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We investigated the off-limb spicules observed in the Mg II h and k lines byIRIS in a solar polar coronal hole. We analyzed the large dataset of obtainedspectra to extract quantitative information about the line intensities, shifts,and widths. The observed Mg II line profiles are broad and double-peaked atlower altitudes, broad but flat-topped at middle altitudes, and narrow andsingle-peaked with the largest Doppler shifts at higher altitudes. We use 1Dnon-LTE vertical slab models (i.e. models which consider departures from LocalThermodynamic Equilibrium) in single-slab and multi-slab configurations tointerpret the observations and to investigate how a superposition of spiculesalong the line of sight (LOS) affects the synthetic Mg II line profiles. Theused multi-slab models are either static, i.e. without any LOS velocities, orassume randomly assigned LOS velocities of individual slabs, representing thespicule dynamics. We conducted such single-slab and multi-slab modeling for abroad set of model input parameters and showed the dependence of the Mg II lineprofiles on these parameters. We demonstrated that the observed line widths ofthe h and k line profiles are strongly affected by the presence of multiplespicules along the LOS. We later showed that the profiles obtained at higheraltitudes can be reproduced by single-slab models representing individualspicules. We found that the multi-slab model with a random distribution of theLOS velocities ranging from -25 to 25 km s$^{-1}$ can well reproduce the widthand the shape of Mg II profiles observed at middle altitudes.

IRIS Mg II Observations and Non-LTE Modeling of Off-limb Spicules in a Solar Polar Coronal Hole