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

We investigated the off-limb spicules observed in the Mg II h and k lines by IRIS in a solar polar coronal hole. The observed h and k line profiles are broad and double-peaked with a smaller shift of the line center at lower altitudes, while profiles at higher altitudes are narrower and single-peaked with a larger Doppler shift. We investigated how a superposition of spicules along the line-of-sight (LOS) can affect the synthetic Mg II h and k line profiles. First, we synthesized the h and k line profiles by solving the non-LTE radiative transfer in an isolated vertical one-dimensional slab. The slab consists of uniform plasma with a given temperature, pressure, geometrical thickness, radial velocity and micro-turbulent velocity and is illuminated from the solar disk. In addition, we calculated synthetic line profiles from a multiple-slab model where each slab has a randomly assigned line-of-sight (LOS) velocity representing the spicule dynamics. We conducted such single-slab and multi-slab modeling for a broad set of model input parameters and show the dependence of the Mg II line profiles on these parameters. We demonstrated that the observed line width of the Mg II h and k line profiles is strongly affected by the presence of multiple spicules along the LOS, especially at the lower altitudes. We also found that the multi-slab model with a random distribution of the LOS velocities from the interval of -25 to 25 km/s can well reproduce the width and the shape of Mg II profiles observed at lower altitudes.