极光（英语：Aurora）是在高纬度（北极和南极）的天空中，带电的高能粒子和高层大气（热层）中的原子碰撞造成的发光现象。带电粒子来自磁层和太阳风，在地球上，它们被地球的磁场带进大气层。大多数的极光发生在所谓的“极光带”，在观察上，这是在所有的经度上距离地磁极10°至20°，纬度宽约3°至6°的带状区域。太阳风受到地球的磁场导引直接进入大气层。当磁暴发生时，在较低的纬度也会出现极光。极光不只在地球上出现，太阳系内的其他一些具有磁场的行星上也有极光。

在英、法等许多西方语言中，人们遵照伽利略的习惯，直接用欧若拉（Aurora）女神的名字来称呼极光现象。

An aurora, sometimes referred to as a polar light, is a natural light display in the sky, predominantly seen in the high latitude (Arctic and Antarctic) regions. Auroras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particles in both solar wind and magnetospheric plasma, mainly in the form of electrons and protons, precipitate them into the upper atmosphere (thermosphere/exosphere), where their energy is lost. The resulting ionization and excitation of atmospheric constituents emits light of varying colour and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles. Precipitating protons generally produce optical emissions as incident hydrogen atoms after gaining electrons from the atmosphere. Proton auroras are usually observed at lower latitudes. Different aspects of an aurora are elaborated in various sections below.