Coronal Loops: Observations and Modeling of Confined Plasma

Abstract

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 loop classification and populations, and then describes the morphology of coronal loops, its relationship with the magnetic field, and the concept of loops as multi-stranded structures. The following part of this section is devoted to the characteristics of the loop plasma and of its thermal structure in particular, according to the classification into hot, warm, and cool loops. Then, temporal analyses of loops and the observations of plasma dynamics and flows are illustrated. In the modeling section 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 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. Finally, a brief discussion about stellar X-ray emitting structures related to coronal loops is included and followed by conclusions and open questions.

The magnetic field in the solar atmosphere

“…Large-scale loop systems (sometimes exhibiting sigmoidal shapes) are often found to connect neighboring ARs and/or ARs with their quiet Sun surrounding (Strong 1994). Following Reale (2010), the observed coronal loop systems span a wide range of length scales, from a few Mm (bright points) up to giant arches which may span 1 Gm (gigameter). Several loop arcades neighboring each other are often found in magnetically complex ARs and often host eruptive processes such as flares or CMEs (see Sect.…”

“…Much of the plasma in the corona is confined by the magnetic field in the form of thin closed flux tubes. The loop plasma may be heated by a number of possible mechanisms (for a review see Reale 2010). As a consequence, pressure and density of the gas within the magnetic loops are enhanced compared to the values reached by the surrounding gas and the coronal loops appear bright in coronal radiation.…”

“…The bulk of the coronal loops hosts plasma of temperatures of ≈10 5 K to ≈10 MK. The lower and upper limits are representative of cool and flaring loops, respectively (Reale 2010). Coronal loops often fan out from relatively compact footpoints at the solar surface, arranging themselves in dome-like structures with multiple layers.…”

Nonlocal heat flux effects on temperature evolution of the solar atmosphere

“…Thus, the differences between the SH model and any other model that considers the A32, page 10 of 12 contribution from the fast electrons will be considerably smaller. Observations have shown that in flaring conditions, the TR is sharp (Reale 2010) and therefore there will be contributions from the tail electrons. The importance of these contributions can only be completely understood when considering other mechanisms that influence the shape of the TR, for example, a better description of the ionization and excitation process in the chromosphere and other microphysical mechanisms that have not been considered in our model.…”

Forward Modelling of a Brightening Observed by AIA

“…Extensive overviews of coronal loops are given by Bray et al (1991), with Reale (2014) specifically addressing them as bright structures confining plasma, while insight into their structure is given by Peter et al (2013). Reale et al (2000) presented detailed modelling of a coronal loop that was found to be impulsively heated.…”