New Windows onto the Sun: Probing the Sun’s magnetic field with an array of new missions and observatories

Project Information

WINSUN

Grant agreement ID: 101097844

DOI

10.3030/101097844

EC signature date 14 May 2023

Start date 1 September 2023

End date 31 August 2028

Funded under

• European Research Council (ERC)

Total cost

€ 2 498 750,00

EU contribution

€ 2 498 750,00

Coordinated by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany

Periodic Reporting for period 1 - WINSUN (New Windows onto the Sun: Probing the Sun’s magnetic field with an array of new missions and observatories)

Reporting period: 2023-09-01 to 2026-02-28

Summary of the context and overall objectives of the project

The Sun provides the energy necessary to sustain life on Earth, making it a star of unique importance for human society. It is also the only star whose surface we can resolve to reveal the richness of the complex processes acting there, creating a highly dynamic and varied environment. Much of the structure and dynamics visible on the Sun is caused by the intricately structured solar magnetic field and its interaction with the plasma that constitutes the solar atmosphere. However, there are considerable gaps in our knowledge of the fundamental physical processes driving the evolution of the solar magnetic field, and how the field drives solar activity and variability, both of which can have an impact on Earth. Solar activity can cause extreme solar events such as Coronal Mass Ejections, which can affect the performance of a wide range of space-borne and ground-based technological systems and pose a danger to human health and safety. Solar variability can have a significant impact on the Earth’s climate.

To fill these gaps in our knowledge, this project will make use of powerful new observational missions and facilities, which will open new windows onto the Sun and its magnetic field: ESA’s Solar Orbiter space mission, the balloon-borne solar observatory Sunrise III, the Daniel K. Inouye Solar Telescope (DKIST) in Hawaii and India’s solar space mission Aditya-L1. The advanced instrumentation, complemented by novel data analysis techniques and state-of-the-art magneto-hydrodynamic simulations, will allow tackling, often in entirely new ways, long-standing difficult problems that have resisted previous attempts at resolving them. Elucidating these will provide deep insights into the life cycle of the solar magnetic field, and how it drives the Sun’s activity and variability. Ultimately, this may help to better cope with the risks that these phenomena pose for the Earth.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

In the first phase of the WINSUN project, a lot of effort was put into building and testing tools, instruments, and data analysis methods. These are essential for collecting and understanding the solar data needed to answer the questions posed the WINSUN project.

For example, the Sunrise III balloon-borne observatory, after a failed launch in 2022, was successfully flown in 2024 and collected 200 terabytes of unique data. These data are now being processed and are already contributing to scientific work. A new software tool was also developed to better interpret this data. Another important source of data is the Solar Orbiter spacecraft, which has now moved slightly out of the plane of Earth’s orbit, allowing, e.g. the SO/PHI instrument to observe the Sun’s poles better than ever before. Its early results are promising and are already being used in scientific studies. Finally, the other main sources of observational data needed for the project, the DKIST ground-based telescope and the SUIT instrument on India’s Aditya satellite have begun collecting data.

Using the data gathered by the above instruments, most of which were developed by or with contributions from my group, as well as simulations with the in-house MURaM code, progress has been made in answering most of the main questions raised in the WINSUN ERC proposal. This includes:
- Work on determining the solar magnetic flux from different viewing angles showed that around a factor of two of magnetic flux is missed by most magnetographs.
- Using Solar Orbiter data, both the fast and the Alfvenic slow solar wind were shown to be driven by small-scale jets.
- With the help of 3D radiative transfer simulations it was shown that even large, 4m diameter solar telescopes should be able see structures in the solar photosphere at their diffraction limit.
- MHD simulations with the MURaM code finally reproduced important observations in the solar chromosphere in the Mg II h and k spectral lines, which had resisted earlier such attempts.
- The project also contributed to understanding how the Sun’s brightness (irradiance) has changed over the past century and even millennia, using both modern and historical data: Total solar irradiance reconstructions were improved by filling gaps in magnetogram time series using Ca II images; a new reconstruction covering the last century was carried out of the Mg II index, an important contributor to UV irradiance and a diagnostic of irradiance variations; the butterfly diagram of solar plage areas was reconstructed over a century from historic Ca II images; and the first reconstruction of sunspot cycles over the last millennium BC based on new yearly 14C measurements was presented.

Overall, WINSUN has successfully transitioned from tool-building to making significant scientific discoveries about the Sun.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Progress has been made in solving the missing flux problem that has bedevilled solar physics for decades. It consists of the fact that the total amount of magnetic flux measured in the heliosphere, at Earth’s orbit is a factor of at least 2 higher than the open magnetic flux at the solar surface, i.e. the magnetic flux that can reach into the heliosphere, detected in magnetograms. Since there are no sources of additional magnetic flux beyond the solar surface, such a result clearly does not agree with physics. A careful study has revealed that most solar magnetograms miss around half of the true magnetic flux everywhere on the solar surface, even in the source regions of the open magnetic flux. This provides a new possible solution of the missing flux problem, which will have to be observationally tested.

Computer simulations using the powerful MURaM code have helped explain key solar processes. This work has for the first time demonstrated that important diagnostics of the conditions in the solar chromosphere, the strengths and widths of the Mg II h and k lines, are reproduced by MHD simulations without requiring any further tuning. This implies that these models have now reached a state of maturity that allows them to be used to explain important chromospheric phenomena and in so doing shed light on the physics of this important and so far poorly understood layer of the solar atmosphere.