Forbush decreases and geomagnetic disturbances: 2. Comparison of solar cycles 23–24 and events with sudden and gradual onset

Abstract

Statistical relationships between the values of geomagnetic indices and the characteristics of cosmic rays and interplanetary disturbances are studied for Forbush decreases with sudden and gradual onset associated with different types of solar sources: a) coronal mass ejections from active regions accompanied by solar flares; b) filament eruptions outside active regions; c) high-speed streams from coronal holes; d) several sources. Using statistical methods, the dependence of geomagnetic indices on cosmic ray and solar wind parameters for Forbush decreases in solar cycles 23 and 24 is also compared. The results obtained showed: a) interplanetary disturbances associated with coronal mass ejections from active regions cause mainly magnetic storms with a sudden onset; b) interplanetary disturbances associated with high-speed streams from coronal holes cause mainly storms with a gradual onset; c) interplanetary disturbances associated with filament eruptions outside active regions cause equally probable storms with a sudden and gradual onset. For sporadic Forbush decreases the values of cosmic ray and geomagnetic activity parameters are, on average, higher for events with a sudden onset; for recurrent Forbush decreases, the nature of the event onset does not affect the value of these parameters. For all types of solar sources the parameters of the disturbed solar wind are, on average, higher in events with a sudden onset. The geoefficiency of interplanetary disturbances is much higher in the 23rd cycle for events associated with ejections from active regions; for other types of disturbances, the difference between the cycles is weak.