Spectroscopic measurements of dynamic fibrils in the Ca {\small{II}} 8662 Å line

Abstract

We present high spatial resolution spectroscopic measurements of dynamic fibrils (DFs) in the Ca {\small{II}} 8662 Å line. These data show clear Doppler shifts in the identified DFs, which demonstrates that at least a subset of DFs are actual mass motions in the chromosphere. A statistical analysis of 26 DFs reveals a strong and statistically significant correlation between the maximal velocity and the deceleration. The range of the velocities and the decelerations are substantially lower, about a factor two, in our spectroscopic observations compared to the earlier results based on proper motion in narrow band images. There are fundamental differences in the different observational methods; when DFs are observed spectroscopically the measured Doppler shifts are a result of the atmospheric velocity, weighted with the response function to velocity over an extended height. When the proper motion of DFs is observed in narrow band images, the movement of the top of the DF is observed. This point is sharply defined because of the high contrast between the DF and the surroundings. The observational differences between the two methods are examined by several numerical experiments using both numerical simulations and a time series of narrow band H$α$ images. With basis in the simulations we conclude that the lower maximal velocity is explained by the low formation height of the Ca IR line. We conclude that the present observations support the earlier result that DFs are driven by magneto-acoustic shocks exited by convective flows and p-modes.