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1. Multispecies Energetic Particle Acceleration Associated with CIR and ICME-driven Shocks

   https://doi.org/10.3847/1538-4357/ad614e  

   Silwal, Ashok; Zhao, Lingling; Zank, Gary P; Wang, Bingbing; Pitña, Alexander; Gautam, Sujan Prasad; Park, Byeongseon; Nakanotani, Masaru; Zhu, Xingyu (September 2024, The Astrophysical Journal)

   Abstract A multispecies energetic particle intensity enhancement event at 1 au is analyzed. We identify this event as a corotating interaction region (CIR) structure that includes a stream interface (SI), a forward-reverse shock pair, and an embedded heliospheric current sheet (HCS). The distinct feature of this CIR event is that (1) the high-energy (>1 MeV) ions show significant flux enhancement at the reverse wave (RW)/shock of the CIR structure, following their passage through the SI and HCS. The flux amplification appears to depend on the energy per nucleon. (2) Electrons in the energy range of 40.5–520 keV are accelerated immediately after passing through the SI and HCS regions, and the flux quickly reaches a peak for low-energy electrons. At the RW, only high-energy electrons (∼520 keV) show significant local flux enhancement. The CIR structure is followed by a fast-forward perpendicular shock driven by a coronal mass ejection (CME), and we observed a significant flux enhancement of low-energy protons and high-energy electrons. Specifically, the 210–330 keV proton and 180–520 keV electron fluxes are enhanced by approximately 2 orders of magnitude. This suggests that the later ICME-driven shock may accelerate particles out of the suprathermal pool. In this paper, we further present that for CIR-accelerated particles, the increase in turbulence power at SI and RWs may be an important factor for the observed flux enhancement in different species. The presence of ion-scale waves near the RW, as indicated by the spectral bump near the proton gyrofrequency, suggests that the resonant wave–particle interaction may act as an efficient energy transferrer between energetic protons and ion-scale waves. 

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2. Return Currents in Collisionless Shocks

   https://doi.org/10.3847/1538-4357/ad3e75  

   Gupta, Siddhartha; Caprioli, Damiano; Spitkovsky, Anatoly (June 2024, The Astrophysical Journal)

   Abstract Collisionless shocks tend to send charged particles into the upstream, driving electric currents through the plasma. Using kinetic particle-in-cell simulations, we investigate how the background thermal plasma neutralizes such currents in the upstream of quasi-parallel non-relativistic electron–proton shocks. We observe distinct processes in different regions: the far upstream, the shock precursor, and the shock foot. In the far upstream, the current is carried by nonthermal protons, which drive electrostatic modes and produce suprathermal electrons that move toward upstream infinity. Closer to the shock (in the precursor), both the current density and the momentum flux of the beam increase, which leads to electromagnetic streaming instabilities that contribute to the thermalization of suprathermal electrons. At the shock foot, these electrons are exposed to shock-reflected protons, resulting in a two-stream type instability. We analyze these processes and the resulting heating through particle tracking and controlled simulations. In particular, we show that the instability at the shock foot can make the effective thermal speed of electrons comparable to the drift speed of the reflected protons. These findings are important for understanding both the magnetic field amplification and the processes that may lead to the injection of suprathermal electrons into diffusive shock acceleration. 

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3. An Anomalous Cosmic-Ray Mediated Termination Shock: Implications for Energetic Neutral Atoms

   https://doi.org/10.3847/2041-8213/acb9e0  

   Kornbleuth, M.; Opher, M.; Zank, G. P.; Wang, B. B.; Giacalone, J.; Gkioulidou, M.; Dialynas, K. (February 2023, The Astrophysical Journal Letters)

