The role of extreme geomagnetic storms in the Forbush decrease profile

TL;DR

This study investigates the relationship between extreme geomagnetic storms and Forbush decrease profiles, highlighting how geomagnetic disturbances influence cosmic ray modulation and emphasizing the importance of including geomagnetic field variations in models.

Contribution

The paper presents a detailed analysis of five ICME-induced extreme storms and their impact on FD profiles, revealing the significant role of geomagnetic conditions in cosmic ray variations.

Findings

Sudden storm commencements coincide with FD onsets

Neutron counts increase during GS main and recovery phases

High-energy neutrons show more pronounced enhancement

Abstract

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, an ICME causes high amplitude FDs and extreme GSs. However, the interlinks between extreme GS and strong FDs are poorly studied. Here, we demonstrate five ICME induced extreme storms and their effects on respective FD profiles. 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. 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.