Finding the Origin of the Slow Solar Wind

Project Information

SLOW_SOURCE

Grant agreement ID: 819189

DOI

10.3030/819189

Project closed

EC signature date 21 March 2019

Start date 1 April 2019

End date 31 March 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 995 902,00

EU contribution

€ 1 995 902,00

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 4 - SLOW_SOURCE (Finding the Origin of the Slow Solar Wind)

Reporting period: 2023-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

The SLOW SOURCE project studied the mechanisms that produce the solar wind, this beam of supersonic charged particles (mainly composed of ionized hydrogen and helium) expelled continuously by the Sun. How stars produce their winds are long-lasting open challenges in astrophysics. Yet stellar winds play a fundamental role in the long-term evolution of stars, the properties of the interstellar medium located between stars and perhaps most importantly the habitability of their orbiting bodies such as our planet Earth. The energy transported by the solar wind - and the occasional solar storms that propagate through the wind - can drive ‘space weather events’ at Earth (so-called geomagnetic storms) that can cause significant perturbations to satellite systems, to ground-based assets such as example electric power lines and general radio transmissions (global positioning, high-frequency communication). Reliable predictions of the occurrence and magnitude of geomagnetic storms depends sensitively on an accurate modelling of the solar wind which in turns requires a better understanding of how it forms and expands in the solar atmosphere.

The SLOW_SOURCE project aimed at developing numerical models capable of describing a comprehensive set of complex physical processes at play during the formation of the solar wind. The latter is observed in at least two states, a fast wind blowing with speeds greater than 500 kilometres per second (km/s) and slower wind with speeds less than 450 km/s. These two winds differ not only in their relative speeds but also in their composition, i.e. the amount of heavy elements such as iron, oxygen and magnesium carried by the wind. The slow wind in particular is highly enriched in iron and magnesium compared to the fast wind. Since the solar wind composition is regulated very close to the solar surface it is thought these composition changes are directly related to the processes that produce the solar wind. The goal of the SLOW_SOURCE project was to explain the bulk properties and peculiar composition of the slow solar wind. One theory relates this varying composition to diffusion processes and/or turbulence developing along an otherwise stationary solar wind. The other theory supposes that the magnetic field of the solar corona is continually reconfigured to allow matter that should normally be confined to the corona (on magnetic loops) into the solar wind.

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

SLOW SOURCE has put in place a broad range of completely new numerical models of the solar atmosphere. These models range in complexity from the simplified isothermal-polytropic models of the corona and solar wind that have provided insights on the acceleration mechanisms of the solar wind to the multi-species high-order moment ISAM model which couples all major and minor species to simulate the solar atmosphere from the chromosphere to the solar wind. The project has also further developed a 3-D magneto-hydrodynamic model of the solar corona (WindPredict-AW) and put in place a completely new model of the inner heliosphere (Heliocast, http://heliocast.irap.omp.eu/) that is continually providing space-weather forecasting.

The project has taken full advantage of the ground-breaking data taken by NASA’s Parker Solar Probe the first mission to enter the solar corona. Closer to the Sun, measurements by Parker Solar Probe have revealed for the first time the highly dynamic state of the solar wind with the omnipresence of jets and magnetic field reversals.

The first theoretical studies of the project tackled the origin of an important component of the slow solar wind in the highly dynamic large-scale structures called solar streamers. Using our new models we were able to compare these simulations with the first images of the solar corona taken from within by the Parker Solar Probe. The second set of theoretical studies investigated the mechanisms that could produce the small micro-eruptions and the associated bursts of jets discovered by Parker Solar Probe (described in the previous section). We showed in several papers how these micro-eruptions are produced by magnetic reconnection between open and closed magnetic field lines in a process known as ‘footpoint exchange’. The third set of theoretical studies put in place new models of the solar wind using a range of different approaches from the simpler models to the state-of-the-art multi-species high-order moment wind models. The exploitation of these completely novel models allowed us to isolate the mechanisms that control the bulk properties of the solar wind and its composition including the oxygen charge-state, alpha particle properties and abundances and heavy-ion abundances. These were the ultimate goals of the project.

The SLOW SOURCE team published over 25 first-author rank-A papers focused on the research goals of the project that have contributed to change our understanding of the solar wind. The team has been extremely active at disseminating the results of the project in conferences through invited presentations including several keynotes, through leading the organisation of the European Space Weather Week 2023 in Toulouse where the team held a dedicated ERC SLOW SOURCE booth. The PI has been interviewed on several radio shows, TV news reports and has given many seminars to the general public and policymakers.

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)

The project has been an incredible adventure in the heart of the solar atmosphere, it has unveiled its full complexity and dynamics addressing a multitude of fundamental processes. First, it provided a theoretical explanation to the transient activity of helmet streamers these white-light structure that we see during solar eclipses. Using our 2.5-D and 3-D magneto-hydrodynamic model of the solar atmosphere, we showed that streamers are dynamic because they produce helical magnetic structures through a process known as magnetic reconnection. We also compared our 3-D simulations of the solar atmosphere with the first images of the solar corona taken from within by the Parker Solar Probe. Second, we carried out the first studies on the dynamic state of the solar wind near the Sun measured by the Parker Solar Probe that does not form inside helmet streamers. We determined the spatial and temporal scales of the bursts of jets and switchbacks discovered by Parker Solar Probe during the SLOW SOURCE project and linked those jets/swithbacks to transient activity very close to the solar surface through the first complete mapping of transient brightpoints at the source of the solar wind measured by Parker Solar Probe and Solar Orbiter. Third, we provided a theoretical explanation to the origin of the bursts of jets and switchbacks measured by PSP as the result of a tearing mode instability developing during interchange magnetic reconnection between solar magnetic loops and magnetic field lines connected to the interplanetary medium. Fourth, we put in place a completely new multi-species model of the solar atmosphere, called ISAM, coupling all minor and major species of the solar atmosphere. The model also couples self-consistently thermal and non-thermal particles. Using ISAM we provided a theoretical explanation for the ionization state of heavy ions and the abundance variations of alpha particles in the solar wind.

Left: Cut of simulated solar streamer. Right: 3-D flux rope released.