On the nature of the chromospheric fine structure - I. Dynamics of dark mottles and grains

We analyze a time series of forty high spatial and temporal resolution two-dimensional intensity and Doppler velocity images at different wavelengths within the Ha line. The observations were obtained with the Multichannel Subtractive Double Pass (MSDP) spectrograph at THEMIS. We study the morphology of dark mottles and grains as seen in different wavelengths and examine their relation to the MDI magnetic field topology. We determine some physical properties of dark mottles with an inversion technique based on an iterative cloud model method with constant source function, giving the optical thickness tau(0), the Doppler width Deltalambda(D), the velocity v and the source function S distribution along a structure. The obtained global properties of mottles as well as the spatial and temporal evolution of several physical parameters along the axes of individual mottles are discussed. The derived velocities in mottles as a function of space and time ( time slice images) exhibit a quasi-periodic, bidirectional pattern. It is suggested that magnetic reconnection is the mechanism responsible for their formation and dynamics. Furthermore, a similar quasi-periodic behaviour of the Doppler velocity variations in dark grains and their morphological characteristics both suggest the similarity of dark mottles and grains.