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.