The role of extreme geomagnetic storms in the Forbush decrease profile observed by neutron monitors

The Forbush decrease (FD) and Geomagnetic storm (GS) are the two distinct space weather events having common causing agents like interplanetary coronal mass ejection (ICME) or corotating interacting region (CIR). Generally, large-amplitude FDs and extreme GSs are caused by ICME. Here, we studied eight ICME induced extreme storms and their effects on respective FD profiles as observed by neutron monitors. We observed the sudden storm commencement of the GS coincides with the FD onset. Interestingly, we also noted a gradual increase in neutron counts during the main and recovery phases of GS. The maximum enhancement in neutron counts coincides with the minimum value of the Sym-H index. The enhancement is visible primarily in all the neutron monitors but significantly pronounced in high-energy neutrons compared to low-energy neutrons. We observed that the enhancement in cosmic ray flux during Forbush decrease as observed by neutron monitors is contributed by magnetospheric disturbance of geomagnetic storm. Thus we conclude that the weakening of Earth's magnetic shield due to ICME-Magnetosphere interaction allows more cosmic rays to reach the ground. Thus, we conclude that the geomagnetic storm conditions highly influence the FD profile along with the external causing agent. Therefore, it is essential to include the effect of geomagnetic field variation in the models that are used to reproduce the FD profile.