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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - RESOLVE (REalistic Simulations and ObservationaL Validation of small-scale Energy channels on the Sun)

Teaser

Summary

The origin of the million Kelvin hot tenuous atmosphere of the Sun, the corona, surrounding the cooler 6000 K photosphere is a long-standing puzzle in modern-day solar and stellar astrophysics. The mechanisms responsible for coronal heating also plays a crucial role in the production of large flares and the acceleration of solar wind, both of which influence the space weather and the Sun-Earth connection. Solar coronal observations in the extreme ultraviolet (EUV) and X-rays reveal loop-like structures of hot plasma confined by the magnetic field. One key to solving the puzzle of the hot corona is understanding the mass and energy transfer through the solar atmosphere in these loops. The magnetic energy required to heat the coronal loops is generated by convective motions beneath the solar surface and then transported through the photosphere into the corona where it is dissipated. The details of how, and at which spatial and temporal scales, the Poynting flux is transported through the solar surface are not well understood. The objective of the project was to characterize the nature of energy transfer from the photosphere at small spatial scales by combining state-of-the-art observations and three-dimensional (3D) radiation magnetohydrodynamic (MHD) simulations. The main outcome of the action is two-fold. (1) By analyzing a variety of observations, we identified that coronal loops often have a complex magnetic topology at their footpoints. This complex topology is driven by transient events of granular-scale magnetic flux emergence and cancellation at the feet of coronal loops. These transient events result from the magnetoconvection near the photosphere. Our MHD simulations reproduced such magnetic transients and predicted that they persist on even smaller spatial scales than what the current photospheric observation resolution of ~100 km. (2) Our observations and simulations strongly suggest that the energy liberated during the reconnection of magnetic field associated with flux emergence and cancellation plays a crucial role in energizing the coronal loops. In conclusion, the action successfully achieved its objectives by revealing new details of the photosphere-corona connection and of the nature of magnetic energy release that it likely responsible for the coronal heating.

Work performed

To achieve the objectives of the action, i.e. to characterize the nature of small-scale energy transfer through the photosphere, we used observations from space- and ground-based solar telescopes and performed 3D radiation MHD simulations. By analyzing multi-wavelength observations of the solar atmosphere including the photosphere and the corona, we first identified that coronal loops are often rooted in regions of mixed-magnetic polarities undergoing transient flux emergence and cancellation at the solar surface. Our observations are supported by simulations that reproduced complex surface magnetic topology driven by transient events at the feet of coronal loops. Our estimates of energy flux associated with these small-scale magnetic transients suggests that flux emergence and cancellation at the solar surface could power the overlying hot corona through magnetic reconnection at the feet of coronal loops. The action and the newly fostered collaborations through it led to a total of 11 peer-reviewed scientific publications (two more projects are in progress). These publications are all available on the arXiv server with free access to the public. In addition, these results have been widely communicated to the scientific community in the form of invited talks, seminars, oral and poster presentations at several international conferences and research institutes in and outside Europe.

Final results

The two-fold outcome of the action can be summarized as (1) there often exists a complex magnetic topology at the feet of coronal loops, and (2) footpoint magnetic reconnection is the likely source of energy to heat the corona. These findings advance our understanding of the physics of the solar atmosphere. It was generally assumed that coronal loops are connected to a pair of opposite-polarity magnetic field patches on the solar surface. In this scenario, each footpoint is represented by a simple configuration of unipolar magnetic field in the photosphere. Contrary to this common view, our high-spatial resolution observations of the solar photosphere revealed that coronal loops are often rooted in regions with complex magnetic topology. At one or both the footpoints, the unipolar magnetic field patch that can extend to several megameters on the solar surface, is observed to be surrounded by small-scale (a few 100 km) magnetic field features with polarity that is opposite to that of the main patch. Models that focus on the photosphere-corona coupling will benefit from the new details on the complex magnetic field structure at the feet of coronal loops identified through this action. The second part of the actionâ€™s outcome deals with coronal heating. In traditional view, coronal heating is facilitated by the horizontal motions of magnetic field in the solar photosphere. These motions launch MHD waves into the solar atmosphere or stress and braid the coronal magnetic field. The energy associated with these waves or braids is then dissipated in the corona to heat the plasma. Although models based on waves and braids have been well studied, the dominant role of either of these processes in heating the corona, is not well established. Our findings highlight the key role of the complexity of the photospheric magnetic field and its interaction between small-scale opposite polarities in heating the solar corona. Based on these results, we point to a specific way, namely, magnetic reconnection due to flux emergence and cancellation at the feet of coronal loops, as a conduit of mass and energy into the corona. The action led to the proposal of a new mechanism of coronal heating through flux cancellation. Our results are expected to motivate further studies in the future to decipher the puzzle of coronal heating in cool-stars like the Sun. Observations of flux emergence and cancellation at the feet of coronal loops with upcoming solar telescopes, capable of resolving the Sun down to 30 km, will improve our understanding of the magnetic reconnection, a fundamental process in the universe.