  Massive gas injection (MGI) experiments have been carried out in many
tokamaks to study disruption dynamics and mitigation schemes. Two events often
observed in those experiments are the excitation of the $m=2, n=1$
magnetohydrodynamic (MHD) mode, and the formation of cold bubble structure in
the temperature distribution before the thermal quench (TQ). Here $m$ is the
poloidal mode number, $n$ the toroidal mode number. The physics mechanisms
underlying those phenomena, however, have not been entirely clear. In this
work, our recent NIMROD simulations of the MGI process in a tokamak have
reproduced the main features of both events, which has allowed us to examine
and establish the causal relation between them. In these simulations, the $3/1$
and $2/1$ islands are found to form successively after the arrival of impurity
ion cold front at the corresponding $q = 3$ and $q = 2$ rational surfaces. At
the interface between impurity and plasma, a local thin current sheet forms due
to an enhanced local pressure gradient and moves inward following the gas cold
front, this may contribute to the formation of a dominant $2/1$ mode. Following
the growth of the $2/1$ tearing mode, the impurity penetration into the core
region inside the $q=2$ surface gives rise to the formation of the cold bubble
temperature structure and initiates the final TQ. A subdominant $1/1$ mode
developed earlier near the $q=1$ surface alone does not cause such a cold
bubble formation, however, the exact manner of the preceding impurity
penetration depends on the nature of the $1/1$ mode: kink-tearing or
quasi-interchange.
