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

Nikos Sioulas, Marco Velli, Chen Shi, Trevor A Bowen, Alfred Mallet, Andrea Verdini, B. D. G Chandran, Anna Tenerani, Jean-Baptiste Dakeyo, Stuart D Bale, …
ArXiv.org
Cornell University
10/11/2025
DOI: 10.48550/arxiv.2510.10106
url
https://doi.org/10.48550/arxiv.2510.10106View
Preprint (Author's original)This preprint has not been evaluated by subject experts through peer review. Preprints may undergo extensive changes and/or become peer-reviewed journal articles. Open Access

Abstract

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$M_a$ , 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$σ_c$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$Λ_d = F_a/Q$remains sufficiently large to permit long-range propagation before appreciable damping occurs. Normalized damping lengths$Λ_d/H_A$ , 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.
 Physics - Solar and Stellar Astrophysics Physics - Space Physics

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