  Owing to our dependance on spaceborne technology, an awareness of
disturbances in the near-Earth space environment is proving to be increasingly
crucial. Earth-directed Coronal mass ejections (CMEs) emanating from the Sun
are the primary drivers of space weather disturbances. Studies of CMEs, their
kinematics, and their near-Earth effects are therefore gaining in importance.
  The effect of CMEs near the Earth is often manifested as transient decreases
in galactic cosmic ray intensity, which are called Forbush decreases (FDs). In
this thesis we probe the structure of CMEs and their associated shocks using FD
observations by the GRAPES-3 muon telescope at Ooty. We have established that
the cumulative diffusion of galactic cosmic rays into the CME is the dominant
mechanism for causing FDs (Chapter 3).
  This diffusion takes place through a turbulent sheath region between the CME
and the shock. One of our main results concerns the turbulence level in this
region. We have quantitatively established that cross-field diffusion aided by
magnetic field turbulence accounts for the observed lag between the FD and the
magnetic field enhancement of the sheath region (Chapter 4 ).
  We have also investigated the nature of the driving forces acting on CMEs in
this thesis. Using CME data from the SECCHI coronagraphs aboard STEREO
sapcecraft, we have found evidence for the non-force-free nature of the
magnetic field configuration inside these CMEs, which is the basis for the
(often-invoked) Lorentz self-force driving (Chapter 5).
  Taken together the work presented in this thesis is a comprehensive attempt
to characterise CME propagation from typical coronagraph fields of view to the
Earth.