   Abstract The Voyager 2 crossing of the termination shock indicated that most of the upstream energy from the thermal solar wind ions was transferred to pickup ions (PUIs) and other energetic particles downstream of the shock. We use hybrid simulations at the termination shock for the Voyager 2, flank, and tail directions to evaluate the distributions of different ion species downstream of the shock over the energy range of 0.52–55 keV. Here, we extend the work of Gkioulidou et al., which showed an energy-dependent discrepancy between modeled and energetic neutral atom (ENA) observations, and fit distributions to a hybrid model to show that a population of PUIs accelerated via diffusive shock acceleration (DSA) to become low-energy anomalous cosmic rays (ACRs) can bridge the gap between modeled and observed ENA fluxes. Our results with the inclusion of DSA via hybrid fitting give entirely new and novel evidence that DSA at the termination shock is likely to be an important physical process. These ACRs carry a significant fraction of the energy density at the termination shock (22%, 13%, and 19% in the Voyager 2, flank, and tail directions, respectively). Using these ACRs in global ENA modeling of the heliosphere from 0.52 to 55 keV, we find that scaling factors as large as 1.8–2.5 are no longer required to match ENA observations at energies of ∼1–4 keV. Large discrepancies between modeled and observed ENAs only remain over energies of 4–20 keV, indicating that there may be a further acceleration mechanism in the heliosheath at these energies. 

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4. Energy Partition at a Collisionless Supercritical Quasi‐Parallel Shock

   https://doi.org/10.1029/2025JA034718  

   Schwartz, Steven J; Trattner, Karlheinz J; Raptis, Savvas; Ergun, Robert E; Wilson, Lynn B; Gomez, Roman G; Cohen, Ian J; Gershman, Daniel J; Goodrich, Katherine A; Kucharek, Harald; et al (March 2026, Journal of Geophysical Research: Space Physics)

   Abstract Shocks in collisionless astrophysical plasmas redistribute some of the incident flow energy into both thermal and non‐thermal energy. Quantifying the partition of that energy amongst various particle species or their sub‐populations, and electromagnetic energy, represents a fundamental goal of shock physics. It embodies the role of the equation of state for the system. Here we apply a framework to assess all the incident and downstream energy fluxes at a crossing of Earth's bow shock for which the upstream magnetic field was roughly aligned with the shock normal direction. Such quasi‐parallel shocks are known to be non‐steady and to produce significant populations of suprathermal particles. We quantify the evolution of all the important carriers of energy flux through the shock region. We sub‐divide the proton population into thermal, suprathermal, and energetic components in order to investigate the shock's efficiency in energizing the nonthermal particles. While the largest energy fluxes are found in the incident proton ram energy and downstream proton thermal enthalpy fluxes, a significant suprathermal population pervades the regions both up‐ and downstream. We also evaluate the energy fluxes attributable to fluctuations in the fluid and field parameters. 

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5. Proton acceleration during the interaction of a coronal-mass-ejection-driven shock and a current sheet

   https://doi.org/10.1051/0004-6361/202556851  

   Zhu, Xingyu; Yang, Liu; Zhao, Lingling; Zank, Gary P; Hou, Chuanpeng; Silwal, Ashok; Subashchandar, Nibuna_S M (February 2026, Astronomy & Astrophysics)

   Shock geometry plays a critical role in the acceleration of energetic particles. To isolate its effect, it is essential to study particle behavior under different shock geometries while maintaining comparable shock properties such as strength, speed, and turbulence conditions within a continuous period of time. We aim to study the physical process of the interaction between an interplanetary shock and a current sheet, and compare the dynamic behavior of energetic protons under different shock geometries separated by the current sheet using the Electron-Proton Telescope on board the Solar Orbiter (SolO) on 14 March 2023. We applied a partial-variance-increment (PVI) method to detect the magnetic structure in the upstream region of the shock. We reconstructed the proton pitch-angle distribution (PAD) in the solar wind frame to analyze the particle dynamics. We calculated the first-order flux anisotropy in the solar-wind frame of reference. We find that the differential flux of 60 keV–2 MeV protons is enhanced by 2.5 times from the current sheet to the shock. During the quasi-parallel shock interval, the first-order flux anisotropy in the solar-wind frame is consistent with diffusive shock acceleration. Nevertheless, the flux anisotropy exhibits a bipolar shape during the quasi-perpendicular shock interval. The coexistence of positive and negative flux anisotropy suggests that the magnetic field is wandering around the shock and connected to the shock surface with different acceleration efficiencies. That the flux anisotropy peaks at a transition energy may indicate that it is influenced by the current sheet. The transition energy is greater during the quasi-perpendicular interval, implying more efficient proton acceleration. We suggest that the current sheet is an important ingredient that affects the local shock geometry and, thus, the particle acceleration efficiency and flux anisotropy. This effect can accumulate as the shock propagates outward from the Sun and should be taken into account when interpreting particle spectral profiles at greater distances. 

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