Anil Raghav

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

We have studied the Forbush decrease (FD) event that 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 imply 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 presently 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.

Forbush Decrease: A New Perspective with Classification

The sudden short duration decrease in cosmic ray flux is known as Forbush decrease which is mainly caused by interplanetary disturbances. A generally accepted view is that the first step of Forbush decrease is due to shock sheath and second step is due to magnetic cloud (MC) of interplanetary coronal mass ejection (ICME). This simplistic picture does not consider several physical aspects, such as, whether the complete shock-sheath or MC (or only part of these) are contributing to the decrease, what effect does the internal structure within the shock-sheath region / MC have on the decrease, etc. We present a summary of the analysis of a total of 18 large (≥ 8%) Forbush decrease events and the associated ICMEs, a majority of which show multiple steps in the Forbush decrease profile. We propose a re-classification of Forbush decrease events depending upon the number of steps observed in their respective profile, and the physical origin of these steps. Our analysis clearly indicates that not only broad regions (shock-sheath and MC), but also localized structures within the shock-sheath and MC, have a very significant role in influencing the Forbush decrease profile. The detailed analysis in the present work is expected to contribute toward understanding the relationship between FD and ICME parameters in better way. key words: Shock-sheath, magnetic cloud (MC), ICME, cosmic ray, Forbush decrease, local magnetic structures. Corresponding author.

The Presence of Turbulent and Ordered Local Structure within the ICME Shock-sheath and Its Contribution to Forbush Decrease

The transient interplanetary disturbances evoke short time cosmic ray flux decrease which is known as Forbush decrease. The traditional model and understanding of Forbush decrease suggest that the substructure of interplanetary counterpart of coronal mass ejection (ICME) independently contributes in cosmic ray flux decrease. These substructures, shock-sheath and magnetic cloud (MC) manifest as classical two-step Forbush decrease. The recent work by Raghav et. al. (2016a) has shown multi-step decreases and recoveries within shock-sheath. However, this can not be explained by ideal shock-sheath barrier model. Further, they suggested that the local structures within the ICMEs sub-structure (MC and shock sheath) could explain this deviation of FD profile from the classical FD. Therefore, present study attempts to investigate the cause of multi-step cosmic ray flux decrease and their respective recovery within shock-sheath in detail. 3D-hodogram method has been utilized to get more details of the local structures within the shock-sheath. A 3D-hodogram method unambiguously suggests the formation of small scale local structures within the ICME (shock-sheath and even in MC). Moreover, the method could differentiate the turbulent and ordered Interplanetary Magnetic Field (IMF) regions within the substructures of ICME. The study explicitly suggests that the turbulent and ordered IMF regions within shock-sheath do influence cosmic-ray variations uniquely.

Torsional Alfvén Wave Embedded ICME Magnetic Cloud and Corresponding Geomagnetic Storm

The energy transfer during the interaction of large-scale solar wind structure and the Earths magnetosphere is the chronic issue in space-weather studies. To understand this, researchers widely studied the geomagnetic storms and sub-storms phenomena. The present understanding suggests that long duration of southward interplanetary magnetic field component is the most important parameter for the geomagnetic storm. Such long duration strong southward magnetic field is often associated with ICMEs, torsional Alfven fluctuations superposed co-rotating interacting regions (CIRs) and fast solar wind streams. Torsional Alfven fluctuations embedded CIRs have been known for a long, however magnetic cloud embedded with such fluctuations are rarely observed. The presence of Alfven waves in the ICME/MC and influence of these waves on the storm evolution remains an interesting topic of study. The present work confirms the torsional Alfven waves in a magnetic cloud associated with a CME launched on 15th February which impacted the Earths magnetosphere on February 18, 2011. Further, observations indicate that these waves inject energy into the magnetosphere during the storm and contribute to the long recovery time of geomagnetic storms. Our study suggests that presence of torsional Alfven waves significantly controls the storm dynamics.

The first in situ observation of torsional Alfvén waves during the interaction of large-scale magnetic clouds

The large-scale magnetic cloud such as coronal mass ejections (CMEs) is the fundamental driver of the space weather. The interaction of the multiple CMEs in interplanetary space affects their dynamic evolution and geo-effectiveness. The complex and merged multiple magnetic clouds appear as the in-situ signature of the interacting CMEs. The Alfven waves are speculated to be one of the major possible energy exchange/dissipation mechanism during the interaction. However, no such observational evidence has been found in the literature. The case studies of CME-CME collision events suggest that the magnetic and thermal energy of the CME is converted into the kinetic energy. Moreover, the magnetic reconnection process is justified to be responsible for the merging of multiple magnetic clouds. Here, we present unambiguous evidence of sunward torsional Alfven waves in the interacting region after the super-elastic collision of multiple CMEs. The Walen relation is used to confirm the presence of Alfven waves in the interacting region of multiple CMEs/magnetic clouds. We conclude that Alfven waves and magnetic reconnection are the possible energy exchange/dissipation mechanisms during large-scale magnetic clouds collisions. The present study has significant implications not only in CME-magnetosphere interactions but also in the interstellar medium where interactions of large-scale magnetic clouds are possible.