Authors: Evan Yerger (Space Science Center, University of New Hampshire)

The heating and acceleration of the solar wind remain an enduring mysteries in heliophysics. A long-standing explanation for both is the turbulent cascade — and ultimate dissipation at small scales — of large-scale Alfvén waves that emanate from the solar surface. Although there is growing evidence that this picture is broadly consistent with observational evidence, basic questions about the relevant turbulent and kinetic processes remain. In particular, it is unclear at which scales the turbulence dissipates and whether the underlying mechanism is inherently resonant or non-resonant. In this scene-setting talk, I will provide a brief primer in plasma turbulence and collisionless heating. With everyone on the same page, I will argue a provocative perspective to frame the remainder of the discussion: that the relative balance between sunward- and anti-sunward-propagating fluctuations determines whether heating is predominantly resonant or non-resonant. I will discuss how this perspective may or may not comport with recent Parker Solar Probe observations — namely, the dominance of proton over electron heating, the near-ubiquity of parallel ion cyclotron waves, and recent constraints on proton heating — and recent theoretical developments, including the helicity barrier and cyclotron breaking. Finally, I will discuss recent work on how intermittency in turbulent fluctuations may blur the lines between resonant and non-resonant processes.