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CHROMPHYS Report Summary

Project ID: 291058
Funded under: FP7-IDEAS-ERC
Country: Norway

Final Report Summary - CHROMPHYS (Physics of the Solar Chromosphere)

The enigmatic chromosphere is the transition between the solar surface and the eruptive outer solar atmosphere. The chromosphere harbours and constrains the mass and energy loading processes that define the heating of the corona, the acceleration and the composition of the solar wind, and the energetics and triggering of solar outbursts (filament eruptions, flares, coronal mass ejections).

The recently (June 27th 2013) launched satellite Interface Region Imaging Spectrograph (IRIS) provides a leap in observational capability of the chromospheric plasma with an unprecedented combination of high spatial, temporal and spectral resolution in lines with diagnostic information all the way from the photosphere to the upper transition region. To fully extract this information it is necessary to combine the observations with numerical simulations that include a realistic description of the complicated physics of the chromosphere.
The most essential part of the CHROMPHYS project is to provide such realistic simulations and combine them with observations to achieve a break-through in our understanding of the solar chromosphere. To this end we have analyzed observations from the Swedish 1-m Solar Telescope and IRIS, further developed the numerical code Bifrost, analyzed physical processes in the simulations and computed and analyzed synthetic observables. In particular we have analyzed the synthetic observables to deduce how to interpret observations from IRIS. A series of papers on the diagnostic interpretation of IRIS observables have been published. The first results from IRIS have been published with members of the CHROMPHYS project as lead authors or co-authors, five of them in Science.

IRIS reveals a chromosphere and transition region that are replete with twist or ubiquitous torsional motions on sub-arcsecond scales, occurring in active regions, quiet Sun and coronal holes alike. We have also identified the so-called “Unresolved Fine Structure” that has been postulated to contain a large part of the transition region plasma at temperatures below 200,000 K. With IRIS these cool loops are now resolved and we have identified loops with similar structure and lifetimes in the numerical simulations. A combination of IRIS observations and numerical simulations of different heating mechanisms give strong evidence of heating by nano-flares and energy transport by beams of electrons at the foot-points of hot coronal loops. We have been able to give constraints on the beam parameters (like total energy, energy spectrum and cut-off energy). In the IRIS observations we also see evidence of locations of strong energy release in the upper photosphere with heating up to 80,000 K. Such events are also evident in simulations with emerging flux interacting with existing flux.

The simulations combined with observations have furthermore shown the importance of 3D radiative transfer for realizing the diagnostic potential of the hydrogen Balmer-alpha line. The long term discrepancy between modelling results and the intensity in the resonance lines from helium (observed intensities have always been 10-20 times higher than models can explain) has finally found its solution: the large intensities are caused by the non-equlibrium ionization of helium. The resonance line from singly ionized helium at 304 Å is one of the most used spectral diagnostics in solar UV imaging and these observations can now finally be translated into a physical understanding.

These findings represent important steps towards a much-improved understanding of the solar chromosphere.

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