Which of the following correctly defines 'Aurora' in the context of Earth's atmosphere?

The definition of 'Aurora' in the context of Earth's atmosphere is indeed an optical phenomenon. Auroras, including the well-known Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights), occur when charged particles from the solar wind interact with the Earth's magnetic field and atmosphere. This interaction causes the particles to excite gases in the Earth's atmosphere, which then emit light, creating stunning displays of colorful rays and bands.

These displays are primarily visible at high latitudes near the polar regions, where the magnetic field lines converge. The colors of the aurora—often green, pink, red, yellow, blue, and violet—depend on the type of gas involved and the altitude at which the interaction occurs. Thus, auroras are categorized as optical phenomena due to their visual nature and the light produced by the interactions in the atmosphere.