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1. Observational Analysis and Numerical Modeling of the Solar Wind Fluctuation Spectra during Intervals of Plasma Instability

   https://doi.org/10.3847/1538-4357/ac9f42  

   Markovskii, S. A.; Vasquez, Bernard J. (December 2022, The Astrophysical Journal)

   Abstract We perform a statistical analysis of observed magnetic spectra in the solar wind at 1 au with localized power elevations above the level of the ambient turbulent fluctuations. We show that the elevations are seen only when the intensity of the ambient fluctuations is sufficiently low. Assuming that the spectral elevations are caused by thermal-ion instabilities, this suggests that on average the effect of the solar wind background is strong enough to suppress the instability or obscure it or both. We then carry out nonlinear numerical simulations with particle ions and an electron fluid to model a thermal-ion instability coexisting with an ambient turbulence. The parameters of the simulation are taken from a known solar wind interval where an instability was assumed to exist based on the linear theory and a bi-Maxwellian fit of the observed distribution with core and secondary-beam protons. The numerical model closely matches the position of the observed spectral elevation in the wavenumber space. This confirms that the thermal-ion instability is responsible for the elevation. At the same time, the magnitude of the elevation turns out to be smaller than in the real solar wind. When higher intensity of the turbulence is used in the simulation, which is typical of solar wind in general, the power elevation is no longer seen. This is in agreement with the reduced observability of the elevations at higher intensities. However, the simulations show that the turbulence does not simply obscure the instability but also lowers its saturation level. 

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2. Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind

   https://doi.org/10.3847/2041-8213/ada3d8  

   Zhao, L-L; Silwal, A; Zhu, X; Li, H; Zank, G P (January 2025, The Astrophysical Journal Letters)

   Abstract 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 numberM_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 80R_⊙. Our results show that (1) the turbulent sonic Mach numberM_tgradually moves toward larger values as it approaches the Sun, until at least 11R_⊙, whereM_tis much larger than the previously observed value of 0.1 at and above 0.3 au; (2) transonic turbulence withM_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 flowM_Ashows 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 andM_tgradually 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. 

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3. High-latitude Observations of Inertial-range Turbulence by the Ulysses Spacecraft During the Solar Minimum of 1993–96

   https://doi.org/10.3847/1538-4357/ac4588  

   Watson, Abigale S.; Smith, Charles W.; Marchuk, Anastasia V.; Argall, Matthew R.; Joyce, Colin J.; Isenberg, Philip A.; Vasquez, Bernard J.; Schwadron, Nathan A.; Bzowski, Maciej; Kubiak, Marzena A.; et al (March 2022, The Astrophysical Journal)

   Abstract We have examined Ulysses magnetic field measurements for the years 1993 through 1996 as the spacecraft moved sunward from 5 au at high southern latitudes, passing through perihelion during the first fast-latitude scan to achieve high northern latitudes, and finally returning to 5 au. These years represent near-solar-minimum activity, providing a clear measure of high-latitude solar-wind turbulence. We apply a series of tests to the data, examining both the magnetic variance anisotropy and the underlying wavevector anisotropy, finding them to be consistent with past 1 au observations. The variance anisotropy depends upon both the thermal proton temperature parameter and the amplitude of the magnetic power spectrum, while the underlying wavevector anisotropy is dominated by the component perpendicular to the mean magnetic field. We also examine the amplitude of the magnetic power spectrum as well as the associated turbulent transport of energy to small scales that results in the heating of the thermal plasma. The measured turbulence is found to be stronger than that seen at low latitudes by the Voyager spacecraft as it traverses the distance from 1 to 5 au during the years approaching solar maximum. If the high- and low-latitude sources are comparable, this would indicate that while the heating processes are active in both regions, the turbulence has had less decay time in the transport of energy to small scales. Alternatively, it may also be that the high-latitude source is stronger. 

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4. Multiscale Solar Wind Turbulence Properties inside and near Switchbacks Measured by the Parker Solar Probe

   https://doi.org/10.3847/1538-4357/abebe5  

   Martinović, Mihailo M.; Klein, Kristopher G.; Huang, Jia; Chandran, Benjamin D.; Kasper, Justin C.; Lichko, Emily; Bowen, Trevor; Chen, Christopher H.; Matteini, Lorenzo; Stevens, Michael; et al (April 2021, The Astrophysical Journal)

   Abstract The Parker Solar Probe (PSP) routinely observes magnetic field deflections in the solar wind at distances less than 0.3 au from the Sun. These deflections are related to structures commonly called “switchbacks” (SBs), whose origins and characteristic properties are currently debated. Here, we use a database of visually selected SB intervals—and regions of solar wind plasma measured just before and after each SB—to examine plasma parameters, turbulent spectra from inertial to dissipation scales, and intermittency effects in these intervals. We find that many features, such as perpendicular stochastic heating rates and turbulence spectral slopes are fairly similar inside and outside of SBs. However, important kinetic properties, such as the characteristic break scale between the inertial to dissipation ranges differ inside and outside these intervals, as does the level of intermittency, which is notably enhanced inside SBs and in their close proximity, most likely due to magnetic field and velocity shears observed at the edges. We conclude that the plasma inside and outside of an SB, in most of the observed cases, belongs to the same stream, and that the evolution of these structures is most likely regulated by kinetic processes, which dominate small-scale structures at the SB edges. 

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5. Phase coherence of solar wind turbulence from the Sun to Earth

   https://doi.org/10.3389/fspas.2023.1161939  

   Nakanotani, Masaru; Zhao, Lingling; Zank, Gary P. (April 2023, Frontiers in Astronomy and Space Sciences)

   The transport of energetic particles in response to solar wind turbulence is important for space weather. To understand charged particle transport, it is usually assumed that the phase of the turbulence is randomly distributed (the random phase approximation) in quasi-linear theory and simulations. In this paper, we calculate the coherence index, C ϕ , of solar wind turbulence observed by the Helios 2 and Parker Solar Probe spacecraft using the surrogate data technique to check if the assumption is valid. Here, values of C ϕ = 0 and 1 indicate that the phase coherence is random and correlated, respectively. We estimate that the coherence index at the resonant scale of energetic ions (10 MeV protons) is 0.1 at 0.87 and 0.65 au, 0.18 at 0.29 au, and 0.3 (0.35) at 0.09 au for super (sub)-Alfvénic intervals, respectively. Since the random phase approximation corresponds to C ϕ = 0, this may indicate that the random phase approximation is not valid for the transport of energetic particles in the inner heliosphere, especially very close to the Sun ( ∼ 0.09  au). 

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