The two sketches above show the outline of the edge at different times as documented by satellite photos, and the graphs in the lower left show measured rotation rates. Details are in the Nature paper.
By analogy with meteorologic cyclones, Chakraborty et al. called the eruption plume a "volcanic mesocyclone". Three key elements interact to produce tornadic structures such as dust devils and waterspouts: updraft in the center, downdraughts, and the rotating mesocyclone. Chakraborty et al. proposed that the mesocyclone pulls the ash radially outwards from the core of the updraught, gathering it in an outer sheath where it discharges to produce the spectacular lightning displays that can accompany volcanic eruptions. The color image shows such lightning in the current eruption of Eyjafjallajokull in Iceland. It was taken by Marco Fulle. The other image shows (a) waterspouts spawned during the eruption of Surtsey volcano on November 14, 1963, and (b) the lightning sheath from Mount Chaiten on May 3, 2008.