We report on a numerical investigation of two coronal mass ejections (CMEs)
which interact as they propagate in the inner heliosphere. We focus on the
effect of the orientation of the CMEs relative to each other by performing four
different simulations with the axis of the second CME rotated by 90 degrees
from one simulation to the next. Each magneto-hydrodynamic (MHD) simulation is
performed in three dimensions (3-D) with the Space Weather Modeling Framework
(SWMF) in an idealized setting reminiscent of solar minimum conditions. We
extract synthetic satellite measurements during and after the interaction and
compare the different cases. We also analyze the kinematics of the two CMEs,
including the evolution of their widths and aspect ratios. We find that the
first CME contracts radially as a result of the interaction in all cases, but
the amount of subsequent radial expansion depends on the relative orientation
of the two CMEs. Reconnection between the two ejecta and between the ejecta and
the interplanetary magnetic field (IMF) determines the type of structure
resulting from the interaction. When a CME with a high inclination with respect
to the ecliptic overtakes one with a low inclination, it is possible to create
a compound event with a smooth rotation in the magnetic field vector over more
than 180 degrees. Due to reconnection, the second CME only appears as an
extended "tail", and the event may be mistaken for a glancing encounter with an
isolated CME. This configuration differs significantly from the one usually
studied of a multiple-magnetic cloud event, which we found to be associated
with the interaction of two CMEs with the same orientation.
