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1. Kinetic-scale Current Sheets in the Solar Wind at 1 au: Properties and the Necessary Condition for Reconnection

   https://doi.org/10.3847/2041-8213/ac3f30  

   Vasko, I. Y.; Alimov, K.; Phan, T. D.; Bale, S. D.; Mozer, F. S.; Artemyev, A. V. (December 2021, The Astrophysical Journal Letters)

   Abstract We present a data set and properties of 18,785 proton kinetic-scale current sheets collected over 124 days in the solar wind using magnetic field measurements at 1/11 s resolution aboard the Wind spacecraft. We show that all of the current sheets are in the parameter range where reconnection is not suppressed by diamagnetic drift of the X-line. We argue this necessary condition for magnetic reconnection is automatically satisfied due to the geometry of current sheets dictated by their source, which is the local plasma turbulence. The current sheets are shown to be elongated along the background magnetic field and dependence of the current sheet geometry on local plasma beta is revealed. We conclude that reconnection in the solar wind is not likely to be suppressed or controlled by the diamagnetic suppression condition. 

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2. Evidence of Interaction between Ion-scale Waves and Ion Velocity Distributions in the Solar Wind

   https://doi.org/10.3847/1538-4357/add467  

   Yogesh; Ofman, Leon; Boardsen, Scott A; Klein, Kristopher; Martinović, Mihailo; Sadykov, Viacheslav M; Verniero, Jaye L; Shankarappa, Niranjana; Jian, Lan K; Mostafavi, Parisa; et al (June 2025, The Astrophysical Journal)

   Abstract Recent in situ observations from Parker Solar Probe (PSP) near perihelia reveal ion beams, temperature anisotropies, and kinetic wave activity. These features are likely linked to solar wind heating and acceleration. During PSP Encounter 17 (at 11.4R_s) on 2023 September 26, the PSP/FIELDS instrument detected enhanced ion-scale wave activity associated with deviations from local thermodynamic equilibrium in ion velocity distribution functions (VDFs) observed by the PSP/Solar Probe Analyzers-Ion. Dense beams (secondary populations) were present in the proton VDFs during this wave activity. Using bi-Maxwellian fits to the proton VDFs, we found that the density of the proton beam population increased during the wave activity and, unexpectedly, surpassed the core population at certain intervals. Interestingly, the wave power was reduced during the intervals when the beam population density exceeded the core density. The drift velocity of the beams decreases from 0.9 to 0.7 of the Alfvén speed, and the proton core shows a higher temperature anisotropy (T_⊥/T_∥ > 2.5) during these intervals. We conclude that the observations during these intervals are consistent with a reconnection event during a heliospheric current sheet crossing. During this event,α-particle parameters (density, velocity, and temperature anisotropy) remained nearly constant. Using linear analysis, we examined how the proton beam drives instability or wave dissipation. Furthermore, we investigated the nonlinear evolution of ion kinetic instabilities using hybrid kinetic simulations. This study provides direct clues about energy transfer between particles and waves in the young solar wind. 

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3. A Comparison of Particle-in-cell and Hybrid Simulations of the Heliospheric Termination Shock

   https://doi.org/10.3847/1538-4357/ad03e2  

   Swisdak, M.; Giacalone, J.; Drake, J. F.; Opher, M.; Zank, G. P.; Zieger, B. (November 2023, The Astrophysical Journal)

   Abstract We compare hybrid (kinetic proton, fluid electron) and particle-in-cell (kinetic proton, kinetic electron) simulations of the solar wind termination shock with parameters similar to those observed by Voyager 2 during its crossing. The steady-state results show excellent agreement between the downstream variations in the density, plasma velocity, and magnetic field. The quasi-perpendicular shock accelerates interstellar pickup ions to a maximum energy limited by the size of the computational domain, with somewhat higher fluxes and maximal energies observed in the particle-in-cell simulation, likely due to differences in the cross-shock electric field arising from electron kinetic-scale effects. The higher fluxes may help address recent discrepancies noted between observations and large-scale hybrid simulations. 

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4. The effect of variations in the magnetic field direction from turbulence on kinetic-scale instabilities

   https://doi.org/10.1051/0004-6361/202345965  

   Opie, Simon; Verscharen, Daniel; Chen, Christopher H.; Owen, Christopher J.; Isenberg, Philip A. (April 2023, Astronomy & Astrophysics)

   At kinetic scales in the solar wind, instabilities transfer energy from particles to fluctuations in the electromagnetic fields while restoring plasma conditions towards thermodynamic equilibrium. We investigate the interplay between background turbulent fluctuations at the small-scale end of the inertial range and kinetic instabilities acting to reduce proton temperature anisotropy. We analyse in situ solar wind observations from the Solar Orbiter mission to develop a measure for variability in the magnetic field direction. We find that non-equilibrium conditions sufficient to cause micro-instabilities in the plasma coincide with elevated levels of variability. We show that our measure for the fluctuations in the magnetic field is non-ergodic in regions unstable to the growth of temperature anisotropy-driven instabilities. We conclude that the competition between the action of the turbulence and the instabilities plays a significant role in the regulation of the proton-scale energetics of the solar wind. This competition depends not only on the variability of the magnetic field but also on the spatial persistence of the plasma in non-equilibrium conditions. 

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5. Solar Orbiter observations of an ion-scale flux rope confined to a bifurcated solar wind current sheet

   https://doi.org/10.1051/0004-6361/202140949  

   Eastwood, J. P.; Stawarz, J. E.; Phan, T. D.; Laker, R.; Robertson, S.; Zhao, L.-L.; Zank, G. P.; Lavraud, B.; Shay, M. A.; Evans, V.; et al (December 2021, Astronomy & Astrophysics)

   Context. Flux ropes in the solar wind are a key element of heliospheric dynamics and particle acceleration. When associated with current sheets, the primary formation mechanism is magnetic reconnection and flux ropes in current sheets are commonly used as tracers of the reconnection process. Aims. Whilst flux ropes associated with reconnecting current sheets in the solar wind have been reported, their occurrence, size distribution, and lifetime are not well understood. Methods. Here we present and analyse new Solar Orbiter magnetic field data reporting novel observations of a flux rope confined to a bifurcated current sheet in the solar wind. Comparative data and large-scale context is provided by Wind. Results. The Solar Orbiter observations reveal that the flux rope, which does not span the current sheet, is of ion scale, and in a reconnection formation scenario, existed for a prolonged period of time as it was carried out in the reconnection exhaust. Wind is also found to have observed clear signatures of reconnection at what may be the same current sheet, thus demonstrating that reconnection signatures can be found separated by as much as ∼2000 Earth radii, or 0.08 au. Conclusions. The Solar Orbiter observations provide new insight into the hierarchy of scales on which flux ropes can form, and show that they exist down to the ion scale in the solar wind. The context provided by Wind extends the spatial scale over which reconnection signatures have been found at solar wind current sheets. The data suggest the local orientations of the current sheet at Solar Orbiter and Wind are rotated relative to each other, unlike reconnection observed at smaller separations; the implications of this are discussed with reference to patchy vs. continuous reconnection scenarios. 

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