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1. 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)

   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 scalesmore »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.« less
2. Pitch-angle Distributions of 0.5–1 GeV Solar Protons Crossing Earth’s Orbit: Influence of the Large-scale Turbulent Interplanetary Magnetic Field

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

   Moradi, Ashraf ; Giacalone, Joe ( July 2023 , The Astrophysical Journal)

   Using numerical simulations, we analyze the time evolution of the pitch-angle distribution of 500 MeV and 1 GeV solar protons, released impulsively near the Sun, at 1 au. The numerical model solves the equations of motion of an ensemble of particles that move in both the average Parker spiral field and a large-scale turbulent interplanetary magnetic field (IMF). Our model also includes the heliospheric current sheet (HCS). The focus of this study is to determine the effect of the large-scale turbulent IMF on the pitch-angle distribution of GV-rigidity protons and its time variations in terms of understanding variations in ground-level enhancement (GLE) events. Our particular interest is to explain the two distinct opposite-directed fluxes of the unusual event on 1989 October 22 (GLE#44). The results show that by adding the large-scale turbulence to the average Parker IMF, the pitch-angle distribution at 1 au depends strongly on the observer’s location relative to the release location of the particles at the Sun. Even a 0.2° displacement in latitude or longitude leads to a significant change in the observed distribution and/or its variation in time. We find that there are some observer locations for which the distinct sunward and antisunward fluxes coexistmore »at certain times of the events. We also find that the HCS has an important effect. For instance, even in locations of poor magnetic connection with the release location at the Sun, but near the HCS, there can be two fluxes moving in different directions at the same time.

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3. Kinetic-scale Current Sheets in the Solar Wind at 1 au: Scale-dependent Properties and Critical Current Density

   https://doi.org/10.3847/2041-8213/ac4fc4  

   Vasko, I. Y. ; Alimov, K. ; Phan, T. ; Bale, S. D. ; Mozer, F. S. ; Artemyev, A. V. ( February 2022 , The Astrophysical Journal Letters)

   We present analysis of 17,043 proton kinetic-scale current sheets (CSs) collected over 124 days of Wind spacecraft measurements in the solar wind at 11 samples s⁻¹magnetic field resolution. The CSs have thickness,λ,from a few tens to one thousand kilometers with typical values around 100 km, or within about 0.1–10λ_pin terms of local proton inertial length,λ_p. We found that the current density is larger for smaller-scale CSs,J₀≈ 6 nAm⁻²· (λ/100 km)^−0.56, but does not statistically exceed a critical value,J_A,corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units:$J_0/J_A\approx 0.17·{(\lambda /{\lambda }_p)}^{-0.51}$. The CSs are statistically force-free with magnetic shear angle correlated with CS spatial scale:$\mathrm{\Delta }\theta \approx 19°·{(\lambda /{\lambda }_p)}^{0.5}$. The observed correlations are consistent with local turbulence being the source of proton kinetic-scale CSs in the solar wind, while the mechanisms limiting the current density remain to be understood.
4. 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 themore »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.« less
5. Suprathermal Ion Energy Spectra and Anisotropies near the Heliospheric Current Sheet Crossing Observed by the Parker Solar Probe during Encounter 7

   https://doi.org/10.3847/1538-4357/ac4961  

   Desai, M. I. ; Mitchell, D. G. ; McComas, D. J. ; Drake, J. F. ; Phan, T. ; Szalay, J. R. ; Roelof, E. C. ; Giacalone, J. ; Hill, M. E. ; Christian, E. R. ; et al ( March 2022 , The Astrophysical Journal)

   Abstract We present observations of ≳10–100 keV nucleon −1 suprathermal (ST) H, He, O, and Fe ions associated with crossings of the heliospheric current sheet (HCS) at radial distances of <0.1 au from the Sun. Our key findings are as follows: (1) very few heavy ions are detected during the first full crossing, the heavy-ion intensities are reduced during the second partial crossing and peak just after the second crossing; (2) ion arrival times exhibit no velocity dispersion; (3) He pitch-angle distributions track the magnetic field polarity reversal and show up to ∼10:1 anti-sunward, field-aligned flows and beams closer to the HCS that become nearly isotropic farther from the HCS; (4) the He spectrum steepens either side of the HCS, and the He, O, and Fe spectra exhibit power laws of the form ∼ E −4 – E 6 ; and (5) maximum energies E X increase with the ion’s charge-to-mass ( Q / M ) ratio as E X / E H ∝ ( Q X / M X ) δ , where δ ∼ 0.65–0.76, assuming that the average Q states are similar to those measured in gradual and impulsive solar energetic particle events at 1 au. Themore »absence of velocity dispersion in combination with strong field-aligned anisotropies closer to the HCS appears to rule out solar flares and near-Sun coronal-mass-ejection-driven shocks. These new observations present challenges not only for mechanisms that employ direct parallel electric fields and organize maximum energies according to E / Q but also for local diffusive and magnetic-reconnection-driven acceleration models. Reevaluation of our current understanding of the production and transport of energetic ions is necessary to understand this near-solar, current-sheet-associated population of ST ions.« less