Ionized-air glow

Ionized-air glow is the luminescent emission of characteristic blue–purple–violet light, often of a color called electric blue, by air subjected to an energy flux either directly or indirectly from solar radiation. When energy is deposited to air, the air molecules become excited. As air is composed primarily of nitrogen and oxygen, excited N2 and O2 molecules are produced. These can react with other molecules, forming mainly ozone and nitrogen(II) oxide. Water vapor, when present, may also play a role; its presence is characterized by the hydrogen emission lines. The reactive species present in the plasma can readily react with other chemicals present in the air or on nearby surfaces. The excited nitrogen deexcites primarily by emission of a photon, with emission lines in ultraviolet, visible, and infrared band: N2* → N2 + hν The blue light observed is produced primarily by this process. The spectrum is dominated by lines of single-ionized nitrogen, with presence of neutral nitrogen lines. The excited state of oxygen is somewhat more stable than nitrogen. While deexcitation can occur by emission of photons, more probable mechanism at atmospheric pressure is a chemical reaction with other oxygen molecules, forming ozone: O2* + 2 O2 → 2 O3 This reaction is responsible for the production of ozone in the vicinity of strongly radioactive materials and electrical discharges. Excitation energy can be deposited in air by a number of different mechanisms: Ionizing radiation is the cause of blue glow surrounding sufficient quantities of strongly radioactive materials in air, e.g. some radioisotope specimens (e.g. radium or polonium), particle beams (e.g. from particle accelerators) in air, the blue flashes during criticality accidents, and the eerie/low brightness "purple" to "blue" glow enveloping the mushroom cloud during the first several dozen seconds after nuclear explosions near sea level.