such low energies excite mainly the red line of oxygen, so that often such auroras are red.
This paper deals with review of aurora borealis spectacular manifestations of solar wind and atmosphere which includes visual forms and colors of auroral radiation, aurora noise, causes of auroras, auroral particles, auroras and the atmosphere, auroras and the ionosphere, interaction of the solar wind with Earth, magnetosphere, auroral particle acceleration, various images of terrestrial auroras - aurora borealis.
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