Abstract:
The cosmic-ray time variation called Forbush decreases are examined in an overview of observational data with attention given to the three species of Forbush decreases. References are made to the satellite observations of magnetic fields associated with Forbush decreases and of the interactions between interplanetary shock and coronal ejection. The 27-day variations of cosmic rays are discussed in terms of the relation between corotating Forbush decreases and corotating streams. The observations of corotating streams allows the determination of the effects of bulk speed and field strength on corotating Forbush decreases. Long-term Forbush decreases are then examined in terms of the quiet state vs the disturbed state of the sun and the interplanetary medium. The interplanetary disturbances associated with Forbush decreases appear to merge and contribute to the long-term solar-cycle modulation of cosmic-ray intensity
