Rayleighs

Rayleighs are a unit of optical brightness used primarily in atmospheric and astronomical contexts to quantify faint, diffuse emissions from the Earth's upper atmosphere, such as airglow and aurorae. The unit is named after Lord Rayleigh, the 19th-century physicist known for work on light scattering.

Definition and usage

One Rayleigh (1 R) is defined as 10^6 photons cm^-2 s^-1 sr^-1, representing a columnar emission rate per unit solid angle. In practice, the Rayleigh measures the radiance of a light-emitting or scattering layer along a line of sight, expressed per square centimeter of atmospheric cross-section and per steradian. It is a photon-based unit, not a luminance or energy flux unit, and conversions to energy or photometric units depend on the wavelength of the emission.

Applications

Rayleighs are commonly used to describe the brightness of the night sky’s airglow and of auroral features, especially for specific emission lines such as oxygen and hydroxyl bands. Because the quantity is tied to photons rather than perceived brightness, Rayleighs are well-suited for comparing diffuse, extended sources in the atmosphere.

Typical ranges

Nightglow intensities usually fall in the range of a few to tens of Rayleighs, while brighter auroral events can reach thousands of Rayleighs. The unit is most informative when discussing line emissions and integrated column effects rather than point sources.

History

The Rayleigh unit honors Lord Rayleigh for his contributions to optical science, including work on scattering, which inspired its use for quantifying atmospheric emissions. It remains a standard measure in observational aeronomy and related atmospheric research.