Thermal-nonthermal energy partition in solar flares derived from X-ray, EUV, and bolometric observations

Abstract

Context. In solar flares, energy is released impulsively and is partly converted into thermal energy of hot plasmas and kinetic energy of accelerated nonthermal particles. It is crucial to constrain the partition of these two energy components to understand energy release and transport as well as particle acceleration in solar flares. Despite numerous efforts, no consensus on quantifying this energy balance has yet been reached. Aims. We aim to understand the reasons for the contradicting results on energy partition obtained by various recent studies. The overarching question we address is whether there is sufficient energy in nonthermal particles to account for the thermal flare component. Methods. We considered five recent studies that address the thermal-nonthermal energy partition in solar flares. Their results are reviewed, and their methods are compared and discussed in detail. Results. The main uncertainties in deriving the energy partition are identified as (a) the derivation of the differential emission measure (DEM) distribution and (b) the role of the conductive energy loss for the thermal component, as well as (c) the determination of the low-energy cutoff for the injected electrons. The bolometric radiated energy, as a proxy for the total energy released in the flare, is a useful independent constraint on both thermal and nonthermal energetics. In most of the cases, the derived energetics are consistent with this constraint. There are indications that the thermal-nonthermal energy partition changes with flare strength: in weak flares, there appears to be a deficit of energetic electrons, while the injected nonthermal energy is sufficient to account for the thermal component in strong flares. This behavior is identified as the main cause of the dissimilar results in the studies we considered. The changing partition has two important consequences: (a) an additional direct (i.e. non-beam) heating mechanism has to be present, and (b) considering that the bolometric emission originates mainly from deeper atmospheric layers, conduction or waves are required as additional energy transport mechanisms.

Fast plasmoid-mediated reconnection in a solar flare

“…High-resolution imaging observations provide rich information about details of the reconnection process which result from its internal dynamics, especially the formation of plasmoids 21 , 22 and turbulent structures 23 , and from the complexity and three-dimensional (3D) nature of the solar magnetic field 24 , 25 . Spectral and multi-wavelength data reveal aspects of the microscopic processes, e.g., particle acceleration 26 , 27 , the nonthermal-thermal energy partition 28 , the plasmoid instability 29 , 30 , and bursty 31 or turbulent reconnection 32 , 33 , in addition to flows 34 . Finally, besides hot flare plasmas, the Sun also allows the reconnection of cool chromospheric structures to be studied 24 , 35 .…”

A model of double coronal hard X-ray sources in solar flares

“…During solar flares, magnetic energy accumulated in the solar corona is explosively released via magnetic reconnection and converted in the form of particle acceleration, plasma heating, and bulk flows (Shibata & Magara 2011;Benz 2017). An enormous number of electrons can be accelerated to high energies (up to tens of MeV) and contain a significant fraction of the dissipated magnetic energy (Lin & Hudson 1976;Emslie et al 2012;Aschwanden et al 2017;Warmuth & Mann 2020). High-energy electrons can further produce hard X-ray (HXR) emission via the bremsstrahlung mechanism and microwave emission via the gyrosynchrotron mechanism.…”

Balmer continuum enhancement detected in a mini flare observed with IRIS

“…In large flares, it has been shown that the injected nonthermal energy derived from RHESSI data is sufficient and even in excess to explain the thermal component of strong flares (Aschwanden et al 2017;Kleint et al 2016). However, it was shown that for weak flares, a deficit of energetic electrons to affect the low levels of the atmosphere in which the bolometric emission (NUV, white light, near-IR radiation) is initiated (Warmuth & Mann 2020). Inglis & Christe (2014), Warmuth & Mann (2020) reported an apparent deficit of nonthermal electrons in weak flares by computing the ratio of thermal energy (and losses) and energy in nonthermal electrons.…”

“…However, it was shown that for weak flares, a deficit of energetic electrons to affect the low levels of the atmosphere in which the bolometric emission (NUV, white light, near-IR radiation) is initiated (Warmuth & Mann 2020). Inglis & Christe (2014), Warmuth & Mann (2020) reported an apparent deficit of nonthermal electrons in weak flares by computing the ratio of thermal energy (and losses) and energy in nonthermal electrons. The interpretation depends on where and in which area the electrons are accelerated.…”