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

摘要

Context. In the past, chromospheric features were mostly studied by analyzing observations performed along the Hprofile, but several aspects related to their formation and dynamicsremained uncertain and poorly understood. Recently, new investigationshave been carried out using data obtained along the CaII line at854.21nm, providing new inputs for clarification of some of theseaspects. Aims. In order to give a further contribution to thecomprehension of the physical processes occurring in small-scalechromospheric features, we analyzed high spatial and temporalresolution images, acquired along the CaII (nm)line with the Interferometric Bidimensional Spectrometer (IBIS). Westudied four chromospheric structures such as mottles and archesbelonging to an arch filament system(AFS) inside a bipolarregion, observed on October3, 2006. Methods. We evaluated the plasma velocity along the line ofsight(LOS) using two methods: the Doppler shift of the centroidof the line profile and the cloud model. Also, we deduced the meantemperature, the Doppler width, the optical thickness and the sourcefunction in the structures to which we could apply the cloud model. Results. The pattern of the LOS velocity in the four mottlesshowed different behaviors. A mottle, initially, showed positive andnegative velocities in eastern and western endpoints, respectively,then the plasma motion seems to reverse over a period of about4mn. In another mottle a motion characterized by alternate upwardand downward plasma flow along the main axis was recorded. Irregularupward and downward motions along the other two mottles confirmprevious results. The LOS velocities measured in the AFS, observedduring the decay phase of the region, are of the same order ofmagnitude as those measured in short-lived active regions during theiremergence phase. Conclusions. The observations carried out in the CaII lineallowed us to obtain information on small-scale magnetic features, likemottles and AFS, observed in the chromosphere. These results, on onehand, confirm previous results obtained using data acquired in the Hline, and on the other hand, provide new clues to the dynamicsimilarities between mottles and dynamic fibrils. Moreover, this studyallowed us to single out the presence of an AFS during a phasecharacterized by decreasing magnetic flux and the approach of theopposite polarities. Key words: Sun: chromosphere - Sun: activity