Determining the real scale of structures in the Sun's corona has proved difficult because of limited spatial resolution. Now high-resolution imaging has allowed dynamic structures on scales of 150 kilometres to be observed. See Letter p.501
The origin of the Sun's outer atmosphere, the corona, is a long-standing scientific problem of great interest and complexity. Why does a star with a surface temperature of roughly 5,700 kelvin have an outer atmosphere with temperatures in excess of 1 megakelvin, and why does the corona exhibit phenomena such as flares? The answer lies in the energy contained in the Sun's magnetic field, which fills the corona, as inferred from coronal images at extreme ultraviolet and X-ray wavelengths. How the magnetic energy is dissipated in the corona and sustains its temperature is controversial, but comes down to a determination of the spatial scales of coronal structures. On page 501 of this issue, Cirtain et al.1 identify dynamic structures on scales of 150 kilometres, which represents a major constraint that theories must now confront.
Before 2012, the best spatial resolution of the solar corona was obtained by NASA's Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory spacecraft, which was launched in 2010. The instrument resolves scales of about 900 km and looks at several wavelength ranges corresponding to different temperatures. However, images of the visible solar surface at a resolution of 100 km show distinct magnetic and plasma structures, and so the question arises as to whether structures with these scales are also present in the corona.
In their study, Cirtain et al. used the High-resolution Coronal Imager (Hi-C), a new extreme-ultraviolet instrument that was launched on a rocket on 11 July 2012 and obtained roughly 5 minutes of data before re-entering Earth's atmosphere. The instrument looks at coronal plasma with temperatures of around 1.5 MK, and is capable of spatial resolution at least five times better than the AIA: the Hi-C can resolve scales on the order of 150 km. To place this in context, it took more than 30 years to improve the spatial resolution from the few thousand kilometres obtained by instruments on NASA's Skylab observatory to that obtained with the AIA. The Hi-C instrumentation performed up to expectation, and images of the Sun show unambiguous structure at the desired resolution — a huge achievement.
A striking feature of Cirtain and colleagues' results is the dynamic structures visible at the limit of resolution, clearly evident by comparing images from the Hi-C and AIA in the paper's Supplementary Videos 1 and 2 (ref. 1). (The reader should also look at other aspects of the videos to note how much else is happening on these small scales, as is also evident in Fig. 1.) The dynamic behaviour of the observed structures is interpreted as evidence for 'magnetic braiding', an effect in which small bundles of magnetic field become wrapped around each other owing to plasma motions at the solar surface2. Whether this is in fact the case is unclear, but there seems little doubt that magnetic-field dissipation on a fundamental scale is seen, with different field elements interacting with one another through magnetic reconnection3, a process that changes the magnetic-field topology through dissipation of electric currents. To me, the Hi-C images are reminiscent of computational models of the kink instability, a process known from plasma physics that is also thought to occur in the corona4. Although such processes have long been conjectured, prima facie evidence for coronal reconnection, as found by Hi-C, is an important result.
A more general point concerns the very presence of structures at this resolution. There has long been a debate about when coronal structures are resolved; that is, what is an elemental structure? In the past, some have stated that structures seen by earlier solar missions are resolved, or 'monolithic'. Others have argued from theory and interpretation of data5,6,7 that scales on the order of 100 km were to be expected, and that such high resolution was needed. Indirect evidence from the AIA had also begun to point the way to such scales8, but the Hi-C results show that any debate on the structure of the corona now needs to address scales of 100–200 km or smaller, as can be seen in Figure 1.
Clearly, even in 5 minutes of observations there is a wealth of data that need to be analysed. The next stage is securing Hi-C, or an instrument with similar or improved performance, on an orbiting spacecraft. This spacecraft must also carry a modern extreme-ultraviolet spectrometer9 — both to complement Hi-C and to provide fundamental plasma measurements of density, temperature, velocities and small-scale turbulence — as well as an instrument capable of measuring signatures of energetic particles, which are known to be a significant product of the magnetic-reconnection process10. Only with such complete instrumentation can a proper understanding of coronal structures be attained.
Has Hi-C really resolved the corona? To do this will require observation of a wider range of solar conditions than is feasible in a short rocket flight, and one should not bet against the existence of further fine structure within the scales detected by Hi-C. But for those who have wanted to see observations on such scales for decades, there is a feeling that things are getting interesting, and quantitative tests of competing theoretical ideas can be undertaken, as is evident from the above discussion of this short data set.
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Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences (2015)