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1. Flux Rope Merging and the Structure of Switchbacks in the Solar Wind

   https://doi.org/10.3847/1538-4357/ac4016  

   Agapitov, O. V. ; Drake, J. F. ; Swisdak, M. ; Bale, S. D. ; Horbury, T. S. ; Kasper, J. C. ; MacDowall, R. J. ; Mozer, F. S. ; Phan, T. D. ; Pulupa, M. ; et al ( February 2022 , The Astrophysical Journal)

   Abstract A major discovery of Parker Solar Probe (PSP) was the presence of large numbers of localized increases in the radial solar wind speed and associated sharp deflections of the magnetic field—switchbacks (SBs). A possible generation mechanism of SBs is through magnetic reconnection between open and closed magnetic flux near the solar surface, termed interchange reconnection, that leads to the ejection of flux ropes (FRs) into the solar wind. Observations also suggest that SBs undergo merging, consistent with an FR picture of these structures. The role of FR merging in controlling the structure of SBs in the solar wind is explored through direct observations, analytic analysis, and numerical simulations. Analytic analysis reveals key features of the structure of FRs and their scaling with heliocentric distance R, which are consistent with observations and demonstrate the critical role of merging in controlling the structure of SBs. FR merging is shown to energetically favor reductions in the strength of the wrapping magnetic field and the elongation of SBs. A further consequence is the resulting dominance of the axial magnetic field within SBs that leads to the observed characteristic sharp rotation of the magnetic field into the axial direction at the SB boundary. Finally,more »the radial scaling of the SB area in the FR model suggests that the observational probability of SB identification should be insensitive to R , which is consistent with the most recent statistical analysis of SB observations from PSP.« less
2. Characteristics of Multi-scale Current Sheets in the Solar Wind at 1 au Associated with Magnetic Reconnection and the Case for a Heliospheric Current Sheet Avalanche

   https://doi.org/10.3847/1538-4357/ac73f6  

   Eriksson, Stefan ; Swisdak, Marc ; Weygand, James M. ; Mallet, Alfred ; Newman, David L. ; Lapenta, Giovanni ; Wilson III, Lynn B. ; Turner, Drew L. ; Larsen, Bjorn ( July 2022 , The Astrophysical Journal)

   Wind spacecraft measurements are analyzed to obtain a current sheet (CS) normal widthd_csdistribution of 3374 confirmed magnetic reconnection exhausts in the ecliptic plane of the solar wind at 1 au. Thed_csdistribution displays a nearly exponential decay from a peak atd_cs= 25d_ito a median atd_cs= 85d_iand a 95th percentile atd_cs= 905d_iwith a maximum exhaust width atd_cs= 8077d_i. A magnetic fieldθ-rotation angle distribution increases linearly from a relatively few high-shear events toward a broad peak at 35° <θ< 65°. The azimuthalϕangles of the CS normal directions of 430 thickd_cs≥ 500d_iexhausts are consistent with a dominant Parker-spiral magnetic field and a CS normal along the ortho-Parker direction. The CS normal orientations of 370 kinetic-scaled_cs< 25d_iexhausts are isotropic in contrast, and likely associated with Alfvénic solar wind turbulence. We propose that the alignment of exhaust normal directions from narrowd_cs∼ 15–25d_iwidths to well beyondd_cs∼ 500d_iwith an ortho-Parker azimuthal direction of a large-scale heliospheric current sheet (HCS) is a consequence of CS bifurcation and turbulence within the HCS exhaust that may trigger reconnection of the adjacent pair of bifurcated CSs. The proposed HCS-avalanche scenario suggests that the underlying large-scale parent HCS closer to the Sun evolves with heliocentric distance to fracture into many, more ormore »less aligned, secondary CSs due to reconnection. A few wide exhaust-associated HCS-like CSs could represent a population of HCSs that failed to reconnect as frequently between the Sun and 1 au as other HCSs.

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3. 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
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.
5. A Solar Source of Alfvénic Magnetic Field Switchbacks: In Situ Remnants of Magnetic Funnels on Supergranulation Scales

   https://doi.org/10.3847/1538-4357/ac2d8c  

   Bale, S. D. ; Horbury, T. S. ; Velli, M. ; Desai, M. I. ; Halekas, J. S. ; McManus, M. D. ; Panasenco, O. ; Badman, S. T. ; Bowen, T. A. ; Chandran, B. D. ; et al ( December 2021 , The Astrophysical Journal)

   Abstract One of the striking observations from the Parker Solar Probe (PSP) spacecraft is the prevalence in the inner heliosphere of large amplitude, Alfvénic magnetic field reversals termed switchbacks . These δ B R / B ∼  ( 1 ) fluctuations occur over a range of timescales and in patches separated by intervals of quiet, radial magnetic field. We use measurements from PSP to demonstrate that patches of switchbacks are localized within the extensions of plasma structures originating at the base of the corona. These structures are characterized by an increase in alpha particle abundance, Mach number, plasma β and pressure, and by depletions in the magnetic field magnitude and electron temperature. These intervals are in pressure balance, implying stationary spatial structure, and the field depressions are consistent with overexpanded flux tubes. The structures are asymmetric in Carrington longitude with a steeper leading edge and a small (∼1°) edge of hotter plasma and enhanced magnetic field fluctuations. Some structures contain suprathermal ions to ∼85 keV that we argue are the energetic tail of the solar wind alpha population. The structures are separated in longitude by angular scales associated with supergranulation. This suggests that these switchbacks originate near the leadingmore »edge of the diverging magnetic field funnels associated with the network magnetic field—the primary wind sources. We propose an origin of the magnetic field switchbacks, hot plasma and suprathermals, alpha particles in interchange reconnection events just above the solar transition region and our measurements represent the extended regions of a turbulent outflow exhaust.« less