Properties of shock waves in the quiet-Sun chromosphere

¹ Indian Institute of Astrophysics, II Block, Koramangala, Bengaluru 560 034, India
e-mail: harsh.mathur@iiap.res.in
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
³ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
⁴ Lockheed Martin Solar & Astrophysics Laboratory, Palo Alto, CA 94304, USA
⁵ Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035, USA

Received: 23 June 2022
Accepted: 3 October 2022

Abstract

Context. Short-lived (100 s or less), sub-arcsec to a couple of arcsec sized features of enhanced brightenings in the narrowband images at the H_2V and K_2V positions of the Ca II H&K lines in the quiet Sun are known as bright grains. These bright grains are interpreted as manifestations of acoustic shock waves in the chromosphere.

Aims. We aim to study time-varying stratified atmospheric properties, such as the temperature, line-of-sight (LOS) velocity, and microturbulence inferred from observations of the bright grains during such acoustic shock events.

Methods. With simultaneous observations of a quiet-Sun internetwork region in the Fe I 6173 Å, Ca II 8542 Å, and Ca II K lines acquired by the CRisp Imaging Spectro-Polarimeter and the CHROMospheric Imaging Spectrometer instruments on the Swedish 1-m Solar Telescope, we performed multi-line non-local thermodynamic equilibrium inversions using the STockholm inversion Code to infer the time-varying stratified atmosphere’s physical properties.

Results. The Ca II K profiles of bright grains show enhancement in the K_2V peak intensities with the absence of the K_2R features. At the time of maximum enhancement in the K_2V peak intensities, we found average enhancements in temperature at lower chromospheric layers (at log τ₅₀₀ ≃ −4.2) of about 1.1 kK, with a maximum enhancement of ∼4.5 kK. These temperature enhancements are co-located with upflows, as strong as −6 km s⁻¹, in the direction of the LOS. The LOS velocities at upper chromospheric layers at log τ₅₀₀ < −4.2 show consistent downflows greater than +8 km s⁻¹. The retrieved value of microturbulence in the atmosphere of bright grains is negligible at chromospheric layers.

Conclusions. This study provides observational evidence to support the interpretation that the bright grains observed in narrowband images at the H_2V and K_2V positions of the Ca II H&K lines are manifestations of upward propagating acoustic shocks against a background of downflowing atmospheres.