Coronal loops have been the subject of in-depth studies for about 50 years. Since they owe their identity to the brightness of the confined plasma, most of the studies have addressed the physics of the confined plasma, i.e., its structure, dynamics, and evolution. Most of the basic laws that rule the confined plasma, such as scaling laws, were developed early after the discovery of loops and are now well-established. Although the observational scenario is ever-enriching with the progress of the solar coronal missions, a variety of questions remain still open, at all levels, starting from loop identification itself. The lack of operative definitions and automatic identification tools, and the difficulty to isolate loops from other surrounding and intersecting structures have prevented systematic studies on large and unbiased loop populations. Detailed morphological and theoretical analyses converge to a fine loop substructuring, which is critical to understand the basic mechanisms but is below the resolution limit of present-day instruments. Also the large scale structure of loops leaves room for further developments and, in particular, the link with the confining magnetic field, which is difficult to measure in the corona. One specific question to be addressed is the role of the loop tapering in the transition region.
Looking inside the loops, open questions involve the detailed thermal structure of the confined plasma. This issue is important to assess the basic loop heating mechanism: a broad multi-component thermal distribution would indicate a structured heating, a simpler distribution a monolithic mechanism. This question is still open for many reasons; spectroscopic methods provide very detailed thermal information, but mostly concentrated in the regime of warm loops. Moreover, the methods of analysis do not appear to provide unique answers yet. Also filter ratio diagnostics from broad-band X-ray and UV imaging telescopes have not been able to provide conclusive results so far. As a consequence, we are unable, right now, to assess the problem of the apparently different nature of warm (TRACE, SoHO/EIT) and hot (Yohkoh/SXT, Hinode/XRT) loops, and whether it is simply a matter of different heating rate or the heating mechanisms are radically different.
The role of the dynamics of plasma confined inside loops is also still under investigation. The measurement of plasma motions is made difficult by the possible ambiguity with the apparent motion of thermal fronts. It will be important in the future to evaluate the relative weight of the different flows, i.e., downflows, evaporation and draining, siphon flows, and their influence on the overall loop budget.
Also the investigation of temporal variations deserves attention. In particular, in narrow band instruments thermal variations might be confused with intensity variations, and make the interpretation difficult. On the other hand, the analysis of emission variations is very important, because it can potentially shed light on heating mechanisms based on short impulsive events (nanoflares) or on wave-like phenomena (Alfvén waves).
On the theoretical side, 1-D loop models are well-established and have provided a wealth of sound and important results. Today, their evolution consists essentially in their transformation into “strand” models, and so a collection of them describes a proper loop. Moreover, loops are now seen much more dynamic than they were in the past, either as the site of flows or of wave perturbations, or of heat pulses. So they need more and more time-dependent modeling to address, for instance, the importance of flows and the relative weight between evaporation-draining flows and siphon flows. The modeling is, therefore, becoming more and more demanding, although computing resources are now much more powerful than in the past and although some approaches are able to squeeze the spatial dimension, yet maintaining the temporal description. At the same time, new data seem to require other model refinements, such as the description of loop expansion in the transition region. The improvement of numerical and computing resources is also allowing more complex and complete modeling of whole loop regions including the 3-D magnetic field structure “ab initio”. This approach is very promising and will surely provide important results in the next future, to complement those obtained with basic single loop models. We all look forward self-consistent descriptions including the generation of heat from magnetic field rearrangements and perturbations.
Special attention still deserves the problem of what heats coronal loops, which means basically what heats the whole corona. This problem has revealed to be particularly difficult, essentially because of intrinsic physical reasons and, in particular, i) the highly effective thermal conduction, which inhibits the identification of the heating site, and ii) the expected small scales of the heating processes, which require so far prohibitive spatial/temporal resolution. Nevertheless, coronal loops remain an obvious excellent laboratory to investigate coronal heating mechanisms, because the dense plasma confined therein make them bright and easy to observe. The most recent challenge offered by coronal loops is probably the increasing evidence that the thin strands they are made of are ignited by small scale, rapid, but intense pulses. Most current efforts are devoted to study this aspect both from the theoretical/modeling and from the observational point of view. However, alternative explanations are actively explored and strongly proposed lately.
The study of coronal loops is very alive and is the subject of Coronal Loop Workshops, taking place every two years, which are site of debate, inspiration of new investigations, and school for young investigators. We look forward further great improvements in our knowledge of coronal loops from the new mission Solar Dynamic Observatory.
Living Rev. Solar Phys. 7, (2010), 5
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