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Context. Solar eruptions and high flare activity often accompany the rapidrotation of sunspots. The study of sunspot rotation and the mechanisms drivingthis motion are therefore key to our understanding of how the solar atmosphereattains the conditions necessary for large energy release.  Aims. We aim to demonstrate and investigate the rotation of sunspots in a 3Dnumerical experiment of the emergence of a magnetic flux tube as it risesthrough the solar interior and emerges into the atmosphere. Furthermore, weseek to show that the sub-photospheric twist stored in the interior is injectedinto the solar atmosphere by means of a definitive rotation of the sunspots.  Methods. A numerical experiment is performed to solve the 3D resistivemagnetohydrodynamic (MHD) equations using a Lagrangian-Remap code. We track theemergence of a toroidal flux tube as it rises through the solar interior andemerges into the atmosphere investigating various quantities related to boththe magnetic field and plasma.  Results. Through detailed analysis of the numerical experiment, we find clearevidence that the photospheric footprints or sunspots of the flux tube undergoa rotation. Significant vertical vortical motions are found to develop withinthe two polarity sources after the field emerges. These rotational motions arefound to leave the interior portion of the field untwisted and twist up theatmospheric portion of the field. This is shown by our analysis of the relativemagnetic helicity as a significant portion of the interior helicity istransported to the atmosphere. In addition, there is a substantial transport ofmagnetic energy to the atmosphere. Rotation angles are also calculated bytracing selected fieldlines; the fieldlines threading through the sunspot arefound to rotate through angles of up to 353 degrees over the course of theexperiment.

Sunspot rotation. I. A consequence of flux emergence