Multi-thermal atmosphere of a mini-solar flare during magnetic reconnection observed with IRIS

Abstract

Context. The Interface Region Imaging Spectrograph (IRIS) with its high spatial and temporal resolution facilitates exceptional plasma diagnostics of solar chromospheric and coronal activity during magnetic reconnection. Aims. The aim of this work is to study the fine structure and dynamics of the plasma at a jet base forming a mini-flare between two emerging magnetic fluxes (EMFs) observed with IRIS and the Solar Dynamics Observatory instruments. Methods. We proceed to a spatio-temporal analysis of IRIS spectra observed in the spectral ranges of Mg II, C II, and Si IV ions. Doppler velocities from Mg II lines were computed using a cloud model technique. Results. Strong asymmetric Mg II and C II line profiles with extended blue wings observed at the reconnection site (jet base) are interpreted by the presence of two chromospheric temperature clouds: one explosive cloud with blueshifts at 290 km s−1 and one cloud with smaller Doppler shift (around 36 km s−1). Simultaneously at the same location (jet base), strong emission of several transition region lines (e.g. O IV and Si IV), emission of the Mg II triplet lines, and absorption of identified chromospheric lines in Si IV broad profiles have been observed and analysed. Conclusions. Such observations of IRIS line and continuum emissions allow us to propose a stratification model for the white light, mini-flare atmosphere with multiple layers of different temperatures along the line of sight in a reconnection current sheet. It is the first time that we could quantify the fast speed (possibly Alfvénic flows) of cool clouds ejected perpendicularly to the jet direction via the cloud model technique. We conjecture that the ejected clouds come from plasma which was trapped between the two EMFs before reconnection or be caused by chromospheric-temperature (cool) upflow material similar to a surge during reconnection.

Balmer continuum enhancement detected in a mini flare observed with IRIS

“…In this paper we extend the IRIS data analysis of the mini flare studied in Joshi et al (2020Joshi et al ( , 2021 and present the Balmer continuum around 2832 Å observed with IRIS (Sect. 2).…”

“…In the 2832 continuum filter, small bright dots are observed at the location of the mini flare (green and blue boxes). For an accurate comparison between the AIA images and IRIS SJIs, we manually aligned these images by shifting the IRIS SJIs by 4 in x-axis and 3 in y-axis, as noted in the previous paper (Joshi et al 2021). The relation between heliographic coordinates and pixels along the slit of the spectra is shown in Fig.…”

“…The IRIS Mg II spectra during the mini flare were analyzed in Joshi et al (2021). Bidirectional flows (±200 km s −1 ) were observed at the flare reconnection site, covering one or two pixels y = 79−80 at 02:03:46 UT.…”

“…The proper motions of the photospheric magnetic polarities suggested that the twist of the flux rope was transferred during reconnection to the jet, generating a twisted jet. A detailed spectroscopic analysis of the mini flare associated with this jet was carried out with IRIS spectra (Joshi et al 2021), Using the cloud model technique applied to the Mg II line profiles, we identified explosive clouds with supersonic Alfvén velocities. A stratification thermal model of the atmosphere at the time of the reconnection was proposed.…”

High Resolution Soft X-ray Spectroscopy and the Quest for the Hot (5–10 MK) Plasma in Solar Active Regions

“…An understanding of the physics of microflares remains elusive, as key spatially-resolved spectroscopic observations have been lacking, and given that they have peak temperatures in the 4-8 MK range (see e.g., Feldman et al, 1996;Hannah et al, 2019;Mitra-Kraev and Del Zanna, 2019;Cooper et al, 2020;Vadawale et al, 2021 1 ) which have largely been unexplored by previous and current imaging spectrometers. Consequently, only a few models of microflare loops and associated events have been developed (see e.g., Testa and Reale, 2020;Joshi et al, 2021). Some information has been provided with imaging spectroscopy of Bremsstrahlung emission with e.g., NuSTAR and FOXSI-2.…”