No abstract

A geomagnetic storm is a phenomenon of weakening of Earth's magnetic field from a few hours to several days, which directly or indirectly affects navigation, transportation, communication, power grids, satellite electronics, etc. In general, the duration of the recovery phase is larger. Thus, its cumulative effect is greater than the main phase. Therefore, it is important to understand the physical processes involved during the recovery phase. In this paper, we have studied the recovery phase of 31 extreme geomagnetic storms that occurred from 1990 to 2020. Each storm demonstrates two distinct features of the recovery phase that is, initial fast and later slow recovery phase. During the fast recovery phase, the rate of recovery either linearly or non‐linearly depends on SYM−H, which has been characterized by an exponential or hyperbolic decay function in various reported studies. In our study, we have noted that the hyperbolic function explains the complete recovery phase of only 11 extreme events. Furthermore, both decay functions fail to explain the late recovery phase of storms. Interestingly, we have found that the rate of recovery during the slow phase shows steady variation (independent to SYM−H), that is, d(SYMbadbreak−H)dt0.3333em=0.3333emconstant. Our analysis suggests that the magnitude of the recovery rate during the late phase is proportional to the magnitude of the storm.

Planar magnetic structures (PMS) are often observed in sheath regions driven by interplanetary coronal mass ejections (ICMEs) and in corotating interaction regions (CIRs). Here, we study plasma properties statistically within planar and non-planar ICME sheath regions using in situ data from the Advanced Composition Explore (ACE) spacecraft. The study includes 420 ICME-driven sheaths from 1998–2017. We found that 146 ($\sim 35{{\ \rm per\ cent}}$) ICME-driven sheaths are planar, whereas 274 ($\sim 65{{\ \rm per\ cent}}$) are non-planar. This study found that the average plasma temperature, density, speed, plasma beta, thermal pressure and magnetic pressure are higher in planar sheaths than in non-planar sheaths. This implies that high compression plays an essential role in the formation of PMS in sheath regions. Interestingly, our analysis reveals explicitly that the strength of the southward/northward magnetic field component is almost double in planar sheath regions compared with non-planar sheath regions. This suggests that planar sheaths are more geoeffective than non-planar sheaths.

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.

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.