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1. Turbulence in the Sub-Alfvénic Solar Wind

   https://doi.org/10.3847/2041-8213/ac51da  

   Zank, G. P. ; Zhao, L. -L. ; Adhikari, L. ; Telloni, D. ; Kasper, J. C. ; Stevens, M. ; Rahmati, A. ; Bale, S. D. ( February 2022 , The Astrophysical Journal Letters)

   The Parker Solar Probe (PSP) entered a region of sub-Alfvénic solar wind during encounter 8, and we present the first detailed analysis of low-frequency turbulence properties in this novel region. The magnetic field and flow velocity vectors were highly aligned during this interval. By constructing spectrograms of the normalized magnetic helicity, cross-helicity, and residual energy, we find that PSP observed primarily Alfvénic fluctuations, a consequence of the highly field-aligned flow that renders quasi-2D fluctuations unobservable to PSP. We extend Taylor’s hypothesis to sub- and super-Alfvénic flows. Spectra for the fluctuating forward and backward Elsässer variables (z^±, respectively) are presented, showing thatz⁺modes dominatez⁻by an order of magnitude or more, and thez⁺spectrum is a power law in frequency (parallel wavenumber)f^−3/2($k_{\parallel }^{-3/2}$) compared to the convexz⁻spectrum withf^−3/2($k_{\parallel }^{-3/2}$) at low frequencies, flattening around a transition frequency (at which the nonlinear and Alfvén timescales are balanced) tof^−1.25at higher frequencies. The observed spectra are well fitted using a spectral theory for nearly incompressible magnetohydrodynamics assuming a wavenumber anisotropy$k_{\perp }\sim k_{\parallel }^{3/4}$, that thez⁺fluctuations experience primarily nonlinear interactions, and that the minorityz⁻fluctuations experience both nonlinear and Alfvénic interactions withz⁺fluctuations. The density spectrum is a powermore »law that resembles neither thez^±spectra nor the compressible magnetic field spectrum, suggesting that these are advected entropic rather than magnetosonic modes and not due to the parametric decay instability. Spectra in the neighboring modestly super-Alfvénic intervals are similar.

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2. MHD and Ion Kinetic Waves in Field-aligned Flows Observed by Parker Solar Probe

   https://doi.org/10.3847/1538-4357/ac28fb  

   Zhao, L.-L. ; Zank, G. P. ; He, J. S. ; Telloni, D. ; Adhikari, L. ; Nakanotani, M. ; Kasper, J. C. ; Bale, S. D. ( November 2021 , The Astrophysical Journal)

   Abstract Parker Solar Probe (PSP) observed predominately Alfvénic fluctuations in the solar wind near the Sun where the magnetic field tends to be radially aligned. In this paper, two magnetic-field-aligned solar wind flow intervals during PSP’s first two orbits are analyzed. Observations of these intervals indicate strong signatures of parallel/antiparallel-propagating waves. We utilize multiple analysis techniques to extract the properties of the observed waves in both magnetohydrodynamic (MHD) and kinetic scales. At the MHD scale, outward-propagating Alfvén waves dominate both intervals, and outward-propagating fast magnetosonic waves present the second-largest contribution in the spectral energy density. At kinetic scales, we identify the circularly polarized plasma waves propagating near the proton gyrofrequency in both intervals. However, the sense of magnetic polarization in the spacecraft frame is observed to be opposite in the two intervals, although they both possess a sunward background magnetic field. The ion-scale plasma wave observed in the first interval can be either an inward-propagating ion cyclotron wave (ICW) or an outward-propagating fast-mode/whistler wave in the plasma frame, while in the second interval it can be explained as an outward ICW or inward fast-mode/whistler wave. The identification of the exact kinetic wave mode is more difficult to confirm owing tomore »the limited plasma data resolution. The presence of ion-scale waves near the Sun suggests that ion cyclotron resonance may be one of the ubiquitous kinetic physical processes associated with small-scale magnetic fluctuations and kinetic instabilities in the inner heliosphere.« less
3. Evolution of anisotropic turbulence in the fast and slow solar wind: Theory and Solar Orbiter measurements

   https://doi.org/10.1051/0004-6361/202140672  

   Adhikari, L. ; Zank, G. P. ; Zhao, L.-L. ; Telloni, D. ; Horbury, T. S. ; O’Brien, H. ; Evans, V. ; Angelini, V. ; Owen, C. J. ; Louarn, P. ; et al ( December 2021 , Astronomy & Astrophysics)

