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Solar magnetic field and its influence on solar variability and activity

Periodic Reporting for period 4 - SOLMAG (Solar magnetic field and its influence on solar variability and activity)

Reporting period: 2021-04-01 to 2021-09-30

For life on Earth, the Sun is the most important astrophysical object in the universe. It is the overwhelming source of energy to the Earth, and it has a potentially threatening effect on Earth. This occurs in the form of space weather, caused by the interplanetary magnetic field originating from the Sun, the fast solar wind, and coronal mass ejections, which affect not only our natural environment, but also sensitive technical systems such as satellites and communications, or the power grid. On longer time scales, the changes in solar irradiance influence the Earth’s atmosphere and may contribute to global climate variability.

The variability and activity of the Sun are caused by the Sun’s highly structured and strongly dynamic magnetic field. The field is responsible for the appearance of dark sunspots and bright faculae, for the high temperature and shape of the corona, and for the acceleration of the solar wind. It produces massive flares and coronal mass ejections. The magnetic field and its cycle is also the driver of the variations in solar irradiance. However, the solar magnetic field remains ill-understood.

The interactions of the magnetic field with the turbulent convection acting in and below the solar atmosphere structure the field on a wide range of spatial scales. Similarly, the field is very dynamic and evolves rapidly. Hence, to get to the heart of the causes of solar activity and variability, the magnetic field must be precisely and reliably measured, not just at the solar surface, but also in the next higher layer, the chromosphere, the most enigmatic part of the solar atmosphere. At the same time, it is necessary to complement such measurements with the best techniques to extract information from the data and the insights gained from numerical simulations.

The purpose of the SOLMAG project is to elucidate the physics underlying the structure and dynamics of the solar magnetic field responsible for the Sun’s activity and variability. This goal will be achieved by following an innovative integral approach, combining novel instruments, the next generation of techniques for data analysis and state-of-the-art 3D numerical simulations.
Novel instruments for doing very precise and/or high resolution spectropolarimetry have been finalized or advanced, e.g. the Fast Solar Polarimeter based on CMOS detectors and the Microlens Hyperspectral Imager. These instruments have been used to probe the turbulent magnetic field of the Sun. An unexpected correlation of the polarization signal with the local brightness was obtained.

A statistical method of correcting stray-light contamination due to residual high-order aberrations, a long-standing issue in solar ground-based observations, has been developed and applied to ground-based slit spectra. Deconvolution raised the granulation contrast to values close to those expected from theory.

A thorough search of the Gregor/GRIS instrument revealed many supersonic downflows associated with various active region features. By far the most dominant mechanism driving such flows appears to be the draining of plasma along the legs of rising magnetic loops.

Magnetic reconnection and associated jets may heat the gas above light bridges. All mixed polarity light bridges investigated display extremely strong magnetic fields reaching strengths of 8.2 kG.

The physical causes of Evershed and counter-Evershed flows were identified: Whereas the Evershed flow is convective in nature, the counter-Evershed flow is a siphon flow.

The MURaM 3D radiation MHD code for the simulation of the surface regions of the Sun has been extended to cover the upper solar atmosphere and in particular the chromosphere. This required adding a considerable amount of new physics into the code.

Our 3D radiative MHD simulations showed that vortices are ubiquitous in small-scale magnetic features and that most of them lie below the resolution of current observations. They are found to play a large role in transporting energy to the upper solar atmosphere and actually carry sufficient energy into the chromosphere to heat it in even in active region plages.

MHD simulations also showed that braiding takes place not between different magnetic features, as proposed in the literature, but rather within individual magnetic features. This changes the picture of coronal heating by braiding significantly. In addition, complex magnetic topology at the footpoints of loops, associated with flux cancellation in the photosphere, is shown to play an important role in energizing active region coronal loops driven by nanoflares.

3D MHD simulations showed that sunspots start breaking up deep in the sun. The breakup process then propagates to the surface, first becoming visible as light bridges.

The superiority of many-line inversions compared with few-line inversions was clearly shown.
In addition, we found that neglecting non-LTE and even horizontal transfer significantly affects inversion results, even for photospheric lines.

Extrapolations of the magnetic field into the upper atmosphere based on the recently developed methods employing magnetohydrostatic solutions were found to be better than non-linear force free field methods.

For the first time the variation of the total solar irradiance was reconstructed without having to calibrate the modelled irradiance with the measured values, a major advance. Using these results, a tight upper limit was set on the amount by which the Sun can have brightened since the 17th century.

Many more results were achieved, but cannot be described due to lack of space.
Instrumentation with unprecedented capabilities for solar observations has been completed and the required tools for data handling and analysis as well as numerical simulations required for reliable interpretation of the data have been extended beyond their previous limitations. E.g. the MURaM radiative MHD code has been significantly extended to include non-LTE radiation transfer and the time-dependent ionization of hydrogen, which makes it a very powerful code that can cover all solar atmospheric layers with much of the relevant physics while at the same time being able to catch the highly dynamic processes in the solar atmosphere.

Novel methods have been developed to determine the Wilson depression in sunspots, to detect the structure of forward scattering polarization in the lower solar atmosphere, and to model the total solar irradiance (TSI) without having to calibrate the result based on observed TSI.

The latter significantly improves our understanding of the role of the Sun as a potential natural driver of global climate change. Of major importance is knowing by how much the solar irradiance has varied in the past centuries, and in particular its increase since the Maunder minimum, a period of very low solar activity. Previously, the estimates of this increase diverged by a full order of magnitude. Using advanced 3D radiation-MHD simulations, we have first proven that magnetic features at the solar surface are the overwhelming cause of solar irradiance variations on timescales between days and decades. This approach clearly goes beyond the state-of-the-art. Using this result, we then put a stringent upper limit of 2.7 W/m2 on the amount by which the Sun was dimmer in the Maunder minimum compared with 2019. This is the first time that such a limit could be set and it limits the contribution from solar variability to global warming.
Magnetic field vector of a sunspot derived from observations. Loeptien et al., A&A (2018).