The effect is however not noticeable to the human eye, since the glare of directly scattered sunlight outshines and obscures it. In daytime, sodium and red oxygen emissions are dominant and roughly 1,000 times as bright as nighttime emissions because in daytime, the upper atmosphere is fully exposed to solar UV radiation. How sodium gets to mesospheric heights is not yet well understood, but it is believed to be a combination of upward transport of sea salt and meteoritic dust. The green oxygen emissions are more spatially distributed. The red oxygen lines originate at altitudes of about 300 km, in the F-layer. The sodium emissions come from a thin sodium layer approximately 10 km thick at an altitude of 90–100 km, above the mesopause and in the D-layer of the ionosphere. Several emission lines are dominant: a green line from oxygen at 557.7 nm, a yellow doublet from sodium at 589.0 and 589.6 nm, and red lines from oxygen at 630.0 and 636.4 nm. The green line Angstrom observed is in fact an emission line with a wavelength of 557.7 nm, caused by the recombination of oxygen in the upper atmosphere.Īirglow is the collective name of the various processes in the upper atmosphere that result in the emission of photons, with the driving force being primarily UV radiation from the Sun. It was not until the 1920s that scientists were beginning to identify and understand the emission lines in aurorae and of the sky itself, and what was causing them. When physicist Anders Ångström examined the spectrum of the aurora borealis, he discovered that even on nights when the aurora was absent, its characteristic green line was still present. Sources of the night sky's intrinsic brightness include airglow, indirect scattering of sunlight, scattering of starlight, and light pollution. When the Sun's depth is more than 18°, the sky generally attains its maximum darkness. At astronomical twilight, the Sun is between –12° and –18°. At nautical twilight, the Sun's altitude is between –6° and –12°. At civil twilight, the center of the Sun's disk appears to be between 1/4° and 6° below the horizon. Twilight (both dusk and dawn) is divided into three 6° segments that mark the Sun's position below the horizon. During twilight (the duration after sunset or before sunrise until or since, respectively, the full darkness of night), the situation is more complicated, and a further differentiation is required. During daytime, when the Sun is above the horizon, the direct scattering of sunlight is the overwhelmingly dominant source of light. The sky's brightness varies greatly over the day, and the primary cause differs as well. the Moon and light pollution) were removed from the night sky, only direct starlight would be visible. The fact that the sky is not completely dark at night is easily visible. Sky brightness refers to the visual perception of the sky and how it scatters and diffuses light. JSTOR ( January 2017) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |