The aurora borealis (in the Northern Hemisphere) or aurora australis (in the Southern) is a natural optical phenomenon in which curtains, arcs, and bands of coloured light — predominantly green, purple, red, and blue — illuminate the night sky at high latitudes. They are among the most impressive natural spectacles on the planet.
Physical mechanism
They occur when charged particles from the solar wind (protons and electrons) are channelled by Earth's magnetic field toward the auroral ovals around the poles. Upon penetrating the upper atmosphere (100-300 km altitude), these particles collide with oxygen and nitrogen atoms, exciting them. As the atoms return to their ground state, they emit photons: oxygen produces bright green (557.7 nm) and red (630 nm), while nitrogen generates blues and purples.
Auroral activity is directly linked to solar activity: geomagnetic storms caused by coronal mass ejections (CMEs) intensify auroras and can extend their visibility to lower latitudes. During severe geomagnetic storms (Kp index ≥ 7), auroras have been observed from Spain, notably in May 2024 during the Solar Cycle 25 maximum.
Although not a meteorological phenomenon in the strict sense (they occur far above the weather atmosphere), associated geomagnetic storms can affect power grids, radio communications, and GPS systems. See also: noctilucent cloud.