Probing nanoflares with observed fluctuations of the coronal EUV emission

Vekstein, G.

doi:10.1051/0004-6361/200911872

Cited by 22 publications

(24 citation statements)

References 29 publications

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“…Shorter cooling times result in more bursty emission and greater fluctuation amplitudes. The increase of the fluctuation amplitude with the increasing intensity is consistent with the observational results obtained recently by Sakamoto et al (2008, and references therein) and discussed also in Vekstein (2009). They also found that the amplitude of the intensity fluctuations of Yohkoh/SXT loops is approximately 8% of the value of the loop intensity.…”

Section: Resultssupporting

confidence: 92%

A Simple Model for the Evolution of Multi-Stranded Coronal Loops

Fuentes¹,

Klimchuk²

2010

ApJ

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We develop and analyze a simple cellular automaton (CA) model that reproduces the main properties of the evolution of soft X-ray coronal loops. We are motivated by the observation that these loops evolve in three distinguishable phases that suggest the development, maintainance, and decay of a self-organized system. The model is based on the idea that loops are made of elemental strands that are heated by the relaxation of magnetic stress in the form of nanoflares. In this vision, usually called "the Parker conjecture" (Parker 1988), the origin of stress is the displacement of the strand footpoints due to photospheric convective motions. Modeling the response and evolution of the plasma we obtain synthetic light curves that have the same characteristic properties (intensity, fluctuations, and timescales) as the observed cases. We study the dependence of these properties on the model parameters and find scaling laws that can be used as observational predictions of the model. We discuss the implications of our results for the interpretation of recent loop observations in different wavelengths.

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Section: Resultssupporting

confidence: 92%

A Simple Model for the Evolution of Multi-Stranded Coronal Loops

Fuentes¹,

Klimchuk²

2010

ApJ

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“…On the other hand, most warm loops are inconsistent with static equilibrium. Recent observations revealed intensity fluctuations of warm loop which can be interpreted as a signature that originates from numerous sporadic coronal heating events (e.g., Sakamoto et al 2008;Vekstein 2008). Many numerical studies have tried to reproduce these two kind of coronal loops, with limited success.…”

Section: Summary and Discussionmentioning

confidence: 99%

Self-Organization of Reconnecting Plasmas to Marginal Collisionality in the Solar Corona

Imada

Zweibel

2012

ApJ

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We explore the suggestions by Uzdensky (2007) and Cassak et al. (2008) that coronal loops heated by magnetic reconnection should self-organize to a state of marginal collisionality. We discuss their model of coronal loop dynamics with a one-dimensional hydrodynamic calculation. We assume that many current sheets are present, with a distribution of thicknesses, but that only current sheets thinner than the ion skin depth can rapidly reconnect. This assumption naturally causes a density dependent heating rate which is actively regulated by the plasma. We report 9 numerical simulation results of coronal loop hydrodynamics in which the absolute values of the heating rates are different but their density dependences are the same. We find two regimes of behavior, depending on the amplitude of the heating rate. In the case that the amplitude of heating is below a threshold value, the loop is in stable equilibrium. Typically the upper and less dense part of coronal loop is collisionlessly heated and conductively cooled. When the amplitude of heating is above the threshold, the conductive flux to the lower atmosphere required to balance collisionless heating drives an evaporative flow which quenches fast reconnection, ultimately cooling and draining the loop until the cycle begins again. The key elements of this cycle are gravity and the density dependence of the heating function. Some additional factors are present, including pressure driven flows from the loop top, which carry a large enthalpy flux and play an important role in reducing the density. We find that on average the density of the system is close to the marginally collisionless value.

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“…However, more recently, Ishikawa et al (2017) employed differential emission measure techniques on hard X-ray observations from the second flight of the Focusing Optics X-ray Solar Imager (FOXSI-2; Krucker et al 2009;Christe et al 2016) sounding rocket, and revealed plasma heated above 10 MK, thus providing yet more evidence for the existence of solar nanoflares. Sakamoto et al (2008) and Vekstein (2009) compared cotemporal intensity time series obtained at EUV and X-ray wavelengths, and estimated that a 'hot' corona could be maintained with nanoflare filling factors on the order of 10%. Subsequent theoretical modeling by Joshi & Prasad (2012) provided corroborating evidence that the X-ray fluctuations observed by Katsukawa & Tsuneta (2001) and Sakamoto et al (2008) could be representative of 10 23 -10 26 erg events released over timescales of ∼100 s. Importantly, the results of Sakamoto et al (2008) demonstrate a lag time between soft X-ray and EUV time series (corresponding to the cooling timescale), which suggests that soft X-ray loops may require higher nanoflare energies than their EUV counterparts, thus perhaps indicating a wavelength dependence on the nanoflare power-law index.…”

Section: Introductionmentioning

confidence: 99%

Statistical Signatures of Nanoflare Activity. I. Monte Carlo Simulations and Parameter-space Exploration

Jess

Dillon

Kirk

et al. 2019

ApJ

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Small-scale magnetic reconnection processes, in the form of nanoflares, have become increasingly hypothesized as important mechanisms for the heating of the solar atmosphere, for driving propagating disturbances along magnetic field lines in the Sun's corona, and for instigating rapid jet-like bursts in the chromosphere. Unfortunately, the relatively weak signatures associated with nanoflares places them below the sensitivities of current observational instrumentation. Here, we employ Monte Carlo techniques to synthesize realistic nanoflare intensity time series from a dense grid of power-law indices and decay timescales. Employing statistical techniques, which examine the modeled intensity fluctuations with more than 10 7 discrete measurements, we show how it is possible to extract and quantify nanoflare characteristics throughout the solar atmosphere, even in the presence of significant photon noise. A comparison between the statistical parameters (derived through examination of the associated intensity fluctuation histograms) extracted from the Monte Carlo simulations and SDO/AIA 171Å and 94Å observations of active region NOAA 11366 reveals evidence for a flaring power-law index within the range of 1.82 ≤ α ≤ 1.90, combined with e-folding timescales of 385 ± 26 s and 262 ± 17 s for the SDO/AIA 171Å and 94Å channels, respectively. These results suggest that nanoflare activity is not the dominant heating source for the active region under investigation. This opens the door for future dedicated observational campaigns to not only unequivocally search for the presence of small-scale reconnection in solar and stellar environments, but also quantify key characteristics related to such nanoflare activity.

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Probing nanoflares with observed fluctuations of the coronal EUV emission

Cited by 22 publications

References 29 publications

A Simple Model for the Evolution of Multi-Stranded Coronal Loops

A Simple Model for the Evolution of Multi-Stranded Coronal Loops

Self-Organization of Reconnecting Plasmas to Marginal Collisionality in the Solar Corona

Statistical Signatures of Nanoflare Activity. I. Monte Carlo Simulations and Parameter-space Exploration

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