Forbush Decreases Associated with Coronal Holes, Coronal Ejections from Active Regions, and Filament Ejections: Comparison in Solar Cycles 23 and 24

In this paper, we study the similarities and differences of Forbush decreases in solar cycles 23 and
24. Groups of events associated with various types of solar sources were analyzed: coronal mass ejections from
active regions accompanied by solar flares (CME1 group), filament ejections outside active regions (CME2
group), and high-velocity streams from coronal holes (CH group). The distributions and relations of various
parameters were studied: the amplitude of Forbush decreases, the maximum values of the hourly decrease in
the cosmic ray density, the equatorial cosmic ray anisotropy, the solar wind velocity, and the magnetic field
strength, as well as the values of the solar wind velocity and the magnetic field strength 1 hour before the onset
of the Forbush decrease during the event. The results showed that the number of events, parameter values,
and their relations depend on the phase and cycle of solar activity. In the 24th cycle, the number of events in
the CME1 group decreased, did not change in CME2, and increased in CH. The values of the parameters
and the difference between them in various groups of events are higher in cycle 23, which is characterized by
greater asymmetry and long “tails” of distributions. The magnitude of the Forbush decreases in the CME1
group in cycle 23 depends more strongly on the solar wind velocity while, in cycle 24, on the magnetic field
strength, as in the CME2 group in both solar cycles. Multiple linear regression describes the dependences of
the parameters of Forbush reductions in the CME1 and CME2 groups in the 23rd cycle and in the CME1
group in the 24th cycle well.