  Spectral observations of the solar transition region (TR) and corona show
broadening of spectral lines beyond what is expected from thermal and
instrumental broadening. The remaining non-thermal broadening is significant
(5-30 km/s) and correlated with the intensity. Here we study spectra of the TR
Si IV 1403 Angstrom line obtained at high resolution with the Interface Region
Imaging Spectrograph (IRIS). We find that the large improvement in spatial
resolution (0.33 arcsec) of IRIS compared to previous spectrographs (2 arcsec)
does not resolve the non-thermal line broadening which in most regions remains
at pre-IRIS levels of about 20 km/s. This invariance to spatial resolution
indicates that the processes behind the broadening occur along the
line-of-sight (LOS) and/or on spatial scales (perpendicular to the LOS) smaller
than 250 km. Both effects appear to play a role. Comparison with IRIS
chromospheric observations shows that, in regions where the LOS is more
parallel to the field, magneto-acoustic shocks driven from below impact the TR
and can lead to significant non-thermal line broadening. This scenario is
supported by MHD simulations. While these do not show enough non-thermal line
broadening, they do reproduce the long-known puzzling correlation between
non-thermal line broadening and intensity. This correlation is caused by the
shocks, but only if non-equilibrium ionization is taken into account. In
regions where the LOS is more perpendicular to the field, the prevalence of
small-scale twist is likely to play a significant role in explaining the
invariance and correlation with intensity.
