Solar microflares: a case study on temperatures and the Fe XVIII emission

Abstract

In this paper, we discuss the temperature distribution and evolution of a microflare, simultaneously observed by Hinode XRT, EIS, and SDO AIA. We find using EIS lines that during peak emission the distribution is nearly isothermal and peaked around 4.5 MK. This temperature is in good agreement with that obtained from the XRT filter ratio, validating the use of XRT to study these small events, invisible by full-Sun X-ray monitors such as GOES. The increase in the estimated Fe xviii emission in the AIA 94 Å band can mostly be explained with the small temperature increase from the background temperatures. The presence of Fe xviii emission does not guarantee that temperatures of 7 MK are reached, as is often assumed. We also revisit with new atomic data the temperatures measured by a SoHO SUMER observation of an active region which produced microflares, also finding low temperatures (3-4 MK) from an Fe xviii/Ca xiv ratio.

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

“…For the flaring emission, there is ample evidence that the cooler lines, e.g., those formed below 3 MK, are mostly not affected during the heating and initial cooling phase of an event. The post-flare loops are seen to be progressively filled in by the hot plasma, and it is only during the following cooling of the plasma that lowertemperature lines are observed, as e.g., shown in the case of a microflare by Mitra-Kraev and Del Zanna (2019). Therefore, with spatially-resolved spectroscopy, we do not expect the background emission to interfere with the emission of the 5-10 MK lines.…”

“…during the peak emission of a microflare recorded by EIS (Mitra-Kraev and Del Zanna, 2019). We note that the actual peak temperature of the post-flare loops was about 5 MK, although short-lived higher temperatures were probably present during the impulsive phase in one of the footpoints (see also Testa and Reale, 2020).…”

“…We note that microflares often are composed of a few loop structures which appear resolved at 1 ′′ resolution in Fe XVIII emission within the AIA 94 Å band (Del Zanna, 2012Mitra-Kraev and Del Zanna, 2019). Therefore, although higher spatial resolution could show unresolved structures (if present), 1 ′′ resolution would be sufficient to follow the evolution of the main structures.…”

“…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).…”

Evolution of Elemental Abundances During B-Class Solar Flares: Soft X-ray Spectral Measurements with Chandrayaan-2 XSM

“…As detailed observations have been lacking, it is not clear if the evolution of these smaller events follows the standard flare model. It is only recently, with the advent of better instrumentation, that in-depth studies of smaller flares are becoming feasible (Kuhar et al 2018;Mitra-Kraev and Del Zanna 2019;Athiray et al 2020;Cooper et al 2020;Glesener et al 2020;Duncan et al 2021;Vadawale et al 2021a). Lately, using X-ray time resolved spectroscopy, Narendranath et al (2020) have carried out abundance studies of flares as small as GOES B9-class.…”