Microflares and the Statistics of X-ray Flares

Hannah, I. G.; Hudson, H. S.; Battaglia, Marina; Christe, Steven; Kašparová, J.; Krucker, S.; Kundu, M. R.; Veronig, Astrid

doi:10.1007/s11214-010-9705-4

Cited by 150 publications

(137 citation statements)

References 148 publications

(205 reference statements)

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“…Its distribution in energy roughly follows a power law [85], which is consistent with observations over a large range of energies and with the concept of self-organized criticality; see e.g. the review [86]. In the coronal part of the volume, the typical energy released in a threedimensional model is of the order of 10 17 J (or 10 24 erg) [85], which is in the same range as the nanoflares originally predicted by Parker [87].…”

Section: (E) Nanoflares Nanoflare Storms and Braiding Of Fieldlinessupporting

confidence: 87%

What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating?

Peter

2015

Phil. Trans. R. Soc. A.

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The upper atmosphere of the Sun is governed by the complex structure of the magnetic field. This controls the heating of the coronal plasma to over a million kelvin. Numerical experiments in the form of threedimensional magnetohydrodynamic simulations are used to investigate the intimate interaction between magnetic field and plasma. These models allow one to synthesize the coronal emission just as it would be observed by real solar instrumentation. Largescale models encompassing a whole active region form evolving coronal loops with properties similar to those seen in extreme ultraviolet light from the Sun, and reproduce a number of average observed quantities. This suggests that the spatial and temporal distributions of the heating as well as the energy distribution of individual heat deposition events in the model are a good representation of the real Sun. This provides evidence that the braiding of fieldlines through magneto-convective motions in the photosphere is a good concept to heat the upper atmosphere of the Sun.

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Section: (E) Nanoflares Nanoflare Storms and Braiding Of Fieldlinessupporting

confidence: 87%

What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating?

Peter

2015

Phil. Trans. R. Soc. A.

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“…These temperatures are significantly higher than those obtained for the hot component in our analysis. Part of this discrepancy might be explained by the different sensitivity of the instruments, due to the different spectral band, that would lead the instruments to be sensitive to different parts a broad emission measure distribution (Peres et al 2000;Hannah et al 2011). The average emission measure obtained from thermal fitting of RHESSI microflares is about 2−3 × 10 46 cm −3 , which is much larger than the emission measure of the hot components that we obtain.…”

Section: Discussioncontrasting

confidence: 63%

“…2 and 5, although in a harder energy band (Hannah et al 2008(Hannah et al , 2011. The different energy bands correspond to different fitting parameters, as expected from experimental bias (Hannah et al 2011). We can equally attempt a comparison if we consider microflares either included in or excluded from our data.…”

Section: Discussionmentioning

confidence: 90%

X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

Miceli

Reale

Gburek

et al. 2012

A&A

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Aims. The detection of very hot plasma in the quiescent corona is important for diagnosing heating mechanisms. The presence and the amount of such hot plasma is currently debated. The SphinX instrument on-board the CORONAS-PHOTON mission is sensitive to X-ray emission of energies well above 1 keV and provides the opportunity to detect the hot plasma component. Methods. We analysed the X-ray spectra of the solar corona collected by the SphinX spectrometer in May 2009 (when two active regions were present). We modelled the spectrum extracted from the whole Sun over a time window of 17 days in the 1.34−7 keV energy band by adopting the latest release of the APED database. Results. The SphinX broadband spectrum cannot be modelled by a single isothermal component of optically thin plasma and two components are necessary. In particular, the high statistical significance of the count rates and the accurate calibration of the spectrometer allowed us to detect a very hot component at ∼7 million K with an emission measure of ∼2.7 × 10 44 cm −3 . The X-ray emission from the hot plasma dominates the solar X-ray spectrum above 4 keV. We checked that this hot component is invariably present in both the high and low emission regimes, i.e. even excluding resolvable microflares. We also present and discuss the possibility of a non-thermal origin (which would be compatible with a weak contribution from thick-target bremsstrahlung) for this hard emission component. Conclusions. Our results support the nanoflare scenario and might confirm that a minor flaring activity is ever-present in the quiescent corona, as also inferred for the coronae of other stars.

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“…Yet in one important aspect they differ significantly; their spectra above 10 keV (usually interpreted as non-thermal emission) are generally steep compared to large flares; interpreted as a power-law the spectral index of microflares is generally between −5 and −8 (see Benz & Grigis 2002;Krucker et al 2002;Christe et al 2008;Hannah et al 2008a), while for large flares it typically ranges from −2.5 to −4, (Saint-Hilaire et al 2008), although there are some notable exceptions (Hannah et al 2008b;O'Flannagain et al 2013). A recent full review of microflare properties may be found in Hannah et al (2011). One possible interpretation of this finding is that microflares are not as efficient particle accelerators as large flares.…”

Section: Introductionmentioning

confidence: 99%

INVESTIGATING THE DIFFERENTIAL EMISSION MEASURE AND ENERGETICS OF MICROFLARES WITH COMBINEDSDO/AIA ANDRHESSIOBSERVATIONS

Inglis

Christe

2014

ApJ

Self Cite

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An important question in solar physics is whether solar microflares, the smallest currently observable flare events in X-rays, possess the same energetic properties as large flares. Recent surveys have suggested that microflares may be less efficient particle accelerators than large flares, and hence contribute less non-thermal energy, which may have implications for coronal heating mechanisms. We therefore explore the energetic properties of microflares by combining EUV and X-ray measurements. We present forward-fitting differential emission measure (DEM) analysis of 10 microflares. The fitting is constrained by combining, for the first time, high-temperature Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations and flux data from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA). Two fitting models are tested for the DEM; a Gaussian distribution and a uniform DEM profile. A Gaussian fit proved unable to explain the observations for any of the studied microflares. However, 8 of 10 events studied were reasonably fit by a uniform DEM profile. Hence microflare plasma can be considered to be significantly multi-thermal, and may not be significantly peaked or contain resolvable fine structure, within the uncertainties of the observational instruments. The thermal and non-thermal energy is estimated for each microflare, comparing the energy budget with an isothermal plasma assumption. From the multi-thermal fits the minimum non-thermal energy content was found to average approximately 30% of the estimated thermal energy. By comparison, under an isothermal model the non-thermal and thermal energy estimates were generally comparable. Hence, multi-thermal plasma is an important consideration for solar microflares that substantially alters their thermal and non-thermal energy content.

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Microflares and the Statistics of X-ray Flares

Cited by 150 publications

References 148 publications

What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating?

What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating?

X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

INVESTIGATING THE DIFFERENTIAL EMISSION MEASURE AND ENERGETICS OF MICROFLARES WITH COMBINEDSDO/AIA ANDRHESSIOBSERVATIONS

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