Properties of Steady Sub-Alfvénic Solar Wind in Comparison with Super-Alfvénic Wind from Parker Solar Probe Measurements

Jiao, Yiming; Liu, Ying D.; Ran, Hao; Cheng, Wenshuai

doi:10.3847/1538-4357/ad0dfe

Cited by 9 publications

(5 citation statements)

References 53 publications

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“…They find that the location of the Alfvén surface for these flows varies between 15 and 24 R e , which is larger than the canonical value of 11-12 R e typically assumed for fast solar wind originating from an open coronal hole. Jiao et al (2024) find that the sub-Alfvénic intervals observed so far all exhibit similar properties and origins.…”

Section: Introductionsupporting

confidence: 57%

“…The first entry of the NASA Parker Solar Probe (PSP) spacecraft into a sub-Alfvénic solar wind flow lasted for 5 hr on 2021 April 28 and has been studied extensively since then (Kasper et al 2021;Alberti et al 2022;Bandyopadhyay et al 2022;Zank et al 2022;Zhang et al 2022;Zhao et al 2022b;Liu et al 2023;Jiao et al 2024). Unlike the super-Alfvénic solar wind, the turbulence properties of this new solar wind environment are connected magnetically to the surface of the Sun.…”

Section: Introductionmentioning

confidence: 99%

“…Sub-Alfvénic solar wind intervals have now been observed since encounters 8-14. Jiao et al (2024) have traced magnetic fields to locate the source of the sub-Alfvénic intervals or streams, finding that the sources are either the boundaries inside coronal holes or small regions of the open magnetic field. They find that the location of the Alfvén surface for these flows varies between 15 and 24 R e , which is larger than the canonical value of 11-12 R e typically assumed for fast solar wind originating from an open coronal hole.…”

Section: Introductionmentioning

confidence: 99%

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Characterization of Turbulent Fluctuations in the Sub-Alfvénic Solar Wind

Zank,

Zhao,

Adhikari

et al. 2024

ApJ

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Parker Solar Probe (PSP) observed sub-Alfvénic solar wind intervals during encounters 8–14, and low-frequency magnetohydrodynamic (MHD) turbulence in these regions may differ from that in super-Alfvénic wind. We apply a new mode decomposition analysis to the sub-Alfvénic flow observed by PSP on 2021 April 28, identifying and characterizing entropy, magnetic islands, forward and backward Alfvén waves, including weakly/nonpropagating Alfvén vortices, forward and backward fast and slow magnetosonic (MS) modes. Density fluctuations are primarily and almost equally entropy- and backward-propagating slow MS modes. The mode decomposition provides phase information (frequency and wavenumber k) for each mode. Entropy density fluctuations have a wavenumber anisotropy of k ∥ ≫ k ⊥, whereas slow-mode density fluctuations have k ⊥ > k ∥. Magnetic field fluctuations are primarily magnetic island modes (δ B i ) with an O(1) smaller contribution from unidirectionally propagating Alfvén waves (δ B A+) giving a variance anisotropy of 〈 δ B i 2 〉 / 〈 δ B A 2 〉 = 4.1 . Incompressible magnetic fluctuations dominate compressible contributions from fast and slow MS modes. The magnetic island spectrum is Kolmogorov-like k ⊥ − 1.6 in perpendicular wavenumber, and the unidirectional Alfvén wave spectra are k ∥ − 1.6 and k ⊥ − 1.5 . Fast MS modes propagate at essentially the Alfvén speed with anticorrelated transverse velocity and magnetic field fluctuations and are almost exclusively magnetic due to β p ≪ 1. Transverse velocity fluctuations are the dominant velocity component in fast MS modes, and longitudinal fluctuations dominate in slow modes. Mode decomposition is an effective tool in identifying the basic building blocks of MHD turbulence and provides detailed phase information about each of the modes.

