  The energy transfer during the interaction of large-scale solar wind
structure and the Earth's 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 Earth's 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.
