Index of variability of the solar ultraviolet (UV) radiation, the composite Mg II core-to-wing ratio, has been used to study relationship between the short-term (&tgr;<27 days) changes of solar activity and solar UV irradiance. Such manifestations of the solar activity have been examined, as the solar central meridian passage of active regions, the solar proton events, and the central meridian passage of hypothetical regions responsible for the Forbush decrease in the galactic cosmic rays. Our results show that all these short-term changes of the solar activity are accompanied by an increase of the solar UV irradiance. The interplanetary magnetic field sector structure is also related to changes in the UV irradiance. After a proper adjustment of the dates of the sector boundary occurrence for the solar disk, the irradiance was found to be maximal on the toward/away boundary and minimal on the away/toward boundary. It has been found that the UV irradiance undergoes quasi-biennial periodicity (QBP), reaching maximum in years of the east QBP phase and decreasing in years of the west QBP phase. Superposition of the quasi-biennial periodicity and effects connected with short-term variations in the solar activity account for the change of the Mg II index up to 2% of the mean level. Thus a new very important agent was found to be responsible for a short-term and medium-term influence of the solar activity upon atmospheric processes and hence on the weather and climate. ¿ 1998 American Geophysical Union