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Section: Introductionsupporting

confidence: 57%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Characterization of Turbulent Fluctuations in the Sub-Alfvénic Solar Wind

Zank,

Zhao,

Adhikari

et al. 2024

ApJ

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“…Use of electron density as a representation of the plasma density may introduce an overestimate of the local Alfvén speed by 2%-5% (Mostafavi et al 2022). We use PSP data from encounters 8-10 and encounters 12-13, during which sub-Alfvénic solar wind measurements are reported (Jiao et al 2024). We exclude encounter 11 due to the absence of the QTN density for this encounter.…”

Section: Observations and Resultsmentioning

confidence: 99%

The Alpha-proton Differential Flow in the Alfvénic Young Solar Wind: from Sub-Alfvénic to Super-Alfvénic

Ran,

Liu,

Chen

et al. 2024

ApJ

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Data obtained from Parker Solar Probe (PSP) since 2021 April have shown the first in situ observation of the solar corona, where the solar wind is formed and accelerated. Here, we investigate the alpha-proton differential flow and its characteristics across the critical Alfvén surface (CAS) using data from PSP during encounters 8–10 and 12–13. We first show the positive correlation between the alpha-proton differential velocity and the bulk solar wind speed at PSP encounter distances. Then we explore how the characteristics of the differential flow vary across the CAS and how they are affected by Alfvénic fluctuations including switchbacks. We find that the differential velocity below the CAS is generally smaller than that above the CAS, and the local Alfvén speed well limits the differential speed both above and below the CAS. The deviations from the alignment between the differential velocity and the local magnetic field vector are accompanied by large-amplitude Alfvénic fluctuations and decreases in the differential speed. Moreover, we observe that V α p increases from M A < 1 to M A ≃ 2 and then starts to decrease, which suggests that alphas may remain preferentially accelerated well above the CAS. Our results also reveal that in the sub-Alfvénic solar wind both protons and alphas show a strong correlation between their velocity fluctuations and magnetic field fluctuations, with a weaker correlation for alphas. By contrast, in the super-Alfvénic regime the correlation remains high for protons, but is reduced for alphas.

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“…Because of the enhanced Alfvén radius of an LMBL, PSP made the first glimpse of the sub-Alfvénic plasma for about 5 hr around a heliocentric distance of about 19 solar radii at the eighth encounter on 2021 April 28 (Kasper et al 2021). Now, more than 10 steady sub-Alfvénic intervals can be identified from PSP measurements since encounter 8, and a statistical analysis of these intervals confirms their nature as LMBLs (Jiao et al 2024). Inside the corona, the sonic critical point, at which the solar wind changes from subsonic to supersonic, is usually thought to be well below the Alfvén critical point, i.e., at a few solar radii (Matthaeus 2021, and references therein).…”

Section: Introductionmentioning

confidence: 84%

Direct In Situ Measurements of a Fast Coronal Mass Ejection and Associated Structures in the Corona

Liu,

Zhu,

Ran

et al. 2024

ApJ

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We report on the first direct in situ measurements of a fast coronal mass ejection (CME) and shock in the corona, which occurred on 2022 September 5. In situ measurements from the Parker Solar Probe spacecraft near perihelion suggest two shocks, with the second one decayed, which is consistent with more than one eruption in coronagraph images. Despite a flank crossing, the measurements indicate unique features of the young ejecta: a plasma much hotter than the ambient medium, suggestive of a hot solar source, and a large plasma β implying a highly non-force-free state and the importance of thermal pressure gradient for CME acceleration and expansion. Reconstruction of the global coronal magnetic fields shows a long-duration change in the heliospheric current sheet (HCS), and the observed field polarity reversals agree with a more warped HCS configuration. Reconnection signatures are observed inside an HCS crossing as deep as the sonic critical point. As the reconnection occurs in the sub-Alfvénic wind, the reconnected flux sunward of the reconnection site can close back to the Sun, which helps balance magnetic flux in the heliosphere. The nature of the sub-Alfvénic wind after the HCS crossing as a low Mach-number boundary layer (LMBL) leads to in situ measurements of the near subsonic plasma at a surprisingly large distance. Specifically, an LMBL may provide favorable conditions for the crossings of the sonic critical point in addition to the Alfvén surface.

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Properties of Steady Sub-Alfvénic Solar Wind in Comparison with Super-Alfvénic Wind from Parker Solar Probe Measurements

Cited by 9 publications

References 53 publications

Characterization of Turbulent Fluctuations in the Sub-Alfvénic Solar Wind

Characterization of Turbulent Fluctuations in the Sub-Alfvénic Solar Wind

The Alpha-proton Differential Flow in the Alfvénic Young Solar Wind: from Sub-Alfvénic to Super-Alfvénic

Direct In Situ Measurements of a Fast Coronal Mass Ejection and Associated Structures in the Corona

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