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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.
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.
SMOOTHED SUNSPOT NUMBER
An average of 13 monthly RI numbers, centered on the month of concern.
Central Meridian Distance (CMD). The angular distance in solar longitude measured from the central meridian.
The approximately 11-year quasi-periodic variation in frequency or number of solar active events.
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.
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.
SOLAR SECTOR BOUNDARY (SSB)
The apparent solar origin, or base, of the interplanetary SECTOR BOUNDARY marked by the larger-scale polarity inversion lines.
The outward flux of solar particles and magnetic fields from the sun. Typically, solar wind velocities are near 350 km/s.
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.
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.
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.)
SUDDEN IONOSPHERIC DISTURBANCE (SID)
HF propagation anomalies due to ionospheric changes resulting from solar FLAREs, PROTON EVENTs and GEOMAGNETIC STORMs.
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.