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

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 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.

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Analysis of the differential emission measure distributions for solar flares observed by RESIK

Journal of Atmospheric and Solar-Terrestrial Physics ◽

10.1016/j.jastp.2018.09.004 ◽

2018 ◽

Vol 179 ◽

pp. 545-552

Author(s):

A. Kepa ◽

B. Sylwester ◽

J. Sylwester ◽

M. Gryciuk ◽

M. Siarkowski

Keyword(s):

Solar Flares ◽

Emission Measure ◽

Differential Emission Measure

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Plasma Diagnostics of Microflares observed by STIX and AIA

10.5194/egusphere-egu21-7966 ◽

2021 ◽

Author(s):

Jonas Saqri ◽

Astrid Veronig ◽

Ewan Dickson ◽

Säm Krucker ◽

Andrea Francesco Battaglia ◽

...

Keyword(s):

Solar Flares ◽

Magnetic Energy ◽

Emission Measure ◽

Transport Processes ◽

X Rays ◽

Solar Dynamics Observatory ◽

Energy Bands ◽

Accurate Analysis ◽

Great Opportunity ◽

Wide Range

<p>Solar flares are generally thought to be the impulsive release of magnetic energy giving rise to a wide range of solar phenomena that influence the heliosphere and in some cases even conditions of earth. Part of this liberated energy is used for particle acceleration and to heat up the solar plasma. The Spectrometer/Telescope for Imaging X-rays (STIX) instrument onboard the Solar Orbiter mission launched on February 10th 2020 promises advances in the study of solar flares of various sizes. It is capable of measuring X-ray spectra from 4 to 150 keV with 1 keV resolution binned into 32 energy bins before downlinking. With this energy range and sensitivity, STIX is capable of sampling thermal plasma with temperatures of&#8819;10 MK, and to diagnose the nonthermal bremsstrahlung emission of flare-accelerated electrons. During the spacecraft commissioning phase in the first half of the year 2020, STIX observed 68 microflares. Of this set, 26 events could clearly be identified in at least two energy channels, all of which originated in an active region that was also visible from earth. These events provided a great opportunity to combine the STIX observations with the multi-band EUV imagery from the Atmospheric Imaging Assembly (AIA) instrument on board the earth orbiting Solar Dynamics Observatory (SDO). For the microflares that could be identified in two STIX science energy bands, it was possible to derive the temperature and emission measure (EM) of the flaring plasma assuming an isothermal source. For larger events where more detailed spectra could be derived, a more accurate analysis was performed by fitting the spectra assuming various thermal and nonthermal sources. These results are compared to the diagnostics derived from AIA images. To this aim, the Differential EmissionMeasure (DEM) was reconstructed from AIA observations to infer plasma temperatures and EM in the flaring regions. Combined with the the relative timing between the emission seen by STIX and AIA, this allows us to get deeper insight into the flare energy release and transport processes.</p>

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[Comment on “The case for protons powering solar flares”] The case for electron dominance in solar flare energy transport

Eos ◽

10.1029/96eo00246 ◽

1996 ◽

Vol 77 (37) ◽

pp. 355 ◽

Cited By ~ 4

Author(s):

A. Gordon Emslie

Keyword(s):

Solar Flare ◽

Solar Flares ◽

Energy Transport ◽

Flare Energy

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Models of Flaring Loops

International Astronomical Union Colloquium ◽

10.1017/s0252921100031833 ◽

1989 ◽

Vol 104 (1) ◽

pp. 105-115

Author(s):

A. Gordon Emslie

Keyword(s):

Solar Flares ◽

Energy Transport ◽

High Energy ◽

Loop Structure ◽

Predominant Role ◽

Release Process ◽

Flare Loop ◽

X Ray ◽

Physical Mechanisms ◽

High Energy Electrons

AbstractWe review the somewhat questionable concept of an isolated flare loop and the various physical mechanisms believed to be responsible, to some degree, for energy transport within the loop structure. Observational evidence suggests a predominant role for high-energy electrons as an energy transport mechanism, and we explore the consequences of such a scenario in some detail, focusing on radiation signatures in the soft X-ray, hard X-ray, and EUV wavebands, as observed by recent satellite observatories. We find that the predictions of flare loop models are in fact in excellent agreement with these observations, reinforcing both the notion of the loop as a fundamental component of solar flares and the belief that electron acceleration is an integral part of the flare energy release process.

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Post-flare Coronal Activity on AU Mic Detected by EUVE

International Astronomical Union Colloquium ◽

10.1017/s0252921100034886 ◽

1995 ◽

Vol 151 ◽

pp. 146-147

Author(s):

Maria Katsova ◽

Jeremy Drake ◽

Moissei Livshits

Keyword(s):

Solar Flares ◽

Extreme Ultraviolet ◽

Emission Measure ◽

Coronal Loops ◽

Stellar Radius ◽

Euv Emission ◽

Long Duration ◽

Red Dwarfs ◽

Coronal Activity ◽

Region Scale

Data of long-duration emission arising after the impulsive rise and decay in a flaring event on the red dwarf star AU Mic are discussed. Intensive EUV emission in the band 65-190 Å was registered by the Extreme Ultraviolet Explorer (EUVE) after both impulses during half a day. A similar behavior of the flux in the Fe XVIII 93.9 Å line is detected after the first powerful impulse. The decay of the intensity in the 65-190 Å band and in the Fe XVIII line during this prolonged event is 10 times slower than the time of radiative cooling of coronal loops with a typical flare plasma density. Some difficulties with two explanations of this event proposed earlier are discussed. Explanation (i) - the radiation of dense loops at main phase of the flare, and (ii) - the emission of the low-dense plasma of coronal transients (CME). The temporal behavior of the emission measure is determined for both the 65-190 Å band and the Fe XVIII line fluxes. The total energy emitted in the 1-2000 Å region for the long-duration event lasting almost 12 hours is 3 · 1035 ergs. The following physical model is proposed to explain the prolonged event (Fig. 1): the source of emission is the system of high coronal loops, the size of which is more than the active region scale, but is less than the stellar radius. The temperature of the plasma in the loops decreases from 107 K slowly, during a few hours. The densities in these loops are in the range 1013 cm−3 to 5 · 109 cm−3. Such systems, when the plasma therein becomes cool, are observed in the Hα line during large solar flares (for instance, June 15, 1991) after CME. Some additional post-flare energy input into this high coronal loop systems can be caused by the reconnection in vertical current sheet, and this post-eruptive energy release provides prolonged and intensive EUV emission.Apparently, we are faced here with a new kind of surface activity on late-type stars which is intermediate between impulsive flares on red dwarfs and long-duration, powerful events on subgiants, which are components of RS CVn binaries.The full version of this contribution will be published in Astronomicheskij Zhurnal (Astronomy Reports), 1995, Vol. 72.

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