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T
TYPE I, II, III, IV See RADIO EMISSION
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U
U BURST A fast radio burst spectrum of a FLARE. It has a U-shaped appear-
ance in an intensity-vs.-frequency plot.
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ULTRA HIGH FREQUENCY (UHF) Those radio frequencies exceeding 300 MHz.
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UMBRA The dark core or cores (umbrae) in a SUNSPOT with PENUMBRA, or a
sunspot lacking penumbra.
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UNIVERSAL TIME (UT) See COORDINATED UNIVERSAL TIME.
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UNSETTLED With regard to geomagnetic levels, a descriptive word speci-
fically meaning that 8 is less than or equal to the Ap INDEX which is less than or equal to15.
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V
VERY HIGH FREQUENCY (VHF) That portion of the radio frequency spectrum
from 30 to 300 MHz.
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VERY LOW FREQUENCY (VLF) That portion of the radio frequency spectrum from
3 to 30 kHz.
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W
WHITE LIGHT (WL) Sunlight integrated over the visible portion of the spec-
trum (4000 - 7000 angstroms) so that all colors are blended to
appear white to the eye.
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WHITE LIGHT FLARE A major FLARE in which small parts become visible in
white light. Such flares are usually strong X-ray, radio, and
particle emitters.
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WOLF NUMBER An historic term for SUNSPOT NUMBER. In 1849, R. Wolf of
Zurich originated the general procedure for computing the sunspot
number.
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X
X-RAY BACKGROUND A daily average background X-ray flux in the 1 to 8
angstrom range. It is a midday minimum designed to reduce the
effects of FLAREs.
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X-RAY BURST A temporary enhancement of the X-ray emission of the sun. The
time-intensity profile of soft X-ray bursts is similar to that of
the H-ALPHA profile of an associated FLARE.
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X-RAY FLARE CLASS Rank of a FLARE based on its X-ray energy output. Flares
are classified by the order of magnitude of the peak burst inten-
sity (I) measured at the earth in the 1 to 8 angstrom band as
follows:
Class (in Watt/sq. Meter)
B I less than (l.t.) 10.0E-06
C 10.0E-06 l.e.= I l.t.= 10.0E-05
M 10.0E-05 l.e.= I l.t.= 10.0E-04
X I g.e.= 10.0E-04
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Z
ZURICH SUNSPOT CLASSIFICATION A sunspot classification system that has been
modified for SESC use.
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