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อภิธานศัพท์: อภิธานศัพท์ฟิสิกส์ (อวกาศ)
S
SECTOR BOUNDARY In the SOLAR WIND, the area of demarcation between sec-
tors, which are large-scale features distinguished by the
predominant direction of the interplanetary magnetic field,
toward or away from the sun.
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SHORT WAVE FADE (SWF) A particular ionospheric solar flare effect under
the broad category of sudden ionospheric disturbances (SIDs)
whereby short-wavelength radio transmissions, VLF, through
HF, are absorbed for a period of minutes to hours.
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SMOOTHED SUNSPOT NUMBER An average of 13 monthly RI numbers, centered on
the month of concern.
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SOLAR COORDINATES
Central Meridian Distance (CMD). The angular distance in solar
longitude measured from the central meridian.
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SOLAR CYCLE The approximately 11-year quasi-periodic variation in frequency
or number of solar active events.
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SOLAR MAXIMUM The month(s) during the SOLAR CYCLE when the 12-month mean
of monthly average SUNSPOT NUMBERS reaches a maximum. The most
recent solar maximum occurred in July 1989.
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SOLAR MINIMUM The month(s) during the SOLAR CYCLE when the 12-month mean
of monthly average SUNSPOT NUMBERS reaches a minimum.
the most recent minimum occurred in September 1986.
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SOLAR SECTOR BOUNDARY (SSB) The apparent solar origin, or base, of the
interplanetary SECTOR BOUNDARY marked by the larger-scale polarity
inversion lines.
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SOLAR WIND The outward flux of solar particles and magnetic fields from the
sun. Typically, solar wind velocities are near 350 km/s.
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SPORADIC E A phenomenon occurring in the E REGION of the
IONOSPHERE, which significantly affects HF radiowave
propagation. Sporadic E can occur during daytime or nighttime
and it varies markedly with latitude.
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SPOTNIL Spotless disk
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SUDDEN COMMENCEMENT(SC, or SSC for Storm Sudden Commencement) An abrupt
increase or decrease in the northward component of the geomagnetic
field, which marks the beginning of a GEOMAGNETIC STORM.
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SUDDEN IMPULSE (SI+ or SI-) A sudden perturbation of several gammas
in the northward component of the low-latitude geomagnetic field,
not associated with a following GEOMAGNETIC STORM. (An SI becomes
an SC if a storm follows.)
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SUDDEN IONOSPHERIC DISTURBANCE (SID) HF propagation anomalies due to
ionospheric changes resulting from solar FLAREs, PROTON EVENTs
and GEOMAGNETIC STORMs.
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SUNSPOT An area seen as a dark spot on the PHOTOSPHERE of the sun. Sunspots
are concentrations of magnetic flux, typically occurring in bipolar
clusters or groups. They appear dark because they are cooler than
the surrounding photosphere.
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