Abstract
In this paper, we study the variations of the solar-wind parameters
(solar wind velocity, plasma density, and IMF-Bz
component) and the Earth’s disturbance storm-time index
(Dst), in relation to cosmic ray flux measurements from
8 neutron monitor stations distributed over Canada, Russia, Finland, and
Greenland, during 3 intense geomagnetic storms occurred during the
24th solar cycle (March 16-18, 2015, June 21-23, 2015,
and September 7-9, 2017). The wavelet analysis of the Forbush decrease
seen in the cosmic ray intensity reveals the clear evolution of the
classical two-step process, and with a peak period of approximately 2.1
h. The correlation-delay analyses show a very strong correlation
(~0.9) between the relative count rate changes cosmic
ray intensity and the indices of solar wind velocity and
Dst. We obtain similar time-delay responses to the solar
wind velocity for all the cases (~4 hours), but large
discrepancies are seen for the Dst index between the
storms. We therefore recommend not using the Dst index
for predicting Forbush decreases. Finally, we employ the resulting
delay-times to parameterize the Forbush decreases in terms of solar
wind, and we obtain a predictive model with R2
parameter of an approximate value of 0.8. Moreover, we observe a
possible dependence on solar wind proton density which modulates the
magnitude of Forbush decreases under similar solar wind velocity
conditions. Our results verify the suitability of using solar wind
parameters to predict Forbush decreases in the cosmic ray flux.