Morphological and dynamical properties of small-scale chromospheric features deduced from IBIS observations

Abstract

Context. In the past, chromospheric features were mostly studied by analyzing observations performed along the H α profile, but several aspects related to their formation and dynamics remained uncertain and poorly understood. Recently, new investigations have been carried out using data obtained along the Ca II line at 854.21 nm, providing new inputs for clarification of some of these aspects. Aims. In order to give a further contribution to the comprehension of the physical processes occurring in small-scale chromospheric features, we analyzed high spatial and temporal resolution images, acquired along the Ca II (λ = 854.21 nm) line with the Interferometric Bidimensional Spectrometer (IBIS). We studied four chromospheric structures such as mottles and arches belonging to an arch filament system (AFS) inside a bipolar region, observed on October 3, 2006. Methods. We evaluated the plasma velocity along the line of sight (LOS) using two methods: the Doppler shift of the centroid of the line profile and the cloud model. Also, we deduced the mean temperature, the Doppler width, the optical thickness and the source function in the structures to which we could apply the cloud model. Results. The pattern of the LOS velocity in the four mottles showed different behaviors. A mottle, initially, showed positive and negative velocities in eastern and western endpoints, respectively, then the plasma motion seems to reverse over a period of about 4 mn. In another mottle a motion characterized by alternate upward and downward plasma flow along the main axis was recorded. Irregular upward and downward motions along the other two mottles confirm previous results. The LOS velocities measured in the AFS, observed during the decay phase of the region, are of the same order of magnitude as those measured in short-lived active regions during their emergence phase. Conclusions. The observations carried out in the Ca II line allowed us to obtain information on small-scale magnetic features, like mottles and AFS, observed in the chromosphere. These results, on one hand, confirm previous results obtained using data acquired in the H α line, and on the other hand, provide new clues to the dynamic similarities between mottles and dynamic fibrils. Moreover, this study allowed us to single out the presence of an AFS during a phase characterized by decreasing magnetic flux and the approach of the opposite polarities.