Abstract
Abstract A Forbush decrease is a sudden decrease in cosmic-ray intensity caused by transient interplanetary disturbances. The substructure of an interplanetary counterpart of a coronal mass ejection (ICME) such as a shock sheath and/or a magnetic cloud independently contributes to cosmic-ray decrease, which is evident as a two-step decrease. Our earlier work has shown multistep decrease and recovery within the ICME-driven shock-sheath region. Further, we have suggested that the presence of a small-scale flux rope within the shock-sheath region causes a steady/gradual recovery in cosmic-ray intensity. Here, we demonstrate the presence of a planar magnetic structure (PMS) and small-scale flux rope within a single shock sheath of an ICME. The plot of the elevation (θ) versus azimuthal (ϕ) angle of the interplanetary magnetic field (IMF) is used for the identification of the PMS. The planarity, efficiency, and a plane-normal vector are estimated by employing a minimum variance analysis (MVA) technique, which confirmed the presence of the PMS. In addition, a 2D-hodogram method in conjunction with the MVA technique is utilized to identify the flux-rope structure and turbulent conditions in the corresponding ICME region. The observation in the visible suggests that the PMS region within the ICME shock sheath caused the decrease in the cosmic-ray flux observed at Earth. It has also been observed that the sharp variations in the IMF (i.e., turbulence) cause a decrease, whereas the flux-rope structure is responsible for the recovery of the CR flux. Further studies are needed to investigate their origins and to confirm their effects on space weather.
Highlights
Various large-scale solar wind disturbances modulate the omnipresent galactic cosmic rays (GCR) in the heliosphere (Engelbrecht & Burger 2013; Zhao et al 2014, 2017, 2018; Strauss et al 2016)
The confirmation of the two-dimensionality of the field vectors in planar magnetic structure (PMS) events is done using the value of ∣Bn∣/B, where B is the magnitude of the interplanetary magnetic field (IMF) and Bn is a component of the magnetic field normal to the PMS plane
Forbush decreases (FDs) is generally understood in terms of physical processes such as diffusion, convection, and turbulence associated with interplanetary counterpart of a coronal mass ejection (ICME) substructures and co-rotating interaction regions (CIRs)
Summary
Various large-scale solar wind disturbances modulate the omnipresent galactic cosmic rays (GCR) in the heliosphere (Engelbrecht & Burger 2013; Zhao et al 2014, 2017, 2018; Strauss et al 2016). The phenomenon of a temporary and rapid decrease in the GCR flux, followed by a comparatively slow recovery lasting for a few days is known as Forbush decreases (FDs; Hess & Demmelmair 1937; Forbush 1938; Lockwood 1971; Cane 2000; Raghav et al 2017, 2014) This phenomenon was first discovered using ground-based measurements by Forbush (1938) and by Hess & Demmelmair in 1937, and has been extensively studied by many researchers over the last few decades (see, e.g., Lockwood 1971; Cane 2000; Belov et al 2001; Raghav et al 2014, 2017; Bhaskar et al 2016a; Jordan et al 2011; Dumbovic et al 2012, and references therein).
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