Measurements of plasma motions in dynamic fibrils

TL;DR

This study combines Doppler and proper motion measurements from high-resolution solar observations to analyze plasma motions in dynamic fibrils, revealing their coherent behavior and clarifying previous observational ambiguities.

Contribution

It provides the first simultaneous measurement of proper motions and Doppler velocities in dynamic fibrils, demonstrating their plasma motion and explaining differences due to atmospheric response functions.

Findings

Doppler velocities are 2-3 times smaller than proper motion velocities.

Doppler signals originate from a wide range of atmospheric heights.

Proper motions are measured from the bright tops of DFs, indicating local velocities.

Abstract

We present a 40 minute time series of filtergrams from the red and the blue wing of the \halpha line in an active region near the solar disk center. From these filtergrams we construct both Dopplergrams and summed ``line center'' images. Several dynamic fibrils (DFs) are identified in the summed images. The data is used to simultaneously measure the proper motion and the Doppler signals in DFs. For calibration of the Doppler signals we use spatially resolved spectrograms of a similar active region. Significant variations in the calibration constant for different solar features are observed, and only regions containing DFs have been used in order to reduce calibration errors. We find a coherent behavior of the Doppler velocity and the proper motion which clearly demonstrates that the evolution of DFs involve plasma motion. The Doppler velocities are found to be a factor 2--3 smaller than velocities derived form proper motions in the image plane. The difference can be explained by the radiative processes involved, the Doppler velocity is a result of the local atmospheric velocity weighted with the response function. As a result the Doppler velocity originates from a wide range in heights in the atmosphere. This is contrasted by the proper motion velocity which is measured from the sharply defined bright tops of the DFs and is therefore a very local velocity measure. The Doppler signal originates from well below the top of the DF. Finally we discuss how this difference together with the lacking spatial resolution of older observations have contributed to some of the confusion about the identity of DFs, spicules and mottles.