  A vortex in a trapped Bose-Einstein condensate can experience at least two
types of instabilities. (1). Macroscopic hydrodynamic motion of the vortex core
relative to the center of mass of the condensate requires some process to
dissipate energy. (2). Microscopic small-amplitude normal modes can also induce
an instability. In one specific example, the vortex core again moves relative
to the overall center of mass, suggesting that there may be only a single
physical mechanism.
