Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind
We use in situ measurements from the first 19 encounters of Parker Solar Probe and the most recent five encounters of Solar Orbiter to study the evolution of the turbulent sonic Mach number M _t (the ratio of the amplitude of velocity fluctuations to the sound speed) with radial distance and its rel...
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IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal Letters |
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| Online Access: | https://doi.org/10.3847/2041-8213/ada3d8 |
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| author | L.-L. Zhao A. Silwal X. Zhu H. Li G. P. Zank |
| author_facet | L.-L. Zhao A. Silwal X. Zhu H. Li G. P. Zank |
| author_sort | L.-L. Zhao |
| collection | DOAJ |
| description | We use in situ measurements from the first 19 encounters of Parker Solar Probe and the most recent five encounters of Solar Orbiter to study the evolution of the turbulent sonic Mach number M _t (the ratio of the amplitude of velocity fluctuations to the sound speed) with radial distance and its relationship to density fluctuations. We focus on the near-Sun region with radial distances ranging from about 11 to 80 R _⊙ . Our results show that (1) the turbulent sonic Mach number M _t gradually moves toward larger values as it approaches the Sun, until at least 11 R _⊙ , where M _t is much larger than the previously observed value of 0.1 at and above 0.3 au; (2) transonic turbulence with M _t ∼ 1 is observed in situ for the first time and is found mostly near the Alfvén critical surface; (3) Alfvén Mach number of the bulk flow M _A shows a strong correlation with the plasma beta, indicating that most of the observed sub-Alfvénic intervals correspond to a low-beta plasma; (4) the scaling relation between density fluctuations and M _t gradually changes from a linear scaling at larger radial distances to a quadratic scaling at smaller radial distances; and (5) transonic turbulence is more compressible than subsonic turbulence, with enhanced density fluctuations and slightly flatter spectra than subsonic turbulence. A systematic understanding of compressible turbulence near the Sun is necessary for future solar wind modeling efforts. |
| format | Article |
| id | doaj-art-6c1f6daebd6e47e0b3329c7719214fc3 |
| institution | Kabale University |
| issn | 2041-8205 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
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| series | The Astrophysical Journal Letters |
| spelling | doaj-art-6c1f6daebd6e47e0b3329c7719214fc32025-01-13T17:02:53ZengIOP PublishingThe Astrophysical Journal Letters2041-82052025-01-019791L410.3847/2041-8213/ada3d8Transonic Turbulence and Density Fluctuations in the Near-Sun Solar WindL.-L. Zhao0https://orcid.org/0000-0002-4299-0490A. Silwal1https://orcid.org/0000-0001-6286-2106X. Zhu2https://orcid.org/0000-0002-1541-6397H. Li3https://orcid.org/0000-0003-3556-6568G. P. Zank4https://orcid.org/0000-0002-4642-6192Center for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville , Huntsville, AL 35805, USA; Department of Space Science, The University of Alabama in Huntsville , Huntsville, AL 35805, USADepartment of Space Science, The University of Alabama in Huntsville , Huntsville, AL 35805, USACenter for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville , Huntsville, AL 35805, USALos Alamos National Laboratory , Los Alamos, NM 87545, USACenter for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville , Huntsville, AL 35805, USA; Department of Space Science, The University of Alabama in Huntsville , Huntsville, AL 35805, USAWe use in situ measurements from the first 19 encounters of Parker Solar Probe and the most recent five encounters of Solar Orbiter to study the evolution of the turbulent sonic Mach number M _t (the ratio of the amplitude of velocity fluctuations to the sound speed) with radial distance and its relationship to density fluctuations. We focus on the near-Sun region with radial distances ranging from about 11 to 80 R _⊙ . Our results show that (1) the turbulent sonic Mach number M _t gradually moves toward larger values as it approaches the Sun, until at least 11 R _⊙ , where M _t is much larger than the previously observed value of 0.1 at and above 0.3 au; (2) transonic turbulence with M _t ∼ 1 is observed in situ for the first time and is found mostly near the Alfvén critical surface; (3) Alfvén Mach number of the bulk flow M _A shows a strong correlation with the plasma beta, indicating that most of the observed sub-Alfvénic intervals correspond to a low-beta plasma; (4) the scaling relation between density fluctuations and M _t gradually changes from a linear scaling at larger radial distances to a quadratic scaling at smaller radial distances; and (5) transonic turbulence is more compressible than subsonic turbulence, with enhanced density fluctuations and slightly flatter spectra than subsonic turbulence. A systematic understanding of compressible turbulence near the Sun is necessary for future solar wind modeling efforts.https://doi.org/10.3847/2041-8213/ada3d8Solar windSpace plasmasInterplanetary turbulence |
| spellingShingle | L.-L. Zhao A. Silwal X. Zhu H. Li G. P. Zank Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind The Astrophysical Journal Letters Solar wind Space plasmas Interplanetary turbulence |
| title | Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind |
| title_full | Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind |
| title_fullStr | Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind |
| title_full_unstemmed | Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind |
| title_short | Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind |
| title_sort | transonic turbulence and density fluctuations in the near sun solar wind |
| topic | Solar wind Space plasmas Interplanetary turbulence |
| url | https://doi.org/10.3847/2041-8213/ada3d8 |
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