  The dynamics of active region (AR) 12673 is qualitatively studied using
observational data obtained with the Helioseismic and Magnetic Imager of the
Solar Dynamics Observatory on August 31--September 8, 2017. This AR was
remarkable for its complex structure and extraodinary flare productivity. The
sunspot group in this AR consisted of (1) an old, well-developed and highly
stable, coherent sunspot, which had also been observed two solar rotations
earlier, and (2) a rapidly developing cluster of umbral and penumbral
fragments. Cluster (2) formed two elongated, arc-shaped chains of spot
elements, skirting around the major sunspot (1), with two chains of magnetic
elements spatially coinciding with the arcs. AR components (1) and (2) were in
relative motion, cluster (2) overtaking spot (1), and their relative velocity
agrees in order of magnitude with the velocity jump over the near-surface shear
layer, or leptocline. The pattern of motion of the features about the main spot
bears amazing resemblance to the pattern of a fluid flow about a roundish body.
This suggests that spot (1) was dynamically coupled with the surface layers,
while cluster (2) developed in deeper layers of the convection zone. The
magnetic-flux emergence in cluster (2) appeared to be associated with fluid
motions similar to roll convection. The mutual approach of components (1) and
(2) gave rise to lights bridges in the umbrae of sunspots with the magnetic
field having the same sign on both sides of the bridge.
