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Space and Solar Definitions - U

U Burst - A fast radio burst spectrum of a flare. It has a u-shaped appear- ance in an intensity-vs.-frequency plot.

UHF - Ultra-high frequency (around 300MHz).

ULS - Ulysses spacecraft.

Ultra High Frequency (Uhf) - Those radio frequencies exceeding 300 mhz.

Umbra - The dark core or cores (umbrae) in a sunspot with penumbra, or a sunspot lacking penumbra.

Universal Time (Ut) - See coordinated universal time.

Unsettled - With regard to geomagnetic levels, a descriptive word specifically meaning that 8 is less than or equal to the ap index which is less than or equal to15.

Uplink - Signal sent to a spacecraft.

Uranus - Seventh planet from the sun, a gas giant or Jovian planet.

USO - Ultra Stable Oscillator, in a spacecraft telecommunications subsystem.

UT - Universal Time, also called Zulu (Z) time, previously Greenwich Mean Time. UT is based on the imaginary "mean sun," which averages out the effects on the length of the solar day caused by Earth's slightly non-circular orbit about the sun. UT is not updated with leap seconds as is UTC.

UTC - Coordinated Universal Time, the world-wide scientific standard of timekeeping. UTC is based upon carefully maintained atomic clocks and is kept accurate to within microseconds worldwide. The addition or subtraction of leap seconds, as necessary, at two opportunities every year adjusts UTC for irregularities in Earth's rotation.

UV - Ultraviolet (meaning "above violet") radiation. Electromagnetic radiation in the neighborhood of 100 nanometers wavelength.

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