Thomas H. Stix Award for Outstanding Early Career Contributions to Plasma Physics Research: Weakly Collisional Heating and Turbulence in Astrophysical Plasmas

Many astrophysical plasmas are weakly collisional, with characteristic macroscopic timescales that rival interparticle collision times. When stirred up by gravitational or magnetic forces, creating turbulence, this leads to rich and interesting plasma physics, with the plasma’s bulk properties – for instance, the relative heating of ions and electrons, or heat and momentum transport – depending on details of the regime and even the stirring mechanism. In turn, changing these bulk properties can completely alter the system’s macroscopic evolution, determining, for example, how the solar wind is accelerated outwards from the sun or the fate of matter falling into a black hole.



In this talk, I will outline some of the entertaining challenges that are involved in the study of weakly collisional plasma turbulence and heating. While 3-D kinetic simulations are becoming increasingly feasible, their key strength – realistically capturing plasma dynamics – is also a weakness, with their complexity sometimes hiding important basic physics principles. Reduced models, such as gyro- and drift-kinetics, therefore enable important progress by revealing conservation laws or other principles that govern the dynamics. Using such models as a foundation, I will discuss some surprising and nontrivial behaviors that occur in different regimes, such as collisionless plasmas that create their own collisionality, and turbulent cascades that become “stuck” at intermediate scales, halting heating entirely. The landscape of weakly collisional turbulent physics across different regimes is only partially explored, and many interesting challenges remain in understanding the role of plasma kinetics in astrophysics.