  Observed spectral profiles of emission lines from the corona are found to
have widths exceeding the thermal line width. To investigate the physical
mechanism, we run a 3D MHD model of a single, straightened loop in which we
partially resolve turbulent motions that form in response to the driving by
self-consistently evolving magneto-convection in the photosphere. The
convective motions shear and twist the magnetic field lines, leading to
heating. From the model we synthesize spectral profiles of emission lines
forming at temperatures around and above 1 MK. The coronal heating process
generates a range of velocity amplitudes and directions structured on a scale
much smaller than the resolving power of current instruments, leading to a
broadening of the spectral lines. Our model includes the mass exchange between
corona and chromosphere, thus we also capture flows parallel to the loop axis.
We find that the spectral lines show a non-thermal line broadening roughly
consistent with observations for a viewing angle perpendicular to the axis. The
broadening through field-parallel flows is comparable, although slightly
smaller. The line broadening is independent of the instrument resolution for a
perpendicular line-of-sight. We can connect the non-thermal line broadening to
heating events and flows. While small-scale velocities along the line-of-sight
are mainly responsible for the broadening observed perpendicular to the loop,
chromospheric evaporation is important for the line broadening observed along
the loop. The model reproduces observed values for non-thermal line widths. In
the model these result from continuous driving by magnetoconvection, without
imposing driving motions or starting from an already braided field.
