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 a
mushroom cloud during the first several dozen seconds after
nuclear explosions near sea level. This post-explosion effect has been observed only at night from
atmospheric nuclear tests owing to its low brightness, with observers noticing it following the pre-dawn
Trinity test, as well as
Upshot-Knothole Annie,
Operation Fishbowl, and the
Cherokee shot of
Operation Redwing. nuclear bomb test • Within minutes after the steam explosion that caused the
Chernobyl accident at 01:23 local time, employees at the power station went outside to get a clearer view of the extent of the damage. One such survivor,
Alexander Yuvchenko, recounts that once he stopped outside and looked up towards the reactor hall he saw a "very beautiful"
laser-like beam of
light bluish light, caused by the ionization of air, that appeared to be "flooding up into infinity". •
Cathode rays in air produce this blue glow. •
Electrical discharge in air is the cause of blue light emitted by
electric sparks,
lightning, and
corona discharges (e.g.
St. Elmo's fire). •
Auroras, the sometimes observable blue-violet hues emitted by nitrogen at lower altitudes.
Colors In dry air, the color of produced light (e.g. by lightning) is dominated by the emission lines of nitrogen, yielding the spectrum with primarily blue emission lines. The lines of neutral nitrogen (NI), neutral oxygen (OI), singly ionized nitrogen (NII) and singly ionized oxygen (OII) are the most prominent features of a lightning
emission spectrum. Neutral nitrogen radiates primarily at one line in the red part of the spectrum. Ionized nitrogen radiates primarily as a set of lines in the blue part of the spectrum. A violet hue can occur when the spectrum contains emission lines of atomic hydrogen. This may happen when the air contains high amount of water, e.g. with lightnings in low altitudes passing through
rain thunderstorms. Water vapor and small water droplets ionize and dissociate easier than large droplets, therefore have higher impact on color. The
hydrogen emission lines at 656.3 nm (the strong
H-alpha line) and at 486.1 nm (H-beta) are characteristic for lightnings.
Rydberg atoms, generated by low-frequency lightnings, emit at red to orange color and can give the lightning a yellowish to greenish tint.(
confusing?) Generally, the radiant species present in atmospheric
plasma are N2, N2+, O2, NO (in dry air) and OH (in humid air). The temperature,
electron density, and
electron temperature of the plasma can be inferred from the distribution of
rotational lines of these species. At higher temperatures, atomic emission lines of N and O, and (in presence of water) H, are present. Other molecular lines, e.g. CO and CN, mark the presence of contaminants in the air.
Cherenkov radiation The emission of blue light is often attributed to
Cherenkov radiation. Cherenkov radiation is produced by charged particles which are traveling through a
dielectric substance at a speed greater than the
speed of light in that medium. Despite the production of similarity-colored light and an association with high-energy particles, Cherenkov radiation is generated by a fundamentally different mechanism. ==See also==