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1. An Empirically Driven MHD Model to Predict the Solar Wind at Parker Solar Probe and Solar Orbiter during the Current Solar Minimum

   Kim, T. K.; Pogorelov, N.; Arge, C. N.; Jones-Mecholsky, S. I. (December 2020, American Geophysical Union, Fall Meeting 2020, abstract #SH021-08)

   null (Ed.)

   Since the launch on 2018 August 12, the Parker Solar Probe (PSP) has completed its first five orbits around the Sun, having reached down to ~28 solar radii at perihelion 5 on 2020 June 7. More recently, the Solar Orbiter (SolO) made its first close approach to the Sun at 0.52 AU on 2020 June 15, nearly 4 months after the launch. Using a 3D heliospheric MHD model coupled with the Wang-Sheeley-Arge (WSA) coronal model using the Air Force Data Assimilative Photospheric flux Transport (ADAPT) magnetic maps as input, we simulate the time-varying inner heliosphere, including the trajectories of PSP and SolO, during the current solar minimum period between 2018 and 2020. Above the ADAPT-WSA model outer boundary at 21.5 solar radii, we solve the Reynolds averaged MHD equations with turbulence and pickup ions taken into account and compare the simulation results with the PSP solar wind and magnetic field data, with particular emphasis on the large-scale solar wind structure and magnetic connectivity during each solar encounter. 

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2. Eruption and Interplanetary Evolution of a Stealthy Streamer-Blowout CME Observed by PSP at ∼0.5 AU

   https://doi.org/10.3389/fspas.2022.903676  

   Pal, Sanchita; Lynch, Benjamin J.; Good, Simon W.; Palmerio, Erika; Asvestari, Eleanna; Pomoell, Jens; Stevens, Michael L.; Kilpua, Emilia K. (May 2022, Frontiers in Astronomy and Space Sciences)

   Streamer-blowout coronal mass ejections (SBO-CMEs) are the dominant CME population during solar minimum. Although they are typically slow and lack clear low-coronal signatures, they can cause geomagnetic storms. With the aid of extrapolated coronal fields and remote observations of the off-limb low corona, we study the initiation of an SBO-CME preceded by consecutive CME eruptions consistent with a multi-stage sympathetic breakout scenario. From inner-heliospheric Parker Solar Probe (PSP) observations, it is evident that the SBO-CME is interacting with the heliospheric magnetic field and plasma sheet structures draped about the CME flux rope. We estimate that 18 ± 11% of the CME’s azimuthal magnetic flux has been eroded