Ca II 8542 Å brightenings induced by a solar microflare ^⋆

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: ckuckein@aip.de
² Universität Potsdam, Institut für Physik und Astronomie, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
³ Astronomical Institute of the Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovak Republic
⁴ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
⁵ Instituto de Astrofísica de Canarias, Avda vía Láctea S/N, 38205 La Laguna, Tenerife, Spain
⁶ Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 298, 25165 Ondřejov, Czech Republic

Received: 6 June 2017
Accepted: 14 September 2017

Abstract

Aims. We study small-scale brightenings in Ca ii 8542 Å line-core images to determine their nature and effect on localized heating and mass transfer in active regions.

Methods. High-resolution two-dimensional spectroscopic observations of a solar active region in the near-infrared Ca ii 8542 Å line were acquired with the GREGOR Fabry-Pérot Interferometer attached to the 1.5-m GREGOR telescope. Inversions of the spectra were carried out using the NICOLE code to infer temperatures and line-of-sight (LOS) velocities. Response functions of the Ca ii line were computed for temperature and LOS velocity variations. Filtergrams of the Atmospheric Imaging Assembly (AIA) and magnetograms of the Helioseismic and Magnetic Imager (HMI) were coaligned to match the ground-based observations and to follow the Ca ii brightenings along all available layers of the atmosphere.

Results. We identified three brightenings of sizes up to 2′′ × 2′′ that appeared in the Ca ii 8542 Å line-core images. Their lifetimes were at least 1.5 min. We found evidence that the brightenings belonged to the footpoints of a microflare (MF). The properties of the observed brightenings disqualified the scenarios of Ellerman bombs or Interface Region Imaging Spectrograph (IRIS) bombs. However, this MF shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and MFs are both apparent in chromospheric and coronal layers according to the AIA channels; and (2) both show flaring arches with lifetimes of about 3.0–3.5 min and lengths of ~20′′ next to the brightenings. The inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. Connecting the footpoints, a dark filamentary structure appeared in the Ca ii line-core images. Before the start of the MF, the spectra of this structure already indicated average blueshifts, meaning upward motions of the plasma along the LOS. During the impulsive phase, these velocities increased up to − 2.2 km s^-1. The structure did not disappear during the observations. Downflows dominated at the footpoints. However, in the upper photosphere, slight upflows occurred during the impulsive phase. Hence, bidirectional flows are present in the footpoints of the MF.

Conclusions. We detected Ca ii brightenings that coincided with the footpoint location of an MF. The MF event led to a rise of plasma in the upper photosphere, both before and during the impulsive phase. Excess mass, previously raised to at most chromospheric layers, slowly drained downward along arches toward the footpoints of the MF.