Magnetic field map of a sunspot. Energy transport in a sunspot. ERC Advanced Grant SOLMAG studies solar magnetic field.

SOLMAG - Solar magnetic field and its influence on solar variability and activity (ERC Advanced Grant)

SOLMAG is funded by an ERC Advanced Grant and led by Sami K. Solanki. The project will follow an integral approach for understanding the physics underlying the structure and dynamics of the solar magnetic field, combining new observational facilities, novel instruments, the next generation of inversion techniques for data analysis and state-of-the-art MHD simulations.

For life on Earth, the Sun is the most important astrophysical object in the universe. For astrophysicists, the atmosphere of the Sun presents an intriguing, complex and extremely varied environment generated by continuous dynamic, small-scale interactions between plasma and intricately structured magnetic fields.

The purpose of this project is to elucidate the physics underlying the structure and dynamics of the solar magnetic field that is responsible for the Sun’s varied activity and its variability. This goal is to be achieved by following an integral approach combining new observational facilities, novel instruments developed in the solar group of the MPS, the next generation of inversion techniques for data analysis and state-of-the-art magnetohydrodynamic simulations. This wide range of expertise present in the solar group of the MPS is unique and well suited to such an approach.

The research proposed here will provide measurements of the Sun’s magnetic field at high spatial and temporal resolution at unprecedented sensitivity to Zeeman splitting and to magnetic flux. Also, the use of a novel polarimetric hyperspectral imager, combined with the next generation of inversion techniques will allow following the 3D structure of the magnetic field and of other physical parameters in time through a sequence of snapshots. This will enable following the build-up of magnetic tension and of waves following the field lines and will set important constraints on the heating mechanism of the solar chromosphere and corona. The planned work, in particular the comparison of measurements with simulations, will also set constraints on the presence and properties of a small-scale turbulent dynamo as well as other fundamental physical processes taking place in the solar atmosphere. The techniques introduced here will enable reliable and robust measurements of chromospheric magnetic fields, shedding new light on this enigmatic but centrally important layer of the solar atmosphere.




The main aim of SOLMAG is to gain deeper insights into the source, nature, structure, dynamics, evolution and influence of the solar magnetic field. Specifically, it is aimed at answering the following set of questions:

1)      What is the total amount of magnetic flux on the Sun and where does it come from?

2)      Which fraction of the surface magnetic flux is due to small-scale turbulent dynamo action?

3)      How is the magnetic field structured in the chromosphere? How common are current sheets in the chromosphere?

4)      How bright are the weak internetwork fields and how strongly do they influence the irradiance of the Sun?

5)      How is energy transported in sunspots?

6)      Which wave modes are excited in magnetic features and travel towards the corona? How much energy do they carry? Which are the mechanisms by which they are excited?

7)      How effectively does the magnetic field get braided? How much energy does this process provide to heat the solar corona?

8)      How is the magnetic field structured in the corona? How can it be best computed starting from measurements in the photosphere and chromosphere?


The objectives of the project will be achieved by following an integrated approach using a wide set of tools. These include a set of novel, unique and innovative polarimetric instruments mounted on the best solar facilities. Such measurements will be of different types. One instrument, TIP II, will combine high spatial resolution with very high sensitivity to the Zeeman splitting and will allow accessing the magnetic field of the upper chromosphere. A second, FSP, will combine very high spatial resolution with a high sensitivity to polarization signals (i.e. to detect weak or hidden fields). A third instrument, MiHI, will provide unique observations allowing the simultaneous recording of the spatial domain in 2D and the spectrum, giving an instantaneous 3D data set from which the 3D structure of the solar atmosphere, including its magnetic field, can be reconstructed. Combined with state-of-the-art data analysis tools, such measurements will lead to a much more complete picture than could be so far obtained of the Sun’s magnetic field and its dynamic evolution in both the solar photosphere and, most importantly, in the chromosphere. In the chromosphere, the field is nearly force-free and its knowledge will enable improved extrapolations of the field into the corona, where it has its largest effect, but is largely unmeasured.

Magnetic field vector of a sunspot derived from observations. Top row: magnetic field intensity, central row: inclination and bottom row: azimuth of the magnetic field. Left to right corresponds to different optical depths (i.e. different heights in the atmosphere). Loeptien et al., A&A (2018).

The research within SOLAMG is of a ground-breaking nature thanks to the combination of methods and instruments never used together before. The knowledge gained through the planned work, the reliable and robust determination of the evolution of the full magnetic vector at unprecedented resolution and sensitivity, and the insights obtained from the MHD simulations will allow fundamental processes to be identified in the solar plasma. Because the magnetic field is the driver of solar variability and activity this will have far-reaching consequences for solar physics, but will also have implications for other fields.

Thus the present research will allow determining the complete amount of magnetic flux on the Sun and which fraction of it is produced by a small-scale turbulent dynamo. It will give new insight into the turbulent dynamo process, which may best be observable on the Sun. It will provide important constraints on the mechanisms responsible for coronal heating, since both the efficiency of field-line braiding and of the excitation of waves travelling along or across the magnetic field will be determined. It will give a better understanding of stellar magnetic fields and will open the way for challenging polarimetric measurements of astronomical objects.

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