Spicules and downflows in the solar chromosphere

Abstract

High-speed downflows have been observed in the solar transition region (TR) and lower corona for many decades. Despite their abundance, it has been hard to find signatures of such downflows in the solar chromosphere. In this work, we target an enhanced network region that shows ample occurrences of rapid spicular downflows in the \halpha\ spectral line that could potentially be linked to high-speed TR downflowing counterparts. We used the $k$-means algorithm to classify the spectral profiles of on-disk spicules in \halpha{} and \cak{} data observed from the Swedish 1-m Solar Telescope (SST) and employed an automated detection method based on advanced morphological image processing operations to detect such downflowing features, in conjunction with rapid blue-shifted and red-shifted excursions (RBEs and RREs). We report the existence of a new category of RREs (termed as downflowing RRE) for the first time that, contrary to earlier interpretation, are associated with chromospheric field-aligned downflows moving towards the strong magnetic field regions. Statistical analysis performed on nearly 20,000 RBEs and 15,000 RREs (including the downflowing counterparts), detected in our 97~min long dataset, shows that the downflowing RREs are very similar to RBEs and RREs except for their oppositely directed plane-of-sky motion. Furthermore, we also find that RBEs, RREs and downflowing RREs can be represented by a wide range of spectral profiles with varying Doppler offsets, and \halpha{} line core widths, both along and perpendicular to the spicule axis, that causes them to be associated with multiple substructures that evolve together. We speculate that these rapid plasma downflows could well be the chromospheric counterparts of the commonly observed TR downflows.

arXiv comment24 pages, 16 figures. Accepted for publication in Astronomy & Astrophysics. Some of the figures have been compressed to comply with arXiv requirements

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