Abstract:
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 at tempt to characterise CME propagation from typical coronagraph fields of
view to the Earth.
