Physics Colloquium – Prof. Stanislav Boldyrev, University of Wisconsin-Madison

Alfvenic Turbulence as a Universal Particle Accelerator

Strong Alfvénic turbulence is arguably ubiquitous in astrophysical plasmas, from the solar wind to black hole accretion flows and pulsar wind nebulae, and it offers a powerful mechanism for heating plasma and accelerating particles to high energies. Yet predicting the resulting non-thermal particle distributions from first principles remains a long-standing challenge. In this talk, I will review recent analytical approaches to this problem in both non-relativistic and relativistic plasmas. I will argue that strong Alfvénic turbulence efficiently energizes particles through curvature acceleration, particularly when their Larmor radii are comparable to the turbulent scales, and present a unified framework that treats both regimes on the same footing. I will then discuss a model in which the acceleration is self-regulating: as the energy density of the energized particles grows, their energy exchange with the turbulence becomes less efficient, and the process saturates. This feedback produces universal power-law distributions of the accelerated particles: in the non-relativistic case, the momentum distribution scales as f(p) ∝ p⁻⁵, while in the ultrarelativistic case, the energy distribution scales as f(E) ∝ E⁻³. These predictions agree with energetic ion spectra observed in the heliosphere and with first-principles kinetic simulations, pointing to a simple, robust principle behind particle energization in turbulent astrophysical plasmas.