Title: The Renaissance of Cosmic Ray Self-Confinement
Abstract: 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 within and beyond galaxies. The cosmic ray fluid may be especially important to driving galactic winds. Individual cosmic ray particles are tied to the magnetic field and advected by large-scale magnetohydrodynamic motions, with transport subject both to large-scale magnetic field geometry and to the motion of the background gas. Additionally, magnetic field perturbations at wavelengths comparable to the gyroradius (micro-parsec for GeV cosmic rays) pitch-angle scatter particles and produce diffusion parallel to the local magnetic field. An important source of waves is an instability driven by streaming of the cosmic ray fluid 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, 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 that are posited to self-confine cosmic rays. However, traditional propagation codes do not take into account details of gas advection or magnetic geometry, even though realistic ISM advection speeds are in fact expected to be comparable to diffusion speeds. Also, the scattering rates are expected to be spatially highly nonuniform because magnetic perturbations are strongly damped in dense, neutral ISM gas. In this talk, I will present results from high-resolution, state-of-the-art simulations of the multiphase, star-forming ISM, with which it is now possible to go beyond the traditional approach, treating the dynamical and diffusive aspects of cosmic ray transport on a more equal footing numerically. When dynamics and diffusion are considered together, along with realistic treatment of multiphase ISM structure, long-standing puzzles regarding empirical energy-dependent transport scalings of cosmic rays are naturally resolved. Under this new hybrid model, the cosmic ray self-confinement paradigm first enunciated nearly 60 years ago is alive and well.