Chromospheric Peculiar Off-limb Dynamical Events from IRIS Observations
Résumé
To study motions and oscillations in the solar chromosphere and at the transition region level we analyze some extreme Doppler shifts observed off-limb with the Interface Region Imaging Spectrograph (IRIS). Raster scans and slit-jaw imaging observations performed in the near-ultraviolet channels were used. Large transverse oscillations are revealed by the far wings profiles after accurately removing the bulk average line profiles of each sequence. Different regions around the Sun are considered. Accordingly, the cool material of spicules is observed in Mg II lines rather dispersed up to coronal heights. In the quiet Sun and especially in a polar coronal hole, we study dynamical properties of the dispersed spicules-material off-limb using high spectral, temporal, and spatial resolutions IRIS observations. We suggest that numerous small-scale jet-like spicules show rapid twisting and swaying motions evidenced by the large distortion and dispersion of the line profiles, including impressive periodic Doppler shifts. Most of these events repeatedly appear in red and blueshifts above the limb throughout the whole interval of the observation data sets, with an average swaying speed of order ±35 km s-1 reaching a maximum value of 50 km s-1 in the polar coronal hole region, well above the 2.2 Mm heights. We identified for the first time waves with a short period of order of 100 s, and less and transverse amplitudes of order of ±20-30 km s-1 with the definite signature of Alfvén waves. No correlation exists between brightness and Doppler shift variations; the phase speed of the wave is very large and cannot definitely be determined from the spectral features seen along the quasi-radial features. Even shorter periods waves are evidenced, although their contrast is greatly attenuated by the overlapping effects along the line of sight.