1 Introduction

The corona is the outer part of the solar atmosphere. Its name derives from the fact that, since it is extremely tenuous with respect to the lower atmosphere, it is visible in the optical band only during the solar eclipses as a faint crown (corona in Latin) around the black moon disk. When inspected through spectroscopy the corona reveals unexpected emission lines, which were first identified as due to a new element (coronium), but which were later ascertained to be due to high excitation states of iron (Grotrian, 1939; Edlén, 1943). It became then clear that the corona is made of very high temperature gas, hotter than 1 MK. Almost all the gas is fully ionized there and thus interacts effectively with the ambient magnetic field. It is for this reason that the corona appears so inhomogeneous when observed in the X-ray band, in which plasma at million degrees emits most of its radiation. In particular, the plasma is confined inside magnetic flux tubes that are anchored on both sides to the underlying photosphere. When the confined plasma is heated more than the surroundings, its pressure and density increase. Since the tenuous plasma is optically thin, the intensity of its radiation is proportional to the square of the density, and the tube becomes much brighter than the surrounding ones and looks like a bright closed arch: a coronal loop.

When observed in the X-ray band, the bright corona appears to be made entirely by coronal loops that can, therefore, be considered as the building blocks of X-ray bright corona. This review specifically addresses coronal loops as bright structures confining plasma. It first provides an observational framework that is the basis for the second part of the review dealing with modeling and interpretation.

The observational section (3) discusses loop classification and populations, and then describes the morphology of coronal loops, its relationship with the magnetic field, regarding the shape and cross-section, and the concept of loops as consisting of bundles of strands, whose thickness may go down to sub-arcsecond scale. The following part of this section is devoted to the characteristics of the loop plasma and of its thermal structure. Diagnostics of the emission measure and of its temperature distribution retrieved from filter ratios and spectroscopy are introduced. The thermal properties of the loops are discussed according to a broad classification into hot, warm, and cool loops. Hot loops are best observed in the soft X-rays and in active regions, and attention is devoted to the possible presence of minor very hot components out of flares. Warm loops are those better observed in several EUV bands with lines emitted around 1 MK and often found to be more isothermal and dense than expected, probably because they are out of equilibrium. The emission measure distribution of loops in the whole coronal temperature range is reviewed paying attention to its shape and broadness, that may indicate or not the coexistence of many heating-cooling cycles. Then, temporal analyses of loop light curves focus on searching for variability in different bands that may indicate a highly variable or more steady heating, and the characteristic timescales. The observations show more and more evidence for significant plasma dynamics and flows, from subsonic to supersonic. Widespread patterns of redshifts and blueshifts are found in different temperature regimes and their spatial distribution is also discussed. Evidence for upflows from the chromosphere and of possible coronal counterparts is addressed. Observations reporting on the detection of waves propagating along loops are also mentioned.

In the modeling section (4) some basics of loop physics are provided, supplying some fundamental scaling laws and timescales, a useful tool for consultation. The concept of loop modeling is introduced and models are distinguished between those treating loops as monolithic and static, and those resolving loops into thin and dynamic strands. Then, more specific discussions address how modeling the loop fine structure is able to explain observed evidence for deviations from equilibrium and different filling factors in different bands, and can help investigating the concept of randomly-distributed heat pulses. Models also address plasma flowing along the loops, both as siphon flows and as motions driven by dynamic heating, i.e., hot upflows from the chromosphere first and downflows from draining afterwards. Special attention is devoted to the question of loop heating, which is strictly connected to the general problem of coronal heating. The conversion of magnetic energy into heat and the problem of the difficult diagnostics of the heating are first discussed in general terms. Impulsive (DC) and wave (AC) heating are separately discussed. DC models have extensively investigated the heating by nanoflares searching for possible signatures and properties such as their frequency and location. AC heating models focus on the way to dissipate waves and to match the observational scenario. Some discussion is devoted to MHD models that describe the solar atmosphere from the chromosphere to the corona on a larger area, and including the magnetic field and the radiative transfer, and to models that describe the magnetic field dissipation through turbulent cascades to very small scales.

There have been several earlier books (Bray et al., 1991*; Golub and Pasachoff, 1997, 2001; Aschwanden, 2004) and reviews (Vaiana and Rosner, 1978*; Peres and Vaiana, 1990*; Golub, 1996; Aschwanden et al., 2001*; Reale, 2005), in particular on coronal heating (Zirker, 1993; Cargill, 1995; Klimchuk, 2006*; De Moortel and Nakariakov, 2012; Parnell and De Moortel, 2012), that have in general a larger or different scope but include information about coronal loops. Interested readers are urged to survey these other reviews in order to complement and fill in any gaps in topical coverage of the present paper.

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