IRIS Observations of Spicules and Structures Near the Solar Limb

We have analyzed Interface Region Imaging Spectrograph (IRIS) spectral and slit-jaw observations of a quiet region near the South Pole. In this article we present an overview of the observations, the corrections, and the absolute calibration of the intensity. We focus on the average profiles of strong (Mg II h and k, C II and Si IV), as well as of weak spectral lines in the near ultraviolet (NUV) and the far ultraviolet (FUV), including the Mg II triplet, thus probing the solar atmosphere from the low chromosphere to the transition region. We give the radial variation of bulk spectral parameters as well as line ratios and turbulent velocities. We present measurements of the formation height in lines and in the NUV continuum from which we find a linear relationship between the position of the limb and the intensity scale height. We also find that low forming lines, such as the Mg II triplet, show no temporal variations above the limb associated with spicules, suggesting that such lines are formed in a homogeneous atmospheric layer and, possibly, that spicules are formed above the height of 2 ''. We discuss the spatio-temporal structure of the atmosphere near the limb from images of intensity as a function of position and time. In these images, we identify p-mode oscillations in the cores of lines formed at low heights above the photosphere, slow-moving bright features in O I and fast-moving bright features in C II. Finally, we compare the Mg II k and h line profiles, together with intensity values of the Balmer lines from the literature, with computations from the PROM57Mg non-LTE model, developed at the Institut d' Astrophysique Spatiale, and estimated values of the physical parameters. We obtain electron temperatures in the range of similar to 8000 K at small heights to similar to 20 000 K at large heights, electron densities from 1.1 x 10(11) to 4 x 10(10) cm(-3) and a turbulent velocity of similar to 24 kms(-1).