Abstract:
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 Alfvén fluctuations superposed corotating
interacting regions (CIRs), and fast solar wind streams. Torsional Alfvén fluctuations embedded CIRs have been
known of for a long time; however, magnetic clouds embedded with such fluctuations are rarely observed. The
presence of Alfvén 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 Alfvén 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
Alfvén waves significantly controls the storm dynamics.