  This study examines the tail disconnection event on April 20, 2007 on comet
2P/Encke, caused by a coronal mass ejection (CME) at a heliocentric distance of
0.34 AU. During their interaction, both the CME and the comet are visible with
high temporal and spatial resolution by the STEREO-A spacecraft. Previously,
only current sheets or shocks have been accepted as possible reasons for comet
tail disconnections, so it is puzzling that the CME caused this event. The MHD
simulation presented in this work reproduces the interaction process and
demonstrates how the CME triggered a tail disconnection in the April 20 event.
It is found that the CME disturbs the comet with a combination of a $180^\circ$
sudden rotation of the interplanetary magnetic field (IMF), followed by a
$90^\circ$ gradual rotation. Such an interpretation applies our understanding
of solar wind-comet interactions to determine the \textit{in situ} IMF
orientation of the CME encountering Encke.
