Microflares and nanoflares in the solar corona

Abstract

Due to the increase in the spatial and temporal resolution of observations of the solar atmosphere, which is mainly associated with progress in space research, we now understand that the Sun’s activity not only is associated with large centers, but also extends to significantly smaller scales. Each new advance in experimental technology over the past 60 years has led to the discovery of more and more numerous and small solar structures: X-ray active regions in the 1960s, hot X-ray points in the 1970s, solar microflares in the 1980s, and finally, from the end of the 20th century, solar nanoflares. At the same time, the total energy release, obtainable from observations, is still insufficient to ensure a balance between heating of the corona and its rapid radiative cooling. For the smallest-scale phenomena, nanoflares, it is still not possible to resolve their structure and mechanism, which raises the question of whether it is correct to classify them as flares. We present a review of the main results obtained so far in the field of small-scale solar activity, mainly microflares and nanoflares, and discuss the main issues that need to be solved in order to move forward.

The faintest solar coronal hard X-rays observed with FOXSI

“…In solar and heliophysics the coronal heating problem relates to the puzzle of identifying and understanding the mechanism(s) causing the corona's temperatures to be multiple thousands of times hotter than the solar surface (e.g., Klimchuk 2006Klimchuk , 2015. Among the various plausible hypotheses proposed, the two strongest candidates are MHD wave dissipation and copious low-energy magnetic reconnections (or nanoflares, a term coined by Parker in 1988) (e.g., Hudson 1991;Bogachev et al 2020). Klimchuk (2006) pointed out that, when examined thoroughly, most plausible coronal heating explanations imply nonthermal heating that happens impulsively on individual flux tubes (strands).…”

Steadiness of Coronal Heating

“…Further implications from the EUI data on reconnection are debatable, because imaging, even at "small" scales of 150 km, is unable to measure scales at which dissipation must occur, except for a possible role for ion viscous dissipation near 10 2 km (Hollweg 1986;Davila 1987). Undaunted by this, we follow arguments using decades of EUV observations (reviewed by Bogachev et al 2020) and explore quantitatively what the findings from EUI imply in terms of elementary units (i.e., "quanta") of energy dissipation.…”

“…In Parker's approach, basic theoretical results in highly conducting plasmas were combined with available observations to propose that flares smaller than those reported by Lin and colleagues may naturally supply a mechanism to deliver ordered magnetic energy into heat via the formation of elementary current sheets (Low 2023). Parker's approach has survived intense scrutiny and prompted a significant community to seek signatures of nanoflares (e.g., Bogachev et al 2020). Nanoflares have therefore been accepted as a likely candidate to explain heating of long-lived coronal structures (e.g., Pontin & Priest 2022).…”

KORTES Mission for Solar Activity Monitoring Onboard International Space Station

“…The monochromatic X-ray Bragg-crystal Imager (MXI) is a simple, yet a very powerful device. The instruments of this type proved their effectiveness for hot plasma imaging in the previous XRAS missions (Zhitnik et al, 2003;Bogachev et al, 2020). MXI is aimed to image the Sun in a single line of hydrogen-like Mg XII-Ly α 8.42 Å.…”