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1. Compression Acceleration of Protons and Heavier Ions at the Heliospheric Current Sheet

   https://doi.org/10.3847/1538-4357/ad6e80  

   Murtas, Giulia ; Li, Xiaocan ; Guo, Fan ( October 2024 , The Astrophysical Journal)

   Recent observations by the Parker Solar Probe (PSP) suggest that protons and heavier ions are accelerated to high energies by magnetic reconnection at the heliospheric current sheet (HCS). By solving the energetic particle transport equation in large-scale MHD simulations, we study the compression acceleration of protons and heavier ions in the reconnecting HCS. We find that the acceleration of multispecies ions results in nonthermal power-law distributions with a spectral index consistent with the PSP observations. Our study shows that the high-energy cutoff of protons can reach$E_{\max }\sim 0.1$–1 MeV depending on the particle diffusion coefficients. We also study how the high-energy cutoff of different ion species scales with the charge-to-mass ratio$E_{\max }\propto {(Q/M)}^{\alpha }$. When determining the diffusion coefficients from the quasi-linear theory with a Kolmogorov magnetic power spectrum, we find thatα∼ 0.4, which is somewhat smaller thanα∼ 0.7 observed by PSP.

    

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2. The near-Sun streamer belt solar wind: turbulence and solar wind acceleration

   https://doi.org/10.1051/0004-6361/202039872  

   Chen, C. H. ; Chandran, B. D. ; Woodham, L. D. ; Jones, S. I. ; Perez, J. C. ; Bourouaine, S. ; Bowen, T. A. ; Klein, K. G. ; Moncuquet, M. ; Kasper, J. C. ; et al ( June 2021 , Astronomy & Astrophysics)

   The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 R ⊙ , allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSP’s fourth solar encounter, which was likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with a spectral index close to –5/3 rather than –3/2), a lower Alfvénicity, and a ‘1∕ f ’ break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ≈4° from the HCS, suggesting ≈8° as the full-width of the streamer belt wind at these distances. While the majority of the Alfvénic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind. 

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3. 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 or 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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4. Prevalence of magnetic reconnection in the near-Sun heliospheric current sheet

   https://doi.org/10.1051/0004-6361/202039863  

   Phan, T. D. ; Lavraud, B. ; Halekas, J. S. ; Øieroset, M. ; Drake, J. F. ; Eastwood, J. P. ; Shay, M. A. ; Pyakurel, P. S. ; Bale, S. D. ; Larson, D. ; et al ( June 2021 , Astronomy & Astrophysics)

   null (Ed.)

   During three of its first five orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale heliospheric current sheet (HCS) multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report the common detections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 29.5–107 solar radii during encounters 1, 4, and 5. Both sunward and antisunward-directed reconnection exhausts were observed. In the sunward reconnection exhausts, PSP detected counterstreaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic field lines with both ends connected to the Sun. In the antisunward exhausts, PSP observed dropouts of strahl electrons, consistent with the reconnected HCS field lines being disconnected from the Sun. The common detection of reconnection in the HCS suggests that reconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossings of the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solar radii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is much thicker than the kinetic scales required for reconnection onset. The observations are also in stark contrast with the apparent absence of reconnection in most of the small-scale and much more intense current sheets encountered near perihelia, many of which are associated with “switchbacks”. Thus, the PSP findings suggest that large-scale dynamics, either locally in the solar wind or within the coronal source of the HCS (at the tip of helmet streamers), plays a critical role in triggering reconnection onset. 

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5. Ion Behaviors in the Reconnection Diffusion Region of a Corrugated Magnetotail Current Sheet

   https://doi.org/10.1029/2019GL082226  

   Wang, Shan ; Chen, Li‐Jen ; Bessho, Naoki ; Hesse, Michael ; Giles, Barbara L. ; Moore, Thomas E. ( May 2019 , Geophysical Research Letters)

   The current sheet structure and ion behaviors in a magnetotail reconnection diffusion region are investigated. The multispacecraft analysis suggests a corrugated current sheet structure, interpreted as due to a flapping motion that propagates along geocentric solar magnetospheric along the +ydirection in the Geocentric Solar Magnetospheric (GSM) coordinate. The electric field (E) and ion distributions have similarities with those in a planar current sheet. Energetic ions move along the current direction, suggesting the acceleration by the observed reconnectionEduring the meandering motion. Counterstreaming ions along the current sheet normal suggest the acceleration by the HallEthat is observed to be the dominant component. However, at certain locations,Eand counterstreaming ions significantly deviate from the local normal direction, and more than one pair of counterstreaming populations exist, possibly because the corrugated current sheet enables ions entering the current sheet at different locations with different velocities to mix together.

    

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