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Interplanetary Coronal Mass Ejections (ICMEs) are prominent drivers of space weather disturbances and mainly lead to intense or extreme geomagnetic storms. The reported studies suggested that the planar ICME sheath and planar magnetic clouds (MCs) cause extreme storms. Here, we investigated the severe two-step geomagnetic storm ($Dst \sim -187$ nT) of 25$^{th}$ solar cycle. Our analysis demonstrates flattened (pancaked) ICME structures, i.e., quasi-planar magnetic structures (PMS). The study corroborates our earlier reported finding that the less adiabatic expansion in quasi-PMS transformed ICME enhanced the strength of the southward magnetic field component. It contributes to the efficient transfer of plasma and energy in the Earth's magnetosphere to cause the observed severe storm.

The dynamic evolution of coronal mass ejection (CME) in interplanetary space generates highly turbulent, compressed, and heated shock-sheath. This region furnishes a unique environment to study the turbulent fluctuations at the small scales and serve an opportunity for unfolding the physical mechanisms by which the turbulence is dissipated and plasma is heated. How does the turbulence in the magnetized plasma control the energy transport process in space and astrophysical plasmas is an attractive and challenging open problem of the 21st century. For this, the literature discusses three types of magnetohydrodynamics (MHD) waves/ fluctuations in magnetized plasma as the magnetosonic (fast), Alfv'enic (intermediate), and sonic (slow). The magnetosonic type is most common in the interplanetary medium. However, Alfv'enic waves/fluctuations have not been identified to date in the ICME sheath. The steepening of the Alfv'en wave can form a rotational discontinuity that leads to an Alfv'enic shock. But, the questions were raised on their existence based on the theoretical ground. Here, we demonstrate the observable in-situ evidence of Alfv'en waves inside turbulent shock-sheath at 1 AU using three different methods desciribed in the literature. We also estimate Els"asser variables, normalized cross helicity, normalized residual energy and which indicate outward flow of Alfv'en waves. Power spectrum analysis of IMF indicates the existence of Alfv'enic turbulence in ICME shock-sheath. The study has strong implications in the domain of interplanetary space plasma, its interaction with planetary plasma, and astrophysical plasma., arXiv admin note: substantial text overlap with arXiv:1810.06004

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., Submitted to MNRAS