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1. Magnetic Reconnection–driven Energization of Protons up to ∼400 keV at the Near-Sun Heliospheric Current Sheet

   https://doi.org/10.3847/2041-8213/ada697  

   Desai, M I; Drake, J F; Phan, T; Yin, Z; Swisdak, M; McComas, D J; Bale, S D; Rahmati, A; Larson, D; Matthaeus, W H; et al (May 2025, The Astrophysical Journal Letters)

   Abstract We report observations of direct evidence of energetic protons being accelerated above ∼400 keV within the reconnection exhaust of a heliospheric current sheet (HCS) crossing by NASA’s Parker Solar Probe (PSP) at a distance of ∼16.25 solar radii (R_s) from the Sun. Inside the exhaust, both the reconnection-generated plasma jet and the accelerated protons up to ∼400 keV propagated toward the Sun, unambiguously establishing their origin from HCS reconnection sites located antisunward of PSP. Within the core of the exhaust, PSP detected stably trapped energetic protons up to ∼400 keV, which is ≈1000 times greater than the available magnetic energy per particle. The differential energy spectrum of the accelerated protons behaved as a pure power law with spectral index of ∼−5. Supporting simulations using thekglobalmodel suggest that the trapping and acceleration of protons up to ∼400 keV in the reconnection exhaust are likely facilitated by merging magnetic islands with a guide field between ∼0.2 and 0.3 of the reconnecting magnetic field, consistent with the observations. These new results, enabled by PSP’s proximity to the Sun, demonstrate that magnetic reconnection in the HCS is a significant new source of energetic particles in the near-Sun solar wind. Our findings of in situ particle acceleration via magnetic reconnection at the HCS provide valuable insights into this fundamental process, which frequently converts the large magnetic field energy density in the near-Sun plasma environment and may be responsible for heating the Sun’s atmosphere, accelerating the solar wind, and energizing charged particles to extremely high energies in solar flares. 

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2. 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)

   Abstract 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 $E_{\max }\propto {\left(Q/M\right)}^{\alpha }$ . 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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3. 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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4. Magnetic Reconnection in the Heliospheric Current Sheet Embedded in an Interplanetary Coronal Mass Ejection Sheath

   https://doi.org/10.3847/2041-8213/ae1d44  

   Ala-Lahti, Matti; Pulkkinen, Tuija I; Suni, Jonas; Hietala, Heli; Good, Simon W; Akhavan-Tafti, Mojtaba; Palmroth, Minna; Kilpua, Emilia_K J (November 2025, The Astrophysical Journal Letters)

   Abstract Solar wind conditions in the heliosphere are altered by solar wind transients such as shocks and interplanetary coronal mass ejections (ICMEs). The interactions of coherent solar wind structures such as the heliospheric current sheet (HCS) with the transients remain incompletely understood, however. Here, we report the first in situ spacecraft observations showing two magnetic reconnection exhausts in the HCS embedded in the sheath region of an ICME. The dropout (presence) of strahl electrons indicates the disconnection (connection) of an exhaust region from (to) Sun. We perform a multispacecraft analysis, which shows that reconnection in the HCS embedded in an ICME sheath can operate in a quasi-steady manner at 1 au, with the observations across the outflow regions showing a good correlation with the Walén relation. The two reconnecting HCS crossings in the ICME sheath are compared to two other HCS crossings, one in the sheath and one in the solar wind upstream of the ICME shock, which show no magnetic reconnection signatures. These results highlight interplanetary transients as space plasma environments that can host dynamics infrequently observed in the solar wind at Earth’s orbit. 

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5. PSP/IS⊙IS Observation of a Solar Energetic Particle Event Associated with a Streamer Blowout Coronal Mass Ejection during Encounter 6

   https://doi.org/10.3847/1538-4357/ac408f  

   Getachew, T.; McComas, D. J.; Joyce, C. J.; Palmerio, E.; Christian, E. R.; Cohen, C. M.; Desai, M. I.; Giacalone, J.; Hill, M. E.; Matthaeus, W. H.; et al (February 2022, The Astrophysical Journal)

   Abstract In this paper we examine a low-energy solar energetic particle (SEP) event observed by IS⊙IS’s Energetic Particle Instrument-Low (EPI-Lo) inside 0.18 au on 2020 September 30. This small SEP event has a very interesting time profile and ion composition. Our results show that the maximum energy and peak in intensity are observed mainly along the open radial magnetic field. The event shows velocity dispersion, and strong particle anisotropies are observed throughout the event, showing that more particles are streaming outward from the Sun. We do not see a shock in the in situ plasma or magnetic field data throughout the event. Heavy ions, such as O and Fe, were detected in addition to protons and 4He, but without significant enhancements in 3He or energetic electrons. Our analysis shows that this event is associated with a slow streamer blowout coronal mass ejection (SBO-CME), and the signatures of this small CME event are consistent with those typical of larger CME events. The time–intensity profile of this event shows that the Parker Solar Probe encountered the western flank of the SBO-CME. The anisotropic and dispersive nature of this event in a shockless local plasma gives indications that these particles are most likely accelerated remotely near the Sun by a weak shock or compression wave ahead of the SBO-CME. This event may represent direct observations of the source of the low-energy SEP seed particle population. 

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