On the Propagation and Damping of Alfvenic Fluctuations in the Outer Solar Corona and Solar Wind
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2025
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| author | Sioulas, Nikos Velli, Marco Shi, Chen Bowen, Trevor A. Mallet, Alfred Verdini, Andrea Chandran, B. D. G. Tenerani, Anna Dakeyo, Jean-Baptiste Bale, Stuart D. Larson, Davin Halekas, Jasper S. Matteini, Lorenzo Réville, Victor Chen, C. H. K. Romeo, Orlando M. Liu, Mingzhe Livi, Roberto Rahmati, Ali Whittlesey, P. L. |
| author_facet | Sioulas, Nikos Velli, Marco Shi, Chen Bowen, Trevor A. Mallet, Alfred Verdini, Andrea Chandran, B. D. G. Tenerani, Anna Dakeyo, Jean-Baptiste Bale, Stuart D. Larson, Davin Halekas, Jasper S. Matteini, Lorenzo Réville, Victor Chen, C. H. K. Romeo, Orlando M. Liu, Mingzhe Livi, Roberto Rahmati, Ali Whittlesey, P. L. |
| contents | 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 $σ_c$ 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2510_10106 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | On the Propagation and Damping of Alfvenic Fluctuations in the Outer Solar Corona and Solar Wind Sioulas, Nikos Velli, Marco Shi, Chen Bowen, Trevor A. Mallet, Alfred Verdini, Andrea Chandran, B. D. G. Tenerani, Anna Dakeyo, Jean-Baptiste Bale, Stuart D. Larson, Davin Halekas, Jasper S. Matteini, Lorenzo Réville, Victor Chen, C. H. K. Romeo, Orlando M. Liu, Mingzhe Livi, Roberto Rahmati, Ali Whittlesey, P. L. Solar and Stellar Astrophysics Space Physics 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 $σ_c$ 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. |
| title | On the Propagation and Damping of Alfvenic Fluctuations in the Outer Solar Corona and Solar Wind |
| topic | Solar and Stellar Astrophysics Space Physics |
| url | https://arxiv.org/abs/2510.10106 |