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Solar magnetic field and its influence on solar variability and activity

Periodic Reporting for period 2 - SOLMAG (Solar magnetic field and its influence on solar variability and activity)

Reporting period: 2018-04-01 to 2019-09-30

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. Space weather not only affects 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 a result of the Sun’s highly structured, complex 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 solar 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 that is responsible for the Sun’s activity and variability. This goal will be achieved by following an innovative integral approach, combining new observational facilities, novel instruments, the next generation of techniques for data analysis and state-of-the-art 3D numerical simulations.
Cameras for a new type of solar infrared spectropolarimeter, which allows polarized spectra to be recorded close to the diffraction limit of the telescope, have been procured and promising test observations have been carried out. Data reduction software has been written/improved for various novel instruments for doing solar spectropolarimetry.

The turbulent magnetic field of the Sun has been probed using the new Fast Solar Polarimeter. The observations have shown the presence of an unexpected correlation of the polarization signal with the local brightness and the presence of substructure in the polarization data.

The MURaM 3D radiation magnetohydrodynamic code for the simulation of the surface regions of the Sun has been extended to cover the upper solar atmosphere. This is a major task that requires to add a considerable amount of new physics into the code. First tests look very promising, showing considerable dynamics in the solar chromosphere.

To make inversions of spectral line profiles more robust and reliable, the correct set of lines needs to be known. Ideal are so-called Zeeman line pairs, i.e. lines which are very similar, except in their magnetic properties. Only a small handful of such pairs were known so far. We could add two extremely promising line pairs. The Sun will be observed in these lines using the new instruments.

A new method for the measurement of the Wilson depression, i.e. the depression of the visible surface of the Sun in sunspots, has been developed and shown to work more accurately than previous methods. It has been applied to a set of recorded sunspots, leading to a number of new findings. The results indicate that the geometry of the magnetic eld in the penumbra is dierent between spots with dierent strengths of the average umbral magnetic eld.

Horizontal vortices are prime candidates to transport energy from the solar surface into the upper solar atmosphere. Radiation MHD simulations have shown that vortices are ubiquitous in small-scale magnetic features and that most of them lie below the spatial resolution of current observations. They are found to play a disproportionately large role in transporting energy to the upper solar atmosphere.

High spatio-temporal resolution is required to capture the finer details of the likely magnetic reconnection-driven, rapidly evolving bursts associated with the heating of the solar chromosphere, and to reveal their role in chromospheric heating. Investigation of an observed UV burst reveals a spatial morphology similar to that of a large-scale solar flare with a circular ribbon. The observations hint at a unified picture of magnetic heating in the solar atmosphere from large-scale flares to small-scale bursts, all associated with a complex fan-spine magnetic topology.

It was shown that magnetic reconnection near the footpoints of hot coronal loops in solar active regions can be responsible for heating and brightening of these loops. A complex magnetic topology at the base of loops with flux cancellation in the photosphere and resulting chromospheric reconnection is proposed to play an important role in energizing active region coronal loops driven by nanoflares.

Counter-Evershed flows have been observed in sunspot penumbrae, i.e. flows directed toward the umbra along penumbral filaments. The driving forces of such counter-Evershed flows in a radiative MHD simulation of a sunspot were identified. It was found that the Evershed flow occurs due to overturning convection in a strongly inclined magnetic field, while the counter-Evershed flows can be described as siphon flows
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.

The MURaM radiation MHD code has been significantly extended to include non-LTE radiation transfer, which makes it a unique 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
- detect the structure of forward scattering polarization in the lower solar atmosphere
- model the total solar irradiance (TSI) without having to calibrate the result based on observed TSI
- perform 3D MHD simulations of a full coronal loop coupled with a fully self-consistent driver in the photosphere.

By the end of the project it is expected that
- the full power of the Microlens Hyperspectral Imager will have been revealed, with many unique results on the structure and dynamics of magnetic features in the photosphere and in the chromosphere,
- a fully working 3D MHD code that covers the photosphere and chromosphere, besides the corona and transition region, will be available,
- the role of the magnetic fields produced by a turbulent small-scale dynamo in forming and driving the solar atmosphere will be better understood,
- important new insights are gained into the heating of the solar chromosphere and the corona.
Inverted magnetic field vector for the Hinode observation of a sunspot. Top row: magnetic field inte