Baral, Rabin, Adhikari, Binod, Calabia, Andres, Shah, Munawar, Mishra, Roshan Kumar, Silwal, Ashok, Bohara, Sudarshan, Manandhar, Roshna, del Peral, Luis, and Rodríguez Frías, María D.
Geomagnetic storms and Forbush decreases (FD) on Earth are primarily caused by interplanetary coronal mass ejections (ICMEs) and stream/corotating interaction regions (SIRs/CIRs) originating in the Sun, which are propagated as a low-energy plasma disturbance through the interplanetary magnetic field (IMF). In this paper, we study the variations of the solar-wind parameters (solar wind velocity, plasma density, and IMF-B z component) and the Earth's disturbance storm-time index (D st) 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 cosmic ray intensity reveals the clear evolution of the classical two-step FD with a peak period of approximately 2.1 h. The correlation-delay analysis shows a very strong correlation (∼0.9) between the relative count rate changes in cosmic ray intensity and the indices of solar wind speed and D st. We obtain similar time-delay responses to the solar wind speed for all the cases (∼4 h), but large discrepancies are seen for the D st index between the storms. We, therefore, recommend not using the D st index for predicting Forbush decreases. Finally, we employ the resulting delay times to parameterise the Forbush decreases in terms of the 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 speed conditions. Our results verify the suitability of using solar wind parameters to predict Forbush decreases in the cosmic ray flux. • A deep Forbush decrease was recorded in the galactic cosmic ray flux, showing a 6% decrease over all the selected stations. • Correlation-delay analyses with space weather and geomagnetic indices have shown that V sw and D st index have significant correlation with cosmic ray flux, with values above 0.9 for the storms of March and June 2015, and about 0.85 for the storm of September 8, 2017. • A possible dependence on solar wind proton density has been observed, which modulates the magnitude of Forbush decreases under similar solar wind velocity conditions. [ABSTRACT FROM AUTHOR]