Researcher at the Institute of Solar Physics (KIS) in Freiburg receives ERC Advanced Grant

The European Research Council (ERC) has honoured the outstanding research of PD Dr Petri Käpylä with an ERC Advanced Grant for the NeoCon project. The five-year grant is endowed with 2.5 million euros. Advanced Grants are reserved for researchers who play a formative role in their field of expertise and whose research experience exceeds 10 years.

In 2024, the European Research Council received a total of 1066 project proposals in the field of physics and engineering. From these, the ERC selected 118 applications for funding, including PD Käpylä's proposal.

Since 2023, Dr Käpylä has been head of the „Solar-Stellar Connections“ department at the Institute of Solar Physics (KIS) in Freiburg. He teaches at the University of Freiburg Institute of Physics which habilitated him at the beginning of 2025 on the basis of his habilitation thesis ‘Ab initio Simulations of Stellar Convection and Dynamos’. Previously, Dr Käpylä conducted research at the University of Göttingen where he was funded by a DFG Heisenberg Fellowship (DFG Heisenberg Fellow). He completed his PhD in astronomy at the University of Helsinki (Doctor of Philosophy in Astronomy), followed by positions in Sweden (Nordita Stockholm), Finland (University of Helsinki, Aalto University), and Germany (Leibniz Institute for Astrophysics Potsdam).

Vigorous fluid motions transport the energy in the outer parts of the Sun. These motions, more commonly referred to as convection, are the ultimate source of the solar cycle and magnetic activity of stars. Furthermore, convection occurs in some evolutionary phase of practically all stars. Understanding convection is therefore of utmost importance for much of astrophysics.

In the past decade and a half, it has become apparent that our understanding of solar and stellar convection is much poorer than previously thought. This is referred to as the convective conundrum which is a major discrepancy between solar observations and current theoretical and numerical models. The essence of the convective conundrum is that current numerical simulations produce much stronger flows on large scales comparable to the depth of the solar convection zone, or giant cell convection. Such giant cells have not been observed in the Sun. Furthermore, the current simulations struggle to reproduce large-scale observables such as the solar differential rotation and dynamo cycle. The NeoCon (“New Paradigm of Stellar Convection”) project posits that the reason for these difficulties is that current models do not capture the true nature of convection in stars such as the Sun.

The main conjecture of the NeoCon project is that deep convection is driven by surface cooling in contrast to textbook wisdom where flows are driven throughout the convection zone by an unstable temperature gradient. While the latter is easily resolved in current models, the former occurs on a very thin surface layer and cannot be taken into account self-consistently. The project aims at developing a solar observation-inspired subgrid-scale model that incorporates the effects of surface cooling in global simulations, thus enabling researchers to obtain realistic models of stars. The model incorporates the cool entropy rain precipitating from the surface. This leads to a highly non-local dynamics of convection that are conjectured to exist in stars. And it may have paradigm-shifting consequences for the theories of stellar convection and dynamos, with potential ramifications for stellar structure and evolution. The project will also study magnetic backreaction on convection as another possible solution to the convective conundrum.

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