We have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 day observation of the Orion Nebula Cluster (ONC), concentrating on the events with the highest statistics (in terms of photon flux and event duration). Analysis of the flare decay allows to determine the physical parameters of the flaring structure, particularly its size and (using the peak temperature and emission measure of the event) the peak density, pressure, and minimum confining magnetic field. A total of 32 events, representing the most powerful '1% of COUP flares, have sufficient statistics and are sufficiently well resolved to grant a detailed analysis. A broad range of decay times are present in the sample of flares, with lc (the 1/e decay time) ranging from 10 to 400 ks. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures that are found, from the analysis of the flare's decay, to confine the plasma are in a number of cases very long, with semilengths up to '10 12 cm, implying the presence of magnetic fields of hundreds of G (necessary to confine the hot flaring plasma) extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures (length L 3 R Ã ) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L R Ã . As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures that confine the flaring plasma are actually the same type of structures that channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk.

Coronal loops are the building blocks of the X-ray bright solar corona. They owe their brightness to the dense confined plasma, and this review focuses on loops mostly as structures confining plasma. After a brief historical overview, the review is divided into two separate but not independent parts: the first illustrates the observational framework, the second reviews the theoretical knowledge. Quiescent loops and their confined plasma are considered and, therefore, topics such as loop oscillations and flaring loops (except for non-solar ones, which provide information on stellar loops) are not specifically addressed here. The observational section discusses the classification, populations, and the morphology of coronal loops, its relationship with the magnetic field, and the loop stranded structure. The section continues with the thermal properties and diagnostics of the loop plasma, according to the classification into hot, warm, and cool loops. Then, temporal analyses of loops and the observations of plasma dynamics, hot and cool flows, and waves are illustrated. In the modeling section, some basics of loop physics are provided, supplying fundamental scaling laws and timescales, a useful tool for consultation. The concept of loop modeling is introduced and models are divided into those treating loops as monolithic and static, and those resolving loops into thin and dynamic strands. More specific discussions address modeling the loop fine structure and the plasma flowing along the loops. Special attention is devoted to the question of loop heating, with separate discussion of wave (AC) and impulsive (DC) heating. Large-scale models including atmosphere boxes and the magnetic field are also discussed. Finally, a brief discussion about stellar coronal loops is followed by highlights and open questions.

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Context. The diagnostics of stellar flaring coronal loops have been so far largely based on the analysis of the decay phase. Aims. We derive new diagnostics from the analysis of the rise and peak phase of stellar flares. Methods. We release the assumption of full equilibrium of the flaring loop at the flare peak, according to the frequently observed delay between the temperature and the density maximum. From scaling laws and hydrodynamic simulations we derive diagnostic formulas as a function of observable quantities and times. Results. We obtain a diagnostic toolset related to the rise phase, including the loop length, density and aspect ratio. We discuss the limitations of this approach and find that the assumption of loop equilibrium in the analysis of the decay leads to a moderate overestimate of the loop length. A few relevant applications to previously analyzed stellar flares are shown. Conclusions. The analysis of the flare rise and peak phase complements and completes the analysis of the decay phase.

Abstract:The physical processes resulting in energy exchange between the Sun's hot corona and its cool lower atmosphere are still poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (~20-60s) of intensity and velocity on small spatial scales (≲500km) at the footpoints of hot and dynamic coronal loops. Comparison with numerical simulations reveal that the observations are consistent with heating by beams of non-thermal electrons and that these beams are generated even in small impulsive (≲30s) heating events called "coronal nanoflares". The accelerated electrons deposit a significant fraction of their energy (≲10 25 erg) in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the non-flaring corona. Main Text:Though it is established that the magnetic field plays a major role in the energetics of the bright corona, determining the details of the physical mechanisms that heat the solar corona remains one of the outstanding open issues in astrophysics. There are several candidate physical processes for heating the corona, including dissipation of magnetic stresses via reconnection, and dissipation of magnetohydrodynamic waves (1,2,3). In many heating models, the energy release is characterized by small spatial and temporal scales. For instance, in the "nanoflare" model, random photospheric motions lead to braiding or shearing of magnetic field lines and to reconnection which yields impulsive heating of the coronal plasma (4,5). Several statistical studies of large numbers of solar flares (6-8) have suggested that the mechanisms producing flares are likely similar within a large range from micro-to X-class flares. If nanoflares behave as a scaled down version of larger flares, particles accelerated in the corona by reconnection processes could play a significant role in the heating of plasma even in absence of large flares. Hard X-ray observations of microflares (E~10 27 erg) in active regions reveal the presence of non-thermal particles (8,9), but nanoflare size events (E~10 24 erg) are not currently accessible to hard X-ray studies due to limited sensitivity. As a result, the properties and generation of non-thermal particles in the solar atmosphere and their role in quiescent coronal heating remain poorly known.The observational tracers of the coronal heating are elusive because the corona is highly conductive, washing out the signatures of heating release. However, the emission of the TR, where the temperature steeply increases to MK values in a narrow layer (~1-3 ×10 8 cm), is instead highly responsive to heating since its density, temperature gradients and spatial dimensions change rapidly during heating events (10)(11)(12). This is the also the case for coronal heating events where ...