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Words begining with "V"

Van Allen Belt or Van Allen Radiation Belt

Doughnut-shaped regions encircling Earth and containing high energy electrons and ions trapped in the Earths magnetic field (the magnetic field has definite boundaries, and is distorted into a tear-drop shape by the solar wind). Explorer 1, launched by NASA in 1958, discovered this intense radiation zone. These regions are called the inner and outer Van Allen radiation belts, named after the scientist who first observed them. See magnetosphere.

Vernal Equinox

The beginning of spring in the Northern Hemisphere. The time/day that the sun crosses the equatorial plane going from south to north.

Very High Frequency (VHF)

Referring to the 50-400 MHz portion of the radio frequency spectrum. Polar-orbiting satellite transmissions (APT) are made in the 136-138 MHz range using FM modulation.

Video

A signal containing information on the brightness levels of different portions of an image along with information on line and frame synchronization. In the case of satellite signals, the video information is transmitted in the form of an AM modulated subcarrier.

Visible

That part of the electromagnetic spectrum to which the human eye is sensitive, between about 0.4 and 0.7 micrometers. See spectrum.

Visible Infrared Spin Scan Radiometer (VISSR)

High-resolution, multi-spectral imaging system flown on the pre-GOES-8 geostationary GOES spacecraft. Similar systems are flown on the METEOSAT and GMS spacecraft.

Volcano

A naturally occurring vent or fissure at the Earth's surface through which erupt molten, solid, and gaseous materials. Volcanic eruptions inject large quantities of dust, gas, and aerosols into the atmosphere. A major component of volcanic clouds is sulfur dioxide, a strong absorber of ultraviolet radiation. Chemical interactions between sulfur dioxide and water cause sulfuric acid aerosols which can scatter some of the incident solar radiation back to space, thus causing a global cooling effect. For example, Mt. Pinatubo in the Philippines erupted in June 1991, and in the following year the global surface temperature was observed to decrease by about O.3 degrees C.

Volt

The unit of electromotive force, or difference of potential, which will cause a current of one ampere to flow through a resistance of one ohm. Named for Italian physicist Alessandro Volta (1745- 1827).


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Jun 12 2014

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