Evidence of the multi-thermal nature of spicular downflows

Impact on solar atmospheric heating^⋆

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
e-mail: souvik.bose@astro.uio.no
² Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
³ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁵ Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA 94304, USA
⁶ Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035, USA

Received: 27 May 2021
Accepted: 4 August 2021

Abstract

Context. Spectroscopic observations of the emission lines formed in the solar transition region commonly show persistent downflows on the order of 10−15 km s⁻¹. The cause of such downflows, however, is still not fully clear and has remained a matter of debate.

Aims. We aim to understand the cause of such downflows by studying the coronal and transition region responses to the recently reported chromospheric downflowing rapid redshifted excursions (RREs) and their impact on the heating of the solar atmosphere.

Methods. We have used two sets of coordinated data from the Swedish 1 m Solar Telescope, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory for analyzing the response of the downflowing RREs in the transition region and corona. To provide theoretical support, we use an already existing 2.5D magnetohydrodynamic simulation of spicules performed with the Bifrost code.

Results. We find ample occurrences of downflowing RREs and show several examples of their spatio-temporal evolution, sampling multiple wavelength channels ranging from the cooler chromospheric to the hotter coronal channels. These downflowing features are thought to be likely associated with the returning components of the previously heated spicular plasma. Furthermore, the transition region Doppler shifts associated with them are close to the average redshifts observed in this region, which further implies that these flows could (partly) be responsible for the persistent downflows observed in the transition region. We also propose two mechanisms – (i) a typical upflow followed by a downflow and (ii) downflows along a loop –from the perspective of a numerical simulation that could explain the ubiquitous occurrence of such downflows. A detailed comparison between the synthetic and observed spectral characteristics reveals a distinctive match and further suggests an impact on the heating of the solar atmosphere.

Conclusions. We present evidence that suggests that at least some of the downflowing RREs are the chromospheric counterparts of the transition region and lower coronal downflows.