4.2 Fine structuring

The description and role of fine structuring of coronal loops is certainly a challenge for coronal physics, also on the side of modeling, essentially because we have few constraints from observations (Section 3.2.2). One of the first times that the internal structuring of coronal loops have been invoked in a modeling context was for the problem of the interpretation of the uniform filter ratio distribution detected with TRACE along warm loops. Lenz et al. (1999) claimed that standard hydrostatic loop models with uniform heating could not explain such indication of uniform temperature distribution. Reale and Peres (2000Jump To The Next Citation Point) showed that a uniform filter ratio could be produced by the superposition of several thin strands described by standard static models of loops at different temperatures. In alternative, Aschwanden (2001Jump To The Next Citation Point) showed that long loops heated at the footpoints result to be mostly isothermal. The problem with this model is that footpoint-heated loops (with heating scale height less than 1/3 of the loop half-length) had been shown to be thermally unstable and, therefore, they cannot be long-lived, as instead observed. A further alternative is to explain observations with steady non-static loops, i.e., with significant flows inside (Winebarger et al., 20012002cJump To The Next Citation Point, see below). Also this hypothesis does not seem to answer the question (Patsourakos et al., 2004Jump To The Next Citation Point).

A first step to modeling fine-structured loops is to use multistrand static models. Such models show some substantial inconsistencies with observations, e.g., in general they predict too large loop cross sections (Reale and Peres, 2000). Such strands are conceptually different from the thin strands predicted in the nanoflare scenario (Parker, 1988Jump To The Next Citation Point), which imply a highly dynamic evolution due to pulsed-heating. The nanoflare scenario is approached in multi-thread loop models, convolving the independent hydrodynamic evolution of the plasma confined in each pulse-heated strand (see Section 4.3). These are able to match some more features of the evolution of warm loops observed with TRACE (Warren et al., 2002Jump To The Next Citation Point2003Jump To The Next Citation PointWinebarger et al., 2003a,bJump To The Next Citation Point). By means of detailed hydrodynamic loop modeling, Warren et al. (2002Jump To The Next Citation Point) found that an ensemble of independently heated strands can be significantly brighter than a static uniformly heated loop and would have a flat filter ratio temperature when observed with TRACE. As an extension, time-dependent hydrodynamic modeling of an evolving active region loop observed with TRACE showed that a loop made as a set of small-scale, impulsively heated strands can generally reproduce the spatial and temporal properties of the observed loops, such as a delay between the appearance of the loop in different filters (Warren et al., 2003Jump To The Next Citation Point). As an evolution of this approach, Warren and Winebarger (2006) modelled an entire active region for comparison with a SoHO/EIT observation. They made potential field extrapolations to compute magnetic field lines and populate these field lines with solutions to the hydrostatic loop equations assuming steady, uniform heating. As a result, they constrained the link between the heating rate and the magnetic field and size of the structures. However, they also found significant discrepancies with the observed EIT emission.

More recently modeling a loop system as a collection of thin unresolved strand with pulsed heating has been used to explain why active regions look fuzzier in harder energy bands, i.e., X-rays, and/or hotter spectral lines, e.g., Fe xvi (Tripathi et al., 2009, Section 3.3.2). The basic reason is that in the dynamic evolution of each strand, the plasma spends a relatively longer time and with a high emission measure at temperature about 3 MK (Guarrasi et al., 2010).

Although multistrand models appear much more complex than single loop models and need further refinements to match all the observational constraints, as mentioned in Section 4.1.2, they certainly represent an important issue for the future of coronal loop comprehension.

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