On the Propagation and Damping of Alfvenic Fluctuations in the Outer Solar Corona and Solar Wind

Abstract

We analyze \textit{Parker Solar Probe} and \textit{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 $M_a$, 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 $M_a<M_a^b$ but steepen beyond this break point, which typically lies near the Alfvén surface ($M_a\approx 1$) yet varies systematically with normalized cross helicity ${\sigma }_c$ and fluctuation scale. In slow, quasi-balanced streams, the transition occurs at $M_a\lesssim 1$; in fast, imbalanced wind, WKB-like scaling persists to $M_a\gtrsim 1$. Outer-scale fluctuations maintain wave-action conservation to larger $M_a$ than inertial-range modes. The turbulent heating rate $Q$ is largest below $M_a^b$, indicating a preferential heating zone shaped by the degree of imbalance. Despite this, the Alfvénic energy flux $F_a$ remains elevated, and the corresponding damping length ${\Lambda }_d=F_a/Q$ remains sufficiently large to permit long-range propagation before appreciable damping occurs. Normalized damping lengths ${\Lambda }_d/H_A$, where $H_A$ is the inverse Alfvén-speed scale height, are near unity for $M_a\lesssim M_a^b$ but decline with increasing $M_a$ 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.