The Dynamics of Astrospheres and Cosmic Ray Propagation in Stellar Wind Environments

Project Description: 

Astrospheres are large-scale regions carved out in the interstellar medium by the outflow of plasma from stars. In the case of the Sun, this structure is known as the heliosphere, formed by the solar wind interacting with the surrounding local interstellar medium (LISM). More generally, stellar winds—continuous streams of charged particles emitted from the outer atmospheres of stars—carry mass, momentum, and magnetic fields into space, shaping the circumstellar environment over vast distances. The structure of an astrosphere is determined by the balance between the outward pressure of the stellar wind and the opposing pressure of the interstellar medium. This interaction produces characteristic boundary regions, including the termination shock (where the stellar wind slows from supersonic to subsonic speeds), the astropause or heliopause (where stellar and interstellar pressures balance), and potentially a bow shock if the star moves supersonically through the surrounding medium. These regions play a key role in modulating cosmic ray propagation and in shielding planetary systems from high-energy galactic radiation. Cosmic ray propagation refers to the transport of high-energy charged particles through the Galaxy and into astrospheres such as the heliosphere. As these particles travel, their trajectories are strongly influenced by magnetic fields carried by stellar winds, causing them to undergo diffusion, scattering, and gradual energy changes. When entering astrospheres, cosmic rays are modulated by expanding stellar wind plasma, which can reduce their intensity and alter their energy spectrum. This modulation is especially important for low-energy cosmic rays and plays a key role in shaping the radiation environment of planetary systems. Observations, numerical simulations, and theoretical models are jointly used to investigate how astrospheres evolve over time and how they interact with the surrounding galactic environment, including the transport and modulation of cosmic rays. Observational data—such as in-situ spacecraft measurements, remote sensing of stellar winds, and energetic particle detections—provide direct constraints on boundary structures and plasma conditions. Numerical simulations, typically based on magnetohydrodynamic (MHD) and particle transport models, are used to reproduce the complex interaction between stellar winds and the interstellar medium, including shock formation and cosmic ray diffusion. Theoretical models then provide the physical framework for interpreting these results, linking macroscopic flow properties to microscopic processes such as particle scattering and energy loss. Together, these approaches enable a self-consistent description of astrospheric structure and dynamics, as well as the modulation of cosmic ray fluxes entering astrospheric environments. (NWU)

Supervisor

North-West University (NWU)

Co-Supervisor