  Investigating the heliospheric evolution and consequences of Coronal mass
ejections (CMEs) is critical to understanding the solar-terrestrial
relationship. For the first time, Heliospheric Imagers (HIs) onboard STEREO,
providing multiple views of CMEs in the heliosphere, observed the vast and
crucial observational gap between the Sun and the Earth. Using J-maps
constructed from coronagraphs (CORs) and HIs observations, we continuously
tracked different density enhanced features of CMEs. We implemented several
reconstruction methods to estimate the three-dimensional (3D) kinematics of
CMEs during their evolution from the Sun to Earth. Our study provides evidence
that the 3D speeds of CMEs near the Sun are not reasonably sufficient for
understanding the propagation and accurate forecasting of the arrival time at
the Earth of a majority of CMEs. This finding can be due to many factors that
significantly change the CME kinematics beyond the COR field of view, such as
the interaction/collision of two or more CMEs or the interaction of CMEs with
the ambient solar wind medium. We attempted to understand the evolution and
consequences of the interacting/colliding CMEs in the heliosphere using
STEREO/HI, WIND, and ACE observations. The study found a significant change in
the dynamics of the CMEs after their collision and interaction. The in situ
observations show the signatures of CME-CME interaction as heating and
compression, formation of magnetic holes (MHs), and interaction region (IR). We
also noticed that long-lasting IR, formed at the rear edge of preceding CME, is
responsible for large geomagnetic perturbations. Our study highlights the
significance of using HIs observations in studying heliospheric evolution of
CMEs, CME-CME collision, identifying and associating the three-part structure
of CMEs in their remote and in situ observations, and hence for improved space
weather forecasting.
