2025/10/11 by Sioulas, Nikos, Velli, Marco, Shi, Chen +17 · 1 citation
#FOS: Physical sciences #Solar and Stellar Astrophysics (astro-ph.SR) #Space Physics (physics.space-ph)
paper · doi:10.48550/arxiv.2510.10106
We analyze Parker Solar Probe and Solar Orbiter observations to investigate the propagation and dissipation of Alfvénic fluctuations from the outer corona to 1~AU. Conservation of wave-action flux provides the theoretical baseline for how fluctuation amplitudes scale with the Alfvén Mach number Ma, once solar-wind acceleration is accounted for. Departures from this scaling quantify the net balance between energy injection and dissipation. Fluctuation amplitudes follow wave-action conservation for Ma < Mab but steepen beyond this break point, which typically lies near the Alfvén surface (Ma ≈ 1) yet varies systematically with normalized cross helicity σc and fluctuation scale. In slow, quasi-balanced streams, the transition occurs at Ma \lesssim 1; in fast, imbalanced wind, WKB-like scaling persists to Ma \gtrsim 1. Outer-scale fluctuations maintain wave-action conservation to larger Ma than inertial-range modes. The turbulent heating rate Q is largest below Mab, indicating a preferential heating zone shaped by the degree of imbalance. Despite this, the Alfvénic energy flux Fa remains elevated, and the corresponding damping length Λd = Fa/Q remains sufficiently large to permit long-range propagation before appreciable damping occurs. Normalized damping lengths Λd/HA, where HA is the inverse Alfvén-speed scale height, are near unity for Ma \lesssim Mab but decline with increasing Ma and decreasing U, implying that incompressible reflection-driven turbulence alone cannot account for the observed dissipation. Additional damping mechanisms -- such as compressible effects -- are likely required to account for the observed heating rates across much of the parameter space.