Auroras (northern lights) are brilliant displays of light that appear in the Arctic and Antarctic regions of the world. Auroras are caused by the collision of charged particles coming from the Sun and atoms in the thermosphere. Auroras most often occur near the equinoxes (spring and autumn). In northern latitudes, the phenomenon is known as aurora borealis, while in the southern regions, it is known as aurora australis.
How often aurora borealis occurs:
Auroras are most often seen when the Earth´s magnetic field is vibrating in response to a solar wind gust (geomagnetic storm). The magnetosphere (Earth´s magnetic field) is crowded with electrons and protons. When a solar wind gust reaches the magnetosphere, they become trapped by Earth´s magnetic field. The collision between ions and atmospheric atoms and molecules produce the release of energy, causing the air to glow in the form of auroras.
Auroras most commonly occur during the most intense stage of the sunspot cycle or during the following three years of this stage. The chances of auroras appearing in the poles depend most commonly on the slant (Bz) of the interplanetary magnetic field (IMF). When the Sun´s magnetic tilt points south (southward Bz) near the magnetopause (boundary where Earth´s magnetic field and the solar wind interact), it may cancel Earth´s magnetic field, opening a gap through which the solar wind energy can reach the Earth´s magnetosphere.
Disturbances in the Earth´s magnetosphere (geomagnetic storms) occur more often during the equinoxes. It has been observed that during the spring and autumn seasons, the IMF and the Earth´s magnetic field intersect at the magnetopause which cancels the Earth´s magnetic field, producing a flow of solar wind into the inner magnetosphere. The diversions of Bz have been observed occurring during the months of April and October, when the Earth´s magnetic field is more directly aligned with the Parker Spiral.
The main source of energy for aurora is the solar wind, therefore, during solar wind low activity periods, little aurora can be seen; however, when the solar wind is very active, aurora can become very intense. Since the Sun turns on its own axis every 27 days, an active region which produced perturbations on the Earth´s magnetosphere may produce perturbations once again 27 days later. Satellite observations permit estimate the occurrence of an aurora, as well as predict the occurrence of a substorm (magnetospheric substorm).
The composition and density of the atmosphere, in addition to the altitude where aurora occurs determines its light emission. Oxygen atoms emit photons in the green and red spectrum. Purple is a mixture of red and blue emissions from nitrogen molecules. Very intense aurora from high energy electrons can be seen as low as 50 miles (80 km) high. Visible aurora can be seen at altitudes of about 120-200 miles (200-2300 km); however, sometimes aurora can be observed at altitude of up to 350 miles (600 km).