Tokamak density limit disruptions are studied by tomographically analysing soft X-ray emission data using a modified circular harmonic reconstruction technique. Both minor and major disruptions are associated with the presence of dominant m=1/n and m=2/n=1 soft X-ray emissivity perturbations leading to the penetration of a cold bubble-like plasma volume to the centre. The role of mode coupling between the various emission components is examined and the observed behaviour is compared with the results of recent theoretical models