Structure of solar coronal loops: from miniature to large-scale

Abstract

Aims. We use new data from the High-resolution Coronal Imager (Hi-C) with its unprecedented spatial resolution of the solar corona to investigate the structure of coronal loops down to 0.2 . Methods. During a rocket flight, Hi-C provided images of the solar corona in a wavelength band around 193 Å that is dominated by emission from Fe  showing plasma at temperatures around 1.5 MK. We analyze part of the Hi-C field-of-view to study the smallest coronal loops observed so far and search for the possible substructuring of larger loops. Results. We find tiny 1.5 MK loop-like structures that we interpret as miniature coronal loops. Their coronal segments above the chromosphere have a length of only about 1 Mm and a thickness of less than 200 km. They could be interpreted as the coronal signature of small flux tubes breaking through the photosphere with a footpoint distance corresponding to the diameter of a cell of granulation. We find that loops that are longer than 50 Mm have diameters of about 2 or 1.5 Mm, which is consistent with previous observations. However, Hi-C really resolves these loops with some 20 pixels across the loop. Even at this greatly improved spatial resolution, the large loops seem to have no visible substructure. Instead they show a smooth variation in cross-section. Conclusions. That the large coronal loops do not show a substructure on the spatial scale of 0.1 per pixel implies that either the densities and temperatures are smoothly varying across these loops or it places an upper limit on the diameter of the strands the loops might be composed of. We estimate that strands that compose the 2 thick loop would have to be thinner than 15 km. The miniature loops we find for the first time pose a challenge to be properly understood through modeling.

What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating?

“…Loops in the model match observations in terms of diameter, or cross section [68,102]. In general, loop structures found in observations, such as thin or thick loops at constant cross section, or expanding envelopes of several thin loops, are also seen in the three-dimensional MHD models [103]. A three-dimensional MHD model that was run to match an actual observation did even reproduce the location of a set of loops in three-dimensional space, as revealed by a comparison to stereoscopic observations [48].…”

The magnetic field in the solar atmosphere

“…1; alternatively, one may state that a diameter of 0. 1 represents an upper limit for the strands which make up an observed loop Peter et al 2013).…”

Case study of a complex active-region filament eruption

“…Consequently, our observation supports that the tether-cutting of magnetic loops occurred before the filament eruption. The coronal loops may be composed of many strands (Peter et al 2013). In this event, we observed that the bright loop split into several strands.…”

“…Priest et al 1989;Rust & Kumar 1994;Amari et al 1999;Gibson & Fan 2006). Van Ballegooijen & Martens (1989) and Aulanier et al (2010) proposed that shearing and converging footpoint motions will force magnetic reconnection between the arcade field lines, progressively transforming the arcade magnetic field lines into a twisted flux tube.…”