Co-chairs: Olga Alexandrova, Lina Hadid, Edith Falgarone, Pierre Lesaffre, Frederic Bournaud, Anaelle Maury, Helene Sol

Turbulence is ubiquitous in all the plasmas of the universe. The goal of turbulence studies is to characterize what kind of order emerges from this apparent chaos and quantify the energy trail in the cascade. There are hints that turbulence statistics could be universal in the limit of large Reynolds numbers. This means that turbulence research can hugely benefit from an interplay across a wide range of fields which is what we are aiming for with this session.

Among the different astrophysical plasmas, the solar wind and planetary magnetospheres, represent excellent laboratories for the observation of collisionless astrophysical plasma turbulence. Thanks to the availability of many space missions, we can study in situ the properties of fully developed turbulence over more than 8 decades in scales (from ~100 m to 10^7 km) and 14 decades in power-spectral-densities (PSD). At the magneto- hydrodynamical scales, the PSD of the magnetic fluctuations follows a Kolmogorov scaling, whereas at kinetic plasma scales (less than 10^3 km), the cascade changes its nature and spectrum, it heats charged particles and can be at the origin of non-adiabatic solar wind expansion.

Outside the solar system, all the plasmas in and around galaxies, whether fully or weakly ionized, are highly turbulent. Their supersonic turbulence (10-50 km/s at 100-10^4 K) is known to play a major role in the structuration of the interstellar medium, up to scales of the order of the kpc, compressing interstellar matter from mean densities of a few cm^-3 at the kpc scale to filaments and dense clumps above 10^7 cm^-3 at the parsec scale and below, setting the conditions for stars to ultimately form. Turbulence may trigger phase transitions between the warm and cold neutral media, seeds the formation of molecules in the atomic medium, support molecular clouds against gravitational collapse, and likely plays a key role in the global inefficiency of star formation. Indeed, the fascinating self- regulation of star formation observed in galaxies, up to high-redshift starburst galaxies, appears driven by the conjunction of cosmological gas infall, gravitational instabilities and stellar feedback, and is mediated by turbulence. Hence understanding the origin and cascade of galaxy-scale turbulence and its precise impact on star formation is important to get realistic models and theories of galaxy formation and evolution.

Another major challenge is the role of turbulence in the transport of angular momentum in protoplanetary disks. While recent observations suggest the turbulence is probably weak in the gas of Myrs-old planet-hosting disks surrounding T-Tauri stars, the conditions could be significantly different in disk winds, and in actively accreting, younger, disks which are the cradles of protoplanets.

Last, understanding the nature of turbulence in galactic and extragalactic plasmas is also a key issue to unravel the origin, transport and acceleration of cosmic rays (CR) at all energy ranges, from hundreds keV up to ultra-high-energy cosmic rays (UHECR) in the 10^20 eV domain. Indeed, more than a century after their discovery, the origin of CR is not yet elucidated. The smooth CR energy spectrum close to a power law from 10^9 eV to UHE suggests that transport properties are not strongly energy-dependent and that common acceleration mechanisms are at work, in shocks, turbulence, and possibly magnetic reconnection. The region of the « knee », where the spectral slope changes in the PeV range, is of special importance since it could mark the transition between galactic and extragalactic CR. The search for PeV-atrons is now possible thanks to new generation instruments such as LHAASO or CTA, and should clarify the role of supernova remnants that have for long been proposed as main sources of galactic CR. As for extragalactic CR, the question remains entirely open and requires new observational and theoretical constraints, several cosmic sites hosting very energetic and turbulent plasma flows being likely to accelerate them up to UHE, such as active galactic nuclei, black holes and relativistic jets, gamma-ray bursts, hypernovae, starburst galaxies, large scale shocks.

The two sessions will be devoted to these different facets of turbulence, with an emphasis on what could be unveiled as universal properties pervading all these cosmic plasmas.