  Context. In the upper atmosphere of the Sun observations show convincing
evidence for crossing and twisted structures, which are interpreted as mutual
helicity and self-helicity. Aims. We use observations with the new Interface
Region Imaging Spectrograph (IRIS) to show the conversion of mutual helicity
into self-helicity in coronal structures on the Sun. Methods. Using far UV
spectra and slit-jaw images from IRIS and coronal images and magnetograms from
SDO, we investigated the evolution of two crossing loops in an active region,
in particular, the properties of the Si IV line profile in cool loops. Results.
In the early stage two cool loops cross each other and accordingly have mutual
helicity. The Doppler shifts in the loops indicate that they wind around each
other. As a consequence, near the crossing point of the loops (interchange)
reconnection sets in, which heats the plasma. This is consistent with the
observed increase of the line width and of the appearance of the loops at
higher temperatures. After this interaction, the two new loops run in parallel,
and in one of them shows a clear spectral tilt of the Si IV line profile. This
is indicative of a helical (twisting) motion, which is the same as to say that
the loop has self-helicity. Conclusions. The high spatial and spectral
resolution of IRIS allowed us to see the conversion of mutual helicity to
self-helicity in the (interchange) reconnection of two loops. This is
observational evidence for earlier theoretical speculations.
