Polar Auroras: Lights of the Solar Wind

Origin of Polar Auroras

Polar auroras are luminous phenomena that mainly appear in regions near the magnetic poles. They result from the interaction between the solar wind and Earth's magnetic field. Charged particles from the solar wind are guided by magnetic field lines toward high latitudes, where they excite atmospheric molecules, causing light emission.

Physical Mechanisms

When electrons and protons from the solar wind penetrate the ionosphere, they collide with oxygen and nitrogen atoms. These collisions raise the electrons of the atoms to an excited state. When these electrons return to their ground state, they emit photons whose wavelengths determine the color of the aurora.

When these electrons return to their ground state, they emit photons whose wavelengths determine the color of the aurora, with green being the most common. The density and energy of solar particles influence the intensity and altitude of auroras.

• Green (about 557.7 nm) is mainly emitted by oxygen atoms (O) at an altitude of 100 to 150 km.
• Red (630.0 nm) is also produced by oxygen (O²), but at altitudes above 200 km.
• Blue (427.8 nm) comes from ionized nitrogen molecules (N₂⁺).
• Purple or violet (390-420 nm) is emitted by neutral nitrogen (N₂), usually during intense solar activity.

Types of Auroras and Variations

Auroras can appear as curtains, arcs, crowns, or spirals. Their occurrence depends on solar activity, measured by the Kp index. Periods of high solar activity, especially during solar flares or CMEs, increase the likelihood of spectacular auroras.

Comparative Table of Main Characteristics

Sources: NASA, NOAA Space Weather, Springer.