Auroras are electromagnetic emissions excited by the interaction of precipitating charged particles with the gases of a planet's upper atmosphere. At Earth, auroral emissions occur principally in two oval-shaped bands lying between ~65 and 75 degrees magnetic latitude and centered on the northern (aurora borealis) and southern (aurora australis) magnetic poles.

Each auroral oval consists of a more or less continuous band of faint diffuse emissions, within which brighter discrete arcs are embedded on both the day side and the night side. The diffuse emissions result from a relatively steady "drizzle" of electrons and protons that are precipitated out of the central plasma sheet by interactions with plasma waves. The particles responsible for these emissions cover a fairly broad range of energies, from a few hundred eV to a few tens of keV. On the other hand, the bright arcs and other discrete features are produced by sheets or beams of energetic electrons that have been narrowly focused in energy (between 1 and 10 keV) and accelerated by field-aligned electric potential drops that form at altitudes of 1 to 3 Earth radii above the Earth.

In addition to the emissions from the auroral zones proper, auroral arcs with a noon-to-midnight orientation are occasionally observed inside the polar cap itself under conditions of northward IMF. Because of their orientation, these emissions are known as "sun-aligned arcs." Auroral emissions, both discrete and diffuse, are excited principally by electron precipitation, although some of the emissions that constitute the diffuse aurora are produced by precipitating protons (the "proton aurora").

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