  Our understanding of the process of fast reconnection has undergone a
dramatic change in the last 10 years driven, in part, by the availability of
high-resolution numerical simulations that have consistently demonstrated the
break-up of current sheets into magnetic islands, with reconnection rates that
become independent of Lundquist number, challenging the belief that fast
magnetic reconnection in flares proceeds via the Petschek mechanism that
invokes pairs of slow-mode shocks connected to a compact diffusion region. The
reconnection sites are too small to be resolved with images but these
reconnection mechanisms, Petschek and the plasmoid instability, have
reconnection sites with very different density and velocity structures and so
can be distinguished by high-resolution line-profiles observations. Using IRIS
spectroscopic observations we obtain a survey of typical line profiles produced
by small-scale events thought to be reconnection sites on the Sun. Slit-jaw
images are used to investigate the plasma heating and re-configuration at the
sites. A sample of 15 events from two active regions is presented. The line
profiles are complex with bright cores and broad wings extending to over 300
km/s. The profiles can be reproduced with the multiple magnetic islands and
acceleration sites that characterise the plasmoid instability but not by
bi-directional jets that characterise the Petschek mechanism. This result
suggests that if these small-scale events are reconnection sites, then fast
reconnection proceeds via the plasmoid instability, rather than the Petschek
mechanism during small-scale reconnection on the Sun.
