An aurora is a natural light display in the sky (from the Latin word aurora, "sunrise" or the Roman goddess of dawn), predominantly seen in the high latitude (Arctic and Antarctic) regions. The name âaurorasâ is now more commonly used for the linguistic plural âauroraeâof âauroraâ, so is adopted throughout the main text of this article. Modern style guides recommend that the names of meteorological phenomena, such as aurora borealis, be uncapitalized. Auroras are caused by charged particles, mainly electrons and protons, entering the atmosphere from above causing ionisation and excitation of atmospheric constituents, and consequent optical emissions. Incident protons also produce emissions, and convert to hydrogen atoms by gaining an electron from the atmosphere.
Most auroras occur in a band known as the auroral zone, which is typically 3° to 6° wide in latitude and between 10° and 20° from the geomagnetic poles at all local times (or longitudes), but most clearly seen at night against a dark sky. A region displaying an aurora at any given time is known as the auroral oval, a band which is displaced towards the nightside of the Earth. The day-to-day positions of the auroral ovals are posted on the internet. A geomagnetic storm causes the auroral ovals (north and south) to expand, and bring the aurora to lower latitudes. Evidence for a geomagnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881) and S. Tromholt (1882) established that the aurora appeared mainly in the "auroral zone", a ring-shaped region with a radius of approximately 2500 km around Earth's magnetic pole. It was hardly ever seen near the geographic pole, which is about 2000 km away from the magnetic pole. The instantaneous distribution of auroras ("auroral oval") is slightly different, centered about 3â5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator when the magnetic pole in question is in between the observer and the Sun. The aurora can be seen best at this time, called magnetic midnight.