الفهرس 
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Abstract
Fifty-five events of Forbush Decreases had been studied together with neutron monitor
data of cosmic rays observed at six stations to determine the effect of Forbush Decreases on solar
diurnal variations of cosmic ray nucleonic components. These fifty-five events had been
classified into four groups according to their shape. These groups are the group of sharp-decrease
and sharp-recovery, the group of sharp-decrease and slow-recovery, the group of slow-decrease
and sharp-recovery, and finally the group of slow-decrease and slow-recovery. Superposed epoch
technique had been used to calculate the vectorial average of the observed diurnal vectors for
every day of the four types of Forbush Decreases. We found that the effect of Forbush Decrease
on the solar diurnal variations of the neutron component in both amplitude and phase is very
remarkable. The solar diurnal amplitudes have larger values for stations having lower
geomagnetic cut-off rigidity. The ratio between the solar diurnal amplitudes before and after the
event was changing according to the profile of the event. There is a shift in the phase towards
earlier hours during the period of Forbush Decreases. The value of that shift also depends on the
profile of the event. The criteria considered for the classification of the Forbush Decrease events
was found not to be the best way for classification. So, we had studied the energy dependence of
Forbush Decreases of cosmic ray intensity, and we concluded the best way for that classification.