6 Conclusions and Perspectives

Coronal loops have been the subject of in-depth studies for over 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. The coronal loops are the building blocks of the bright solar corona and, as such, they are important as the basic laboratory to investigate the mechanisms of coronal magnetism, dynamics, and heating. Our knowledge on coronal loops has progressed with the development of the instrument capabilities. The starting point has been the observation, mostly in the X-ray band, that coronal loops are globally steady on timescales longer than the plasma cooling times. This has allowed to develop the loop scaling laws, which work well to describe the hydrostatic properties of coronal plasma confined in a magnetic flux tube (Section 4.1.1). However, since the 1990s observations have more and more revealed that coronal loops are dynamic and structured, both spatially and thermally. Imaging instruments have shown bundles of fine strands whose thickness is not well resolved up-to-date, although we begin to have constraints from the latest observations with the Hi-C (∼ 100 km). The lack of agreement about the thickness values might indicate a distribution of strand sizes (Section 3.2.2). EUV spectroscopy shows a variety of thermal structuring, from nearly isothermal loops to broad distributions over a temperature decade (6 < logT < 7). The width of the distributions might depend on the heating intensity (warm vs hot loops), on the age of the active region and/or on the magnetic field (Section 3.3). The presence, whenever confirmed, of minor but widespread and filamented, very hot plasma components (logT ∼ 7) out of flares might be a tracer of small-scale impulsive heating. Coronal rain might mark thermal instability on small scales of thickness. High-resolution spectroscopy has also detected widespread flows (∼ 10 km s–1) with a complex pattern, especially low in the corona and transition region (Section 3.5). The pattern is described quite well as switching from redshifts to blueshifts with increasing temperature, thus indicating the coexistence of cool downfalling plasma and warm evaporation. This complex pattern is also a signature of fine structure, at least in the low corona. Higher in the corona, flow patterns are less defined, simply because of geometrical reasons. Fast upflows (∼ 100 km s–1) from the chromosphere might play a role in feeding coronal loops (Section 3.5.1). Continuous monitoring of the full solar disk has also allowed very detailed photometric analysis (Section 3.4). The detailed analysis of the light curves seems to indicate the presence of systematic cooling patterns, both in the X-rays and, from comparison of different channels, in the EUV band. The monitoring has also allowed the detection of MHD waves propagating along coronal loops (Section 3.5.2).

The physics of the plasma confined inside coronal loops is well described with compressible hydrodynamics with a very important role of thermal conduction and radiative losses from optically thin medium. The mass and energy are transported mostly along the field lines, with little direct role of the magnetic field. The initial hydrostatic single loop scenario is now replaced by a much more dynamic one with loops structured into bundle of finer strands (Section 4.1.2). The physics of the plasma inside each strand does not change much, but one important issue becomes how these strands combine into the unresolved observed structures. Multi-strand loop modeling is able to explain the fuzzier and fuzzier appearance of loops observed in channels sensitive to hotter and hotter emission, and flat filter ratios along loops (Section 4.2). A proper MHD description is required when we need to address directly the interaction of plasma with the ambient magnetic field, both regarding the confinement itself, e.g., the expansion across the transition region, and the conversion of the magnetic energy to power the plasma temperature and dynamics. Although flows along loops have long been modeled (Section 4.3), the recent evidence for upflows from the chromosphere has provided a new boost to models and, since it has been suggested that the flows might also carry the energy to heat the loop plasma, it has also revitalized the investigation of coronal heating, the central issue of coronal loop physics. The most basic question regarding the conversion of magnetic energy into heat remains probably whether this conversion occurs on small or large timescales, both in the charging and in the release (Section 4.4). This may make the difference between energy provided directly through fast magnetic reconnection (nanoflares) and energy dissipated more gradually by MHD waves, but new modeling seems to make the boundary much more blurred, wheareas MHD waves are dissipated impulsively through turbulent cascades. So, at the moment, there is some convergence to heating mechanisms released on short timescales inside thin strands composing the loops. The charge-and-release processes are also deserving attention regarding both the connection to the magnetic field, and the cadence and distribution of the events in the single strand. Turbulent cascades might provide a natural explanation for the dissipation of magnetic energy in anomalously small scales.

Key issues for the future remain the loop fine structure and dynamics. We need to address what are the ultimate elementary loop components, and whether they are unique or determined by local conditions, i.e., what determines the section scale size. We need the highest possible spatial resolution, probably in different bands. Further investigation of the plasma fine thermal structure and dynamics requires also high spectral resolution. High resolution broad-band X-ray spectroscopy is foreseeable to probe the hot components, signature of impulsive heating. Also the investigation of temporal variations still deserves attention. If loops are really so dynamic and subject to a distribution of heating events, whatever it is, the light curves are very difficult to interpret and signatures of any possible small-scale events are confused in a storming activity and by the plasma inertia. 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). The continuous monitoring by the SDO mission is a very powerful tool for temporal variability studies. The analysis of imaging multi-channel observations seems to indicate widespread plasma cooling, which needs further independent investigation. Observations from IRIS are providing very detailed information about the interaction of the corona with the chromosphere and, in particular, about the intriguing transition region. Highest-resolution observations in the EUV seem also to emphasize the importance of the transition region, because it is very sensitive to changes of the physical conditions and, therefore, it can be a tracer of basic loop processes.

Great help is expected from modeling. Multi-stranded time-dependent loop models are still providing a wealth of information and might contribute to study specific issues such as the sequence and the relative weight of evaporation-draining cycles, and the vertical structure and dynamics of the thin transition region. The coupling with the chromosphere is becoming increasingly important. Improving numerical and computing resources are allowing to address the more basic question of the coupling of the plasma with the magnetic field and of the conversion of the magnetic energy into heat. The approaches involve both full MHD models and other models that couple different regimes, such as the large-scale magnetic field and the locally confined plasma. A totally self-consistent description is still out-of-reach, but 3D MHD models are beginning to attack some very basic issues, regarding the heat release, such as the role of the MHD instabilities and their switch-on. Turbulence seems a key to the anomalous dissipation needed to explain the loop ignition.

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.

  Go to previous page Scroll to top Go to next page