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Solar magnetic field and its influence on solar variability and activity

Periodic Reporting for period 3 - SOLMAG (Solar magnetic field and its influence on solar variability and activity)

Reporting period: 2019-10-01 to 2021-03-31

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.
Various novel instruments for doing very precise and/or high resolution spectropolarimetry have been finalized or advanced, e.g. the Fast Solar Polarimeter (FSP) and the Microlens Hyperspectral Imager. These instruments have been used for high-resolution measurements, e.g. to probe the turbulent magnetic field of the Sun. The observations have shown an unexpected correlation of the polarization signal with the local brightness. Combined with a novel analysis technique the data for the first time reveal the spatial distribution and structure of forward scattering-induced polarization and how it is related to the granulation pattern.

Crucial tools for numerical modeling have been significantly advanced. In particular, 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, primarily non-LTE radiative energy transfer and time-dependent ionization and recombination of hydrogen. The computations with this code show the solar chromosphere to be highly dynamic, much more than previous simulations did.

Vortices propagating along magnetic field concentrations are prime candidates to transport energy from the solar surface into the upper atmosphere. 3D radiative MHD simulations have shown 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 active region plages.

High spatio-temporal resolution is required to capture the details of the likely magnetic reconnection-driven bursts associated with the heating of the chromosphere. Investigation of an observed UV burst reveals a spatial morphology that indicates a unified picture of magnetic heating in the solar atmosphere from large-scale flares to small-scale bursts.

A complex magnetic topology at the footpoints of loops, associated with flux cancellation in the photosphere, is proposed to play an important role in energizing active region coronal loops driven by nanoflares. A majority of the brightenings of hot loops in active region cores are found to be associated with complex magnetic morphologies at their footpoints.

Further achievements include the identification of the role of the fluting instability for the decay of sunspots, an quantification of the extent to which observational results based on the inversions of photospheric spectral lines are affected by non-LTE and horizontal transfer effects, the finding that in many cases the spatial resolution of solar observations is insufficient to obtain the correct power law index often used to characterize the form of turbulence in a particular system, and a new and more accurate method for the measurement of the Wilson depression, i.e. the depression of the visible surface of the Sun in sunspots.
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 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, to model the total solar irradiance (TSI) without having to calibrate the result based on observed TSI, and to perform 3D MHD simulations of a full coronal loop with a fully self-consistent driver in the photosphere.

For a better understanding of the causes of global climate change, it is necessary to refine our knowledge of natural drivers, including the Sun. 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 MHD simulations, we have 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 limits the contribution from solar variability to global warming.

Achievements still expected until the end of the project include first simulations with the new 3D MHD code that covers all layers of the solar atmosphere, a better understanding of the role of the magnetic fields produced by a turbulent small-scale dynamo in driving the solar atmosphere and new insights into the heating of the solar chromosphere and corona.
Magnetic field vector of a sunspot derived from observations. Loeptien et al., A&A (2018).