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Astrophysics Science Division | Sciences and Exploration

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NIGHTGLOW Science for Scientists

NIGHTGLOW Science for the Public

What is Nightglow?

Even at night, the Earth's atmosphere glows from reflected starlight, moonlight, man-made ultraviolet (UV) light and molecular processes. Our instrument is designed to measure this UV "nightglow", hence the mission name NIGHTGLOW! The image at the right (from Frank and Craven, 1988, Rev. Geophysics, 26, 249) taken in the far UV (1000 - 2000 Angstroms) shows the night side of the Earth. The bright ring near the North Pole is the aurora; the bands of glow over the lower left part of the disk are due to HI Lyman alpha. This is similar to what NIGHTGLOW will observe in the 3000 - 4000 Angstrom range.

Earth and nightglow

Measuring Nightglow

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View the existing data from a brief 1962 rocket flight. Higher resolution images are also available, showing a larger part of the UV spectrum and the time variations of the UV nightglow observed from the ground.

In the wavelength range of 2000 - 4000 Angstroms, NO and O2 molecular processes dominate the nightglow from the atmosphere. The nightglow is produced by atom-atom interactions and ionic recombination. In particular, the O2 Herzberg bands are produced by 3-body recombination of ground state O atoms (the third body in the reaction being mainly N2).

Man-made lights also emit in the near UV, and cities can be a large source of "light pollution" in this range just as in the visible. The image of the United States at night in visible light demonstrates this problem.

Surprisingly, there are not very many measurements of the near UV nightglow, and no long-term studies have been undertaken. Most researchers have concentrated on measuring the dayglow rather than the nightglow and many instruments lack the sensitivity needed to see the nightglow. The dayglow is dominated (in this wavelength regime) by surface and cloud reflections, and Rayleigh and aerosol scattering of sunlight. Ozone absorption by the atmosphere is an important component of the dayglow, especially below about 3200 Angstroms.

The NASA Orbiting Wide-angle Light collector (OWL) mission will image cosmic ray air showers (from particles with energies greater than 1019 eV). These particles excite the nitrogen in the atmosphere, which then fluoresces in the near UV. OWL will measure the amount of UV light emitted from these large air showers with one or two large aperture orbiting telescopes. This nitrogen fluorescence light must be viewed against the "noise" from the other sources of UV light talked about above. NIGHTGLOW will measure the component of the UV light (from sources other than the ultra-high energy cosmic ray showers) which constitutes a background for OWL.

NIGHTGLOW data

NIGHTGLOW PUBLICATIONS

About the NIGHTGLOW instrument...

This file was last modified January 9, 2003