Torsional Alfvén wave embedded ICME magnetic cloud and corresponding geomagnetic storm

Energy transfer during the interaction of large-scale solar wind structure and the Earth's magnetosphere is a chronic issue in space-weather studies. To understand this, researchers widely studied the geomagnetic storm and substorm phenomena. The present understanding suggests that the long duration of the southward interplanetary magnetic field component is the most important parameter for the geomagnetic storm. Such a long duration strong southward magnetic field is often associated with ICMEs, torsional Alfven fluctuations superposed corotating interacting regions (CIRs), and fast solar wind streams. Torsional Alfven fluctuations embedded CIRs have been known of for a long time; however, magnetic clouds embedded with such fluctuations are rarely observed. The presence of Alfven waves in the ICME/MC and the 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 2011 February 15, which impacted the Earth's magnetosphere on 2011 February 18. Furthermore, 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 the presence of torsional Alfven waves significantly controls the storm dynamics.