Excess-density-driven snakes in tokamaks

``Snakes'' refer to sinusoidal patterns observed on space-time plots of soft-X-ray signals in tokamak plasmas. They are generally attributed to persistent and localized density perturbations that form at a rational surface after pellet injection (Weller, JET 1987), and Parker, Alcator-C 1987), or impurity accumulation (Naujoks, ASDEX 1996, Delgado-Aparicio, C-Mod 2011). It is not clear whether all snake observations have a unique origin. A likely explanation is that material trapped inside an island driven by a temperature hole leads to the observed soft-X-ray signals (Wesson 1995). More recently, it has been suggested that they could be the result of saturated nonlinear internal kinks in low, or reversed-shear geometries (Cooper 2011). We have started an examination of some of these issues using ideas from neoclassical transport theory (Shaing 2007) in conjunction with various magnetohydrodynamic models. In a RMHD model, we demonstrated that excess-density-driven bootstrap current can stabilize a resistive $m=1$ island at a small amplitude, leaving a radially and poloidally localized snake-like structure. Extension of this work to more sophisticated models that include diamagnetic effects, and possibly more realistic geometries, will be presented.