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A nanoflare based cellular automaton model and the observed properties of the coronal plasma  

Marcelo Lopez-Fuentes   Submitted: 2016-08-04 12:21

We use the cellular automaton model described in L\'opez Fuentes & Klimchuk (2015, ApJ, 799, 128) to study the evolution of coronal loop plasmas. The model, based on the idea of a critical misalignment angle in tangled magnetic fields, produces nanoflares of varying frequency with respect to the plasma cooling time. We compare the results of the model with active region (AR) observations obtained with the Hinode/XRT and SDO/AIA instruments. The comparison is based on the statistical properties of synthetic and observed loop lightcurves. Our results show that the model reproduces the main observational characteristics of the evolution of the plasma in AR coronal loops. The typical intensity fluctuations have an amplitude of 10 to 15% both for the model and the observations. The sign of the skewness of the intensity distributions indicates the presence of cooling plasma in the loops. We also study the emission measure (EM) distribution predicted by the model and obtain slopes in log(EM) versus log(T) between 2.7 and 4.3, in agreement with published observational values.

Authors: M. López Fuentes, J.A. Klimchuk
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2016-08-05 15:28
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Two-dimensional cellular automaton model for the evolution of active region coronal plasmas  

Marcelo Lopez-Fuentes   Submitted: 2016-08-04 12:17

We study a 2D cellular automaton (CA) model for the evolution of coronal loop plasmas. The model is based on the idea that coronal loops are made of elementary magnetic strands that are tangled and stressed by the displacement of their footpoints by photospheric motions. The magnetic stress accumulated between neighbor strands is released in sudden reconnection events or nanoflares that heat the plasma. We combine the CA model with the Enthalpy Based Thermal Evolution of Loops (EBTEL) model to compute the response of the plasma to the heating events. Using the known response of the XRT telescope on board Hinode we also obtain synthetic data. The model obeys easy to understand scaling laws relating the output (nanoflare energy, temperature, density, intensity) to the input parameters (field strength, strand length, critical misalignment angle). The nanoflares have a power-law distribution with a universal slope of -2.5, independent of the input parameters. The repetition frequency of nanoflares, expressed in terms of the plasma cooling time, increases with strand length. We discuss the implications of our results for the problem of heating and evolution of active region coronal plasmas.

Authors: M. López Fuentes, J.A. Klimchuk
Projects: None

Publication Status: Published in ApJ, Volume 799, Issue 2, article id. 128
Last Modified: 2016-08-05 15:28
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A simple model for the evolution of multi-stranded coronal loops  

Marcelo Lopez-Fuentes   Submitted: 2010-06-16 09:23

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.

Authors: M. C. Lopez Fuentes, J. A. Klimchuk
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2010-06-16 18:23
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Are constant loop widths an artifact of the background and the spatial resolution?  

Marcelo Lopez-Fuentes   Submitted: 2007-04-10 08:05

We study the effect of the coronal background in the determination of the diameter of EUV loops, and we analyze the suitability of the procedure followed in a previous paper (Lopez Fuentes, Klimchuk & Demoulin 2006) for characterizing their expansion properties. For the analysis we create different synthetic loops and we place them on real backgrounds from data obtained with the Transition Region and Coronal Explorer (TRACE). We apply to these loops the same procedure followed in our previous works, and we compare the results with real loop observations. We demonstrate that the procedure allows us to distinguish constant width loops from loops that expand appreciably with height, as predicted by simple force-free field models. This holds even for loops near the resolution limit. We find no statistical correlation between variations of the measured width and of the background. Therefore, we confirm that the background does not significantly influence the determination of the overall expansion properties of loops.

Authors: M.C. Lopez Fuentes, P. Demoulin, J.A. Klimchuk
Projects: None

Publication Status: Submitted to ApJ
Last Modified: 2007-04-10 09:03
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The temporal evolution of coronal loops observed by GOES-SXI  

Marcelo Lopez-Fuentes   Submitted: 2006-11-10 08:29

We study the temporal evolution of coronal loops using data from the Solar X-ray Imager (SXI) on board of GOES-12. This instrument allows us to follow in detail the full lifetime of coronal loops. The observed light curves suggest three somewhat distinct evolutionary phases: rise, main, and decay. The durations and characteristic timescales of these phases are much longer than a cooling time and indicate that the loop-averaged heating rate increases slowly, reaches a maintenance level, and then decreases slowly. This suggests that a single heating mechanism operates for the entire lifetime of the loop. For monolithic loops, the loop-averaged heating rate is the intrinsic energy release rate of the heating mechanism. For loops that are bundles of impulsively heated strands, it is an indication of the frequency of occurrence of individual heating events, or nanoflares. We show that the timescale of the loop-averaged heating rate is proportional to the timescale of the observed intensity variation. The ratios of the radiative to conductive cooling times in the loops are somewhat less than 1, putting them intermediate between the values measured previously for hotter and cooler loops. Our results provide further support for the existence of a trend suggesting that all loops are heated by the same mechanism, or that different mechanisms have fundamental similarities (e.g., are all impulsive or are all steady with similar rates of heating).

Authors: M.C. Lopez Fuentes, J.A. Klimchuk, C.H. Mandrini
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2006-11-10 10:17
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The Magnetic Structure of Coronal Loops Observed by TRACE  

Marcelo Lopez-Fuentes   Submitted: 2005-07-28 13:42

Previous studies have found that coronal loops have a nearly uniform thickness, which seems to disagree with the characteristic expansion of active region magnetic fields. This is one of the most intriguing enigmas in solar physics. We here report on the first comprehensive one-to-one comparison of observed loops with corresponding magnetic flux tubes obtained from cotemporal magnetic field extrapolation models. We use EUV images from TRACE, magnetograms from the MDI instrument on SOHO, and linear force-free field extrapolations. For each loop, we find the particular value of the force-free parameter α that best matches the observed loop axis and then construct flux tubes using different assumed cross sections at one footpoint (circle and ellipses with different orientations). We find that the flux tubes expand with height by typically twice as much as the corresponding loops. We also find that many flux tubes are much wider at one footpoint than the other, whereas the corresponding loops are far more symmetric. It is clear that the actual coronal magnetic field is more complex than the models we have considered. We suggest that the observed symmetry of loops is related to the tangling of elemental magnetic flux strands produced by photospheric convection.

Authors: M. C. Lopez Fuentes, J. A. Klimchuk, P. Demoulin
Projects:

Publication Status: Accepted for publication in ApJ
Last Modified: 2005-11-03 12:56
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Abstracts by Author
A nanoflare based cellular automaton model and the observed properties of the coronal plasma
Two-dimensional cellular automaton model for the evolution of active region coronal plasmas
A simple model for the evolution of multi-stranded coronal loops
Are constant loop widths an artifact of the background and the spatial resolution?
The temporal evolution of coronal loops observed by GOES-SXI
The Magnetic Structure of Coronal Loops Observed by TRACE

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