Possible Detection of Torsional Alfvén Waves within an Interplanetary Magnetic Cloud

Although it is believed that Alfvén waves can be present in the form of torsional modes in interplanetary magnetic flux ropes, convincing observational evidence remains elusive. In this Letter, we report the detection of Alfvén waves embedded within an interplanetary magnetic cloud (MC) on 2003 March 20, which exhibited features quite different from those upstream and downstream. The magnetic field inside the MC underwent alternate rotations along an arc through a relatively small angle in the plane perpendicular to the minimum variance direction, which seems consistent with the appearance of torsional modes. A significant poloidal motion of plasma existed in the MC, thus it is possible that the field-aligned helical plasma flow was mixed with Alfvén waves exhibiting high correlation between plasma velocity and the magnetic field.