Understanding Forbush decrease drivers based on shock-only and CME-only models using global signature of February 14, 1978 event

TL;DR

This study analyzes the 1978 Forbush decrease event using neutron monitor data to understand the roles of shock and CME drivers, revealing their distinct contributions and dependencies on rigidity, local time, and solar wind speed.

Contribution

It applies shock-only and CME-only models to separate the effects of shocks and CMEs on Forbush decreases, confirming their respective roles in different phases.

Findings

Shock and FD amplitudes follow similar power laws.

Shock phase and recovery times show rigidity dependence.

Solar wind speed correlates with recovery phase.

Abstract

We have studied Forbush decrease (FD) event occurred on February 14, 1978 using 43 neutron monitor observatories to understand the global signature of FD. We have studied rigidity dependence of shock amplitude and total FD amplitude. We have found almost the same power law index for both shock phase amplitude and total FD amplitude. Local time variation of shock phase amplitude and maximum depression time of FD have been investigated which indicate possible effect of shock/CME orientation. We have analyzed rigidity dependence of time constants of two phase recovery. Time constants of slow component of recovery phase show rigidity dependence and implies possible effect of diffusion. Solar wind speed was observed to be well correlated with slow component of FD recovery phase. This indicates solar wind speed as possible driver of recovery phase. To investigate the contribution of interplanetary drivers, shock and CME in FD, we have used shock-only and CME-only models. We have applied these models separately to shock phase and main phase amplitudes respectively. This confirms present accepted physical scenario that the first step of FD is due to propagating shock barrier and second step is due to flux rope of CME/magnetic cloud.