Aurora australis and aurora borealis, often called the Northern Lights or Southern Lights depending on hemisphere where it occurs is an optical phenomenon in Earth's atmosphere, mainly characterized by bright bands of red-green-blue called auroral arcs. Auroras can still occur with a wide range of shapes and colors rapidly changing in time and space.
The phenomenon is caused by the interaction of charged particles protons and electrons solar wind with the ionosphere Earth atmosphere between 100 - 500 km. These particles excite the atoms of the atmosphere that subsequently emit light of various wavelengths . Because of the geometry of the Earth's magnetic field, auroras are visible in two narrow bands around the magnetic poles of the Earth, called the auroral oval. Auroras visible to the naked eye are produced by electrons, while those of protons can be seen only with the aid of special tools, either from the ground by both space.
Often the polar aurora is visible even in areas not near the poles, like Scotland, or many parts of the Scandinavian peninsula, Finland, Iceland, Sweden, Norway, Finnmark fjords, Greenland, Canada and Alaska. Auroras are more frequent and intense during periods of intense solar activity, periods when the interplanetary magnetic field can vary greatly in intensity and direction, increasing the possibility of a coupling magnetic reconnection with the Earth's magnetic field.
The origin of the aurora is 149 million of km from the Earth, ie the Sun. The appearance of a large group of sunspots is the first sign of a coronal mass expulsion activities intense. The particle energy emitted by the Sun travel in space forming the solar wind. This moves through interplanetary space and then to Earth, which can reach in 50 hours with speeds typically between 400 and 800 km/s, taking with it part of the magnetic field solar interplanetary magnetic field.
The solar wind interacts with the Earth's magnetic field also called magnetosphere, it distorts creating a sort of bubble magnetic shape similar to a comet. The Earth's magnetosphere works like a shield, shielding the Earth from the direct impact of the charged particles (plasma) that make up the solar wind. In first approximation, these particles slide along the outer edge of the magnetosphere (magnetopause) and pass beyond the Earth.
In reality, due to a process known as magnetic reconnection (the interplanetary magnetic field pointing in a direction opposite to that of the Earth), the solar wind plasma can penetrate into the magnetosphere and, after complex processes of acceleration, interact with the Earth's ionosphere, depositing immense amounts of protons and electrons in the upper atmosphere, and giving rise, in this way, the phenomenon of the aurora.
It is noteworthy that the Arctic areas, possessing a magnetic protection less, are the most exposed to this phenomenon and often, for a few days after the event, the ozone is reduced by about five percent.
Auroras are more intense when magnetic storms are being caused by a strong activity of sunspots . The distribution of the intensity of the aurora in altitude shows that are formed mainly at an altitude of 100 km above the Earth's surface. Are usually visible in the regions near the poles, but can occasionally be seen far to the south, up to 40º of latitude .
The particles that move toward the Earth hit the atmosphere around the poles forming a kind of ring, called the auroral oval. This ring is centered on the magnetic pole moved approximately 11º respect the geographic pole and has a diameter of 3000 km during quiet times, and grow when the magnetosphere is disturbed. Auroral ovals are generally between 60 ° and 70 ° north and south.
The aurora is formed by the interaction of high-energy particles usually electrons with atoms neutral upper atmosphere of the Earth. These particles can excite by collisions valence electrons of the atom neutral. After a characteristic time interval, these electrons return to their initial state, emitting photons particles of light. This process is similar to the download to the plasma of a neon lamp .
The particular color of the aurora depends on which gases are present in the atmosphere, from their electrical state and energy of the particles that strike them. Atomic Oxygen is responsible for the green color (wavelength 557.7 nm) and molecular oxygen for the red (630 nm). Nitrogen causes the blue color.
The shape of the polar aurora is very diverse. Arches and bright rays of light begin to 100 k m above the earth's surface and extend upward along the magnetic field, for hundreds of miles. The arches can be very thin, just 100 meters, while extending from horizon to horizon. They can be almost motionless and then, as if a hand had passed on a long tent, start moving and turning. After midnight, the aurora can take a form to each of the spots and stains often flashes about every 10 seconds until dawn.
Most of the visible light in an aurora is a greenish yellow, but sometimes the rays can become red at the top and along the bottom edge. On very rare occasions, sunlight can hit the top of the rays creating a faint blue color. Even more rarely (once every 10 years or more) the aurora can be blood red from top to bottom. In addition to producing light, the energetic particles that form the aurora bring warmth. This radiation is dissipated as infrared or transported away by the strong winds of the upper atmosphere.
The Sun is a star with some very variables that change with periods ranging from a few hours to hundreds of years. The direction of the interplanetary magnetic field, and the speed and density of the solar wind, all depend on the activity of the sun. They can change drastically in a short time and influence the geomagnetic activity. When this increase, the southern edge of the atmosphere boreal moves southward. Although the amounts of emissions of the solar corona cause auroral oval larger. If the interplanetary magnetic field is directed in the opposite direction to the terrestrial energy transfer is greater, and therefore the aurora are more pronounced.
The disturbances of the Earth's magnetosphere are called geomagnetic storms. They can produce sudden changes in the shape and motion of the aurora, called auroral substorms. The magnetic fluctuations of these storms can disrupt the grid electricity, sometimes doing some hearing damage and causing blackouts extended. They can also affect the operation of radio communications via satellite . Magnetic storms can last several hours or even days, and auroral substorms can occur many times a day. Each substorm generates hundreds of terajoules of energy, so much as they consume the entire United States in ten hours.
Sometimes, during the appearance of dawn, you can hear sounds that resemble hissing. It is sounds electrophonic, a phenomenon that may occur, although much more rarely, even during the appearance of racing cars. The origin of these sounds is still not clear and is believed that they are caused by disturbances in the Earth's magnetic field local, caused by increased ionization of the atmosphere above it.
Often listening to such sounds is facilitated by the presence of metal objects in the immediate vicinity of the witness. On 28 August 1859 were sighted some auroras over a large area of the territory American. Science centers around the world, the instrumentation was severely and unexplained variations and currents spurious formed in the telegraph lines.
The following day, the astronomer English Richard Christopher Carrington noticed a group of sunspots of unusually large size, from which started a flash of white light, which after a few hours produced a second wave of auroras of great intensity. With the Big Aurora of 1859, the models of explanation of the phenomena of solar activity evolved rapidly and the old assumptions of lightning at high altitude, or reflected light from icebergs were replaced by those most relevant to solar events and disturbance.
It is estimated that the storms of such intensity machine comes every 500 years. The last event of an intensity equal to half that of 1859 happened in 1960 causing radio interruptions all over the planet. Experts believe that the costs of a possible superstorm could be comparable to those of a large earthquake, should they lack the appropriate countermeasures, such as procrastinating some critical assets carried by satellites, move airways, early detection of vulnerabilities of networks.
The solar magnetic activity, and thus the formation of sunspots varies cyclically every eleven years. In January of 2008 began the new cycle then we can expect for the coming years, an increase of activity. Over the past eleven years, scholars have found about 21000 flares and 13000 plasma clouds escaped from the solar surface.