  Aims: Using 2D Mgii h&k solar prominence modelling, our aim is to understand
the formation of complex line profiles and how these are seen by the Interface
Region Imaging Spectrograph (IRIS). Additionally, we see how the properties of
these simulated observations are interpreted through the use of traditional 1D
prominence modelling.
  Methods: We used a cylindrical non-local thermodynamic equilibrium (NLTE) 2D
complete redistribution (CRD) code to generate a set of cylindrical prominence
strands, which we stacked behind each other to produce complex line profiles.
Then, with the use of the point spread functions (PSFs) of IRIS, we were able
to predict how IRIS would observe these line profiles. We then used the 1D NLTE
code PROM in combination with the Cross Root Mean Square method (xRMS) to find
the properties recovered by traditional 1D prominence modelling.
  Results: Velocities of magnitude lower than 10 km/s are sufficient to produce
asymmetries in the Mgii h&k lines. However, convolution of these with the PSFs
of IRIS obscures this detail and returns standard looking single peaks. By
increasing the velocities by a factor of three, we recover asymmetric profiles
even after this convolution. The properties recovered by xRMS appear adequate
at first, but the line profiles chosen to fit these profiles do not
satisfactorily represent the line profiles. This is likely due to the large
line width of the simulated profiles.
  Conclusions: Asymmetries can be introduced by multithread models with
independent Doppler velocities. The large line width created by these models
makes it difficult for traditional 1D forward modelling to find good matches.
This may also demonstrate degeneracies in the solution recovered by
single-species 1D modelling.
