Transport of magnetic fields and cosmic rays in the turbulent interstellar medium

Author(s)

Camila Naomi Koshikumo

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto Astronômico e Geofísico (IAG/SBD)
• Defense date: 2024-07-29

Examining board members

Reinaldo Santos de Lima; Ibere Luiz Caldas; Diego Antonio Falceta Gonçalves; Elisabete Maria de Gouveia Dal Pino

• Advisor: Reinaldo Santos de Lima

Abstract

Turbulence, magnetic fields, and cosmic rays are components of the interstellar medium of our Galaxy and are strongly coupled through complex plasma processes. The magnetic flux transport in molecular clouds is essential in understanding different processes involvedin stellar formation. The details of the cosmic-ray transport in the vicinities of supernova shocks are necessary to understand how the acceleration process of the galactic cosmic rays occurs. Both transports are controlled by turbulence. In this thesis, we investigate the basic mechanisms of the magnetic flux transport in the presence of magneto-hydrodynamic (MHD) turbulence, and we also investigate how the cosmic rays that diffuse in a shock front can amplify the magnetic fields and affect the efficiency of the confinement and acceleration of these particles in that region via Diffusive Shock Acceleration (DSA). For that, we developed numerical experiments by 3D MHD simulations and Particle-in-Cell-MHD 2D to characterize, quantitatively, the turbulence effect in the transport of magnetic fields and cosmic rays (CRs). In the first part of this thesis, we investigate the diffusion coefficient of magnetic fields in sub-Alfvénic (M_A < 1) 3D MHD turbulence, characterized by different sonic Mach numbers M S . The theory of turbulent Reconnection Diffusion (RD), based on statistics of incompressible Alfvénic turbulence, predicts the dependence of the diffusion coefficient of the magnetic field with the Alfvénic Mach number M_A . However, this theory does not take into account the effects of compressibility which should be important in the regime of supersonic MHD turbulence present in molecular clouds. We performed direct numerical simulations of forced turbulence in periodic domains from the incompressible limit (M_S = 0) to the supersonic regime (M_S = 3). The measured diffusion rate provided by incompressible turbulence agrees with the suppression predicted by the RD theory in the presence of strong magnetic fields: D M_A^3 . Our simulations also indicate an increase in RD efficiency when the turbulence is compressible. The dependency on M_A and M_S from the simulations can be described by the relation D M_A^ , where (M_S ) 3/(1 + M_S ). This quantitative characterization of D is critical for modeling star formation in turbulent molecular clouds and evaluating the efficiency of this transport compared to other mechanisms, such as Ambipolar Diffusion. In the second part of this thesis, we focus on investigating the acceleration of CRs via DSA in shocks produced by young supernova remnants. Theoretical and observational evidence requires an amplification of the magnetic field during this process. While CR streaming instabilities can efficiently amplify magnetic fluctuations in the shock precursor, it is not clear whether this can guarantee an efficient confinement of CRs of the highest ( PeV) energies. An alternative process for the amplification of fields at larger scales relies on the turbulent dynamo powered by the pressure of the accelerated CRs interacting with the density inhomogeneities of the interstellar medium. The efficiency of this process was previously studied using MHD simulations, adopting a prescription for the CR force acting on the fluid. We revisited this process, using a simplified kinetic description for the CR protons, and considering an acceleration efficiency more realistic than those adopted previously. We used 2D simulations in scales 0.1 pc around a non-relativistic shock, employing a modified technique of Particle-In-Cell-Magneto-hydrodynamic in which the particles are evolved using the relativistic guiding center equations extended with stochastic terms for representing the particles coordinates diffusion, generated by the sub-grid fields. For a strong shock, we obtained an amplification factor 5 6 in the perpendicular limit (magnetic field perpendicular to the shock velocity), 910 in the oblique case (45º), and 2 3 in the parallel limit. These amplification efficiencies are below the values obtained in previous MHD studies, although they can offer an important pre-amplification factor, increasing the total amplification even more due to the CR instabilities. Additionally, the approach introduced in this study offers a computationally viable way of connecting the transport of CR distributions with fluid phenomena, enabling applications in global and multidimensional studies of particle acceleration and transport, such as the non-linear DSA during long periods. (AU)

• FAPESP's process: 22/10102-4 - Transport of magnetic fields and cosmic rays in the turbulent interstellar medium
• Grantee: Camila Naomi Koshikumo
• Support Opportunities: Scholarships in Brazil - Master