   Aims. Solar Orbiter (SolO) was launched on February 9, 2020, allowing us to study the nature of turbulence in the inner heliopshere. We investigate the evolution of anisotropic turbulence in the fast and slow solar wind in the inner heliosphere using the nearly incompressible magnetohydrodynamic (NI MHD) turbulence model and SolO measurements. Methods. We calculated the two dimensional (2D) and the slab variances of the energy in forward and backward propagating modes, the fluctuating magnetic energy, the fluctuating kinetic energy, the normalized residual energy, and the normalized cross-helicity as a function of the angle between the mean solar wind speed and the mean magnetic field ( θ UB ), and as a function of the heliocentric distance using SolO measurements. We compared the observed results and the theoretical results of the NI MHD turbulence model as a function of the heliocentric distance. Results. The results show that the ratio of 2D energy and slab energy of forward and backward propagating modes, magnetic field fluctuations, and kinetic energy fluctuations increases as the angle between the mean solar wind flow and the mean magnetic field increases from θ UB  = 0° to approximately θ UB  = 90° and then decreases as θ UB  → 180°.more »We find that solar wind turbulence is a superposition of the dominant 2D component and a minority slab component as a function of the heliocentric distance. We find excellent agreement between the theoretical results and observed results as a function of the heliocentric distance.« less
4. Applicability of Taylor’s hypothesis during Parker Solar Probe perihelia

   https://doi.org/10.1051/0004-6361/202039879  

   Perez, Jean C. ; Bourouaine, Sofiane ; Chen, Christopher H. ; Raouafi, Nour E. ( June 2021 , Astronomy & Astrophysics)

   We investigate the validity of Taylor’s hypothesis (TH) in the analysis of velocity and magnetic field fluctuations in Alfvénic solar wind streams measured by Parker Solar Probe (PSP) during the first four encounters. The analysis is based on a recent model of the spacetime correlation of magnetohydrodynamic (MHD) turbulence, which has been validated in high-resolution numerical simulations of strong reduced MHD turbulence. We use PSP velocity and magnetic field measurements from 24 h intervals selected from each of the first four encounters. The applicability of TH is investigated by measuring the parameter ϵ  =  δu 0 /√2 V ⊥ , which quantifies the ratio between the typical speed of large-scale fluctuations, δu 0 , and the local perpendicular PSP speed in the solar wind frame, V ⊥ . TH is expected to be applicable for ϵ ≲ 0.5 when PSP is moving nearly perpendicular to the local magnetic field in the plasma frame, irrespective of the Alfvén Mach number M A = V SW ∕ V A , where V SW and V A are the local solar wind and Alfvén speed, respectively. For the four selected solar wind intervals, we find that between 10 and 60% of the time,more »the parameter ϵ is below 0.2 and the sampling angle (between the spacecraft velocity in the plasma frame and the local magnetic field) is greater than 30°. For angles above 30°, the sampling direction is sufficiently oblique to allow one to reconstruct the reduced energy spectrum E ( k ⊥ ) of magnetic fluctuations from its measured frequency spectra. The spectral indices determined from power-law fits of the measured frequency spectrum accurately represent the spectral indices associated with the underlying spatial spectrum of turbulent fluctuations in the plasma frame. Aside from a frequency broadening due to large-scale sweeping that requires careful consideration, the spatial spectrum can be recovered to obtain the distribution of fluctuation’s energy across scales in the plasma frame.« less
5. 2D and Slab Turbulent Cascade Rates in the Inner Heliosphere

   https://doi.org/10.3847/1538-4357/ac9234  

   Adhikari, L. ; Zank, G. P. ; Zhao, L.-L. ; Telloni, D. ( October 2022 , The Astrophysical Journal)

   Abstract We present a theoretical and observational study of 2D and slab turbulence cascade (or heating) rates of transverse total turbulence energies, transverse cross helicity, transverse outward and inward Elsässer energy, transverse fluctuating magnetic energy density, and transverse fluctuating kinetic energy from the perihelion of the first Parker Solar Probe (PSP) orbit at ∼36.6 R ⊙ to Solar Orbiter (SolO) at ∼177 R ⊙ . We use the Adhikari et al. (2021a) approach to calculate the observed transverse turbulence heating rate, and the nearly incompressible magnetohydrodynamic (NI MHD) turbulence transport theory to calculate the theoretical turbulence cascade rate. We find from the 1 day long PSP measurements at 66.5 R ⊙ , and the SolO measurements at 176.3 R ⊙ that various transverse turbulent cascade rates increase with increasing angle, from 10° to 98°, between the mean solar wind speed and mean magnetic field ( θ UB ), indicating that the 2D heating rate is largest in the inner heliosphere. Similarly, we find from the theoretical and observed results that the 2D heating rate is larger than the slab heating rate as a function of heliocentric distance. We present a comparison between the theoretical and observed 2D and slab turbulencemore »cascade rates as a function of heliocentric distance.« less