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In the interstellar medium (ISM), the total energy density of cosmic rays is comparable to magnetic, kinetic, and thermal energy densities, such that cosmic-rays can be important partners in gas dynamics, including driving galactic winds. Cosmic rays are tied to the magnetic field and advected by large-scale magnetohydrodynamic motions, with transport parallel to field lines subject to large-scale magnetic field geometry. Additionally, magnetic field perturbations at wavelengths comparable to the gyroradius (micro-parsec for GeV cosmic rays) pitch-angle scatter particles and produce parallel diffusion. An important source of waves is an instability driven by streaming of the cosmic rays themselves along the magnetic field, which led in the 1970s to the concept of cosmic-ray self-confinement. Cosmic ray propagation codes are used to place empirical constraints on theoretical models; standard codes do not take into account details of gas advection or magnetic geometry, simply solving diffusion equations to obtain phenomenological scattering rates and confinement times for cosmic ray species as a function of energy. Since the 1970s, a conundrum has been that empirical constraints on energy scalings of cosmic ray residency times are incompatible with the theoretical predictions of streaming-driven scattering rates. However, realistic ISM advection speeds are expected to be comparable to diffusion speeds, and the scattering rate will be spatially highly nonuniform because magnetic perturbations are strongly damped in dense, neutral ISM gas. With high-resolution, state-of-the-art simulations of the multiphase, star-forming ISM, it is now possible to treat the dynamical and diffusive aspects of cosmic ray transport on a more equal footing numerically. When dynamics and diffusion are considered together in models of the multiphase ISM, long-standing puzzles regarding empirical energy-dependent transport scalings of cosmic rays are naturally resolved. Remarkably, under this new hybrid model, the cosmic ray self-confinement paradigm first enunciated over 75 years ago is alive and well.
LOCATION:Phillips Auditorium STATUS:CONFIRMED DTSTART:20260430T150000Z DTEND:20260430T160000Z END:VEVENT END:VCALENDAR