farultraviolet

Far ultraviolet (FUV) is the portion of the ultraviolet spectrum roughly spanning 122 to 200 nanometers. It lies shorter than near ultraviolet and longer than extreme ultraviolet. Because Earth’s atmosphere absorbs strongly at these wavelengths, FUV observations must be conducted with space-based instruments or in specialized ground-based facilities with suitable shielding, making it a domain dominated by satellite and sounding-rocket programs.

FUV hosts many atomic and molecular transitions that are valuable for diagnosing hot plasmas and low-density gas. The hydrogen Lyman-alpha line at 121.6 nm is a well-known feature, and numerous lines arise from ionized species such as carbon, oxygen, nitrogen, and sulfur. Molecular hydrogen exhibits absorption bands in the FUV, providing key information about temperature, density, composition, and dynamics in stars, the interstellar medium, and the intergalactic medium.

In astronomy, FUV observations are essential for studying hot, young stars, accretion processes, active galactic nuclei, and the chemical and physical conditions of astronomical environments. In planetary science, FUV photometry and spectroscopy shed light on upper atmospheres, atmospheric escape, and surface processes of bodies in the Solar System. Solar and stellar physics also rely on FUV to trace high-energy radiation and heating in stellar atmospheres.

Instrumentation for FUV work uses optics and detectors optimized for vacuum operation. Mirrors are typically aluminum with fluoride overcoats to maximize reflectivity, and detectors include photon-counting devices such as microchannel plates. Notable missions include the Far Ultraviolet Spectroscopic Explorer (FUSE) and ultraviolet spectrographs on space telescopes, which have advanced high-resolution FUV spectroscopy and imaging.