Radial Evolution of MHD Turbulence Anisotropy in Low Mach Number Solar Wind
The Parker Solar Probe (PSP) and Wind spacecraft observed the same plasma flow during PSP encounter 15. The solar wind evolves from a sub-Alfvénic flow at 0.08 au to become modestly super-Alfvénic at 1 au. We study the radial evolution of the turbulence properties and deduce the spectral anisotropy...
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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/ada354 |
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| author | Xingyu Zhu Gary P. Zank Lingling Zhao Ashok Silwal |
| author_facet | Xingyu Zhu Gary P. Zank Lingling Zhao Ashok Silwal |
| author_sort | Xingyu Zhu |
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| description | The Parker Solar Probe (PSP) and Wind spacecraft observed the same plasma flow during PSP encounter 15. The solar wind evolves from a sub-Alfvénic flow at 0.08 au to become modestly super-Alfvénic at 1 au. We study the radial evolution of the turbulence properties and deduce the spectral anisotropy based on the nearly incompressible (NI) MHD theory. We find that the spectral index of the z ^+ spectrum remains unchanged (∼−1.53), while the z ^− spectrum steepens, the index of which changes from −1.35 to −1.47. The fluctuating kinetic energy is on average greater than the fluctuating magnetic field energy in the sub-Alfvénic flow while smaller in the modestly super-Alfvénic flow. The NI MHD theory well interprets the observed Elsässer spectra. The contribution of 2D fluctuations is nonnegligible for the observed z ^− frequency spectra for both intervals. Particularly, the magnitudes of 2D and NI/slab fluctuations are comparable in the frequency domain for the modestly super-Alfvénic flow, resulting in a slightly concave shape of z ^− spectrum at 1 au. We show that, in the wavenumber domain, the power ratio of the observed forward NI/slab and 2D fluctuations is ∼15 at 0.08 au, while it decreases to ∼3 at 1 au, suggesting the growing significance of the 2D fluctuations as the turbulence evolves in low Mach number solar wind. |
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| institution | Kabale University |
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| language | English |
| publishDate | 2025-01-01 |
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| spelling | doaj-art-741fe34c054d4782b79fbdb49e5ff9a12025-01-08T08:46:18ZengIOP PublishingThe Astrophysical Journal Letters2041-82052025-01-019782L3410.3847/2041-8213/ada354Radial Evolution of MHD Turbulence Anisotropy in Low Mach Number Solar WindXingyu Zhu0https://orcid.org/0000-0002-1541-6397Gary P. Zank1https://orcid.org/0000-0002-4642-6192Lingling Zhao2https://orcid.org/0000-0002-4299-0490Ashok Silwal3https://orcid.org/0000-0001-6286-2106Center for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville , Huntsville, AL 35805, USA ; xz0017@uah.eduCenter for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville , Huntsville, AL 35805, USA ; xz0017@uah.edu; Department 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, USA ; xz0017@uah.edu; Department 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, USA ; xz0017@uah.eduThe Parker Solar Probe (PSP) and Wind spacecraft observed the same plasma flow during PSP encounter 15. The solar wind evolves from a sub-Alfvénic flow at 0.08 au to become modestly super-Alfvénic at 1 au. We study the radial evolution of the turbulence properties and deduce the spectral anisotropy based on the nearly incompressible (NI) MHD theory. We find that the spectral index of the z ^+ spectrum remains unchanged (∼−1.53), while the z ^− spectrum steepens, the index of which changes from −1.35 to −1.47. The fluctuating kinetic energy is on average greater than the fluctuating magnetic field energy in the sub-Alfvénic flow while smaller in the modestly super-Alfvénic flow. The NI MHD theory well interprets the observed Elsässer spectra. The contribution of 2D fluctuations is nonnegligible for the observed z ^− frequency spectra for both intervals. Particularly, the magnitudes of 2D and NI/slab fluctuations are comparable in the frequency domain for the modestly super-Alfvénic flow, resulting in a slightly concave shape of z ^− spectrum at 1 au. We show that, in the wavenumber domain, the power ratio of the observed forward NI/slab and 2D fluctuations is ∼15 at 0.08 au, while it decreases to ∼3 at 1 au, suggesting the growing significance of the 2D fluctuations as the turbulence evolves in low Mach number solar wind.https://doi.org/10.3847/2041-8213/ada354Solar windInterplanetary turbulenceSolar coronaSolar physics |
| spellingShingle | Xingyu Zhu Gary P. Zank Lingling Zhao Ashok Silwal Radial Evolution of MHD Turbulence Anisotropy in Low Mach Number Solar Wind The Astrophysical Journal Letters Solar wind Interplanetary turbulence Solar corona Solar physics |
| title | Radial Evolution of MHD Turbulence Anisotropy in Low Mach Number Solar Wind |
| title_full | Radial Evolution of MHD Turbulence Anisotropy in Low Mach Number Solar Wind |
| title_fullStr | Radial Evolution of MHD Turbulence Anisotropy in Low Mach Number Solar Wind |
| title_full_unstemmed | Radial Evolution of MHD Turbulence Anisotropy in Low Mach Number Solar Wind |
| title_short | Radial Evolution of MHD Turbulence Anisotropy in Low Mach Number Solar Wind |
| title_sort | radial evolution of mhd turbulence anisotropy in low mach number solar wind |
| topic | Solar wind Interplanetary turbulence Solar corona Solar physics |
| url | https://doi.org/10.3847/2041-8213/ada354 |
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