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Heating and structure of Stellar Chromospheres

Final Activity Report Summary - CHROMORAD (Heating and structure of stellar chromospheres)

The project aims at a better understanding of the structure, dynamics and heating of the chromospheres of our Sun and other stars. The chromosphere of the Sun is a thin layer of the solar atmosphere, embedded between the photosphere (the visible "surface") and the corona above. This layer was only poorly understood in the past because it is difficult to observe. One open question was how the high temperatures implied by observations are maintained. It would require heating mechanisms with consequences for the solar atmosphere as a whole.

In this project the solar chromosphere was studied by means of numerical simulations in comparison with observations. We took advantage of new instruments, which are currently world-leading in high-resolution observations of the Sun: IBIS at the Dunn Solar Telescope (NM, USA), CRISP at the Swedish Solar Telescope (La Palma, Spain), and the Japanese-led Hinode satellite.

The recent progress in instrumentation enabled solar observations with hitherto unreached combination of high resolution simultaneously in the spatial, temporal, and spectral domains. The individual results provided new constraints for a revision of our picture of the solar chromosphere. The structure and the dynamics are much more intermittent on small spatial and temporal scales than previously known. The solar chromosphere is no longer regarded as a separate layer but rather as part of a compound of dynamically coupled atmospheric domains. Moreover, even quiet regions of the Sun are likely to be permeated with weak magnetic fields, making the related heating processes more complex. It may therefore not be possible to separate a purely acoustic contribution to the atmospheric heating.

Two individual findings are particularly noteworthy. Firstly, we succeeded to settle a long-lasting apparent conflict concerning the continuum radiation that emerges from the solar photosphere at visible wavelengths. Modern numerical simulations produced synthetic radiation intensity maps that usually had a contrast (the so-called "granulation contrast") that was much higher than actually observed. The simulations were therefore often considered being unrealistic. By properly accounting for the influence of instrumental image degradation, we were now able to reach agreement between our models and modern space-borne observations with Hinode. This result proves that the numerical models produced in this project and with it similar state-of-the-art models are realistic enough to be used for quantitative applications such as the current discussion on elemental abundances in the universe and here for the determination of atmospheric heating.

Secondly, we discovered a new process that potentially contributes to the heating of the solar atmosphere. High-resolution observations with the CRISP instrument at the Swedish Solar Telescope revealed hitherto unknown swirl motions in the chromosphere with diameter of the order of only app.1500 km. We interpret these swirl motions as a direct indication of upper-atmospheric magnetic field twisting and braiding -- a mechanism, which is one of the prime candidates for the heating of stellar coronae.