Sometimes, it can get pretty mathematical in solar physics too. Opposite formula describes the dynamical evolution of a vortical or swirling motion, of which there exists various types in the solar atmosphere. A vortex or swirl can be intuitively described as the rotation of fluid parcels around a common axis. Despite this simple concept, a rigorous mathematical definition is still an open issue in fluid mechanics. An effective physical quantity for characterising swirls is the swirling strength for which we have now derived the corresponding dynamical equation.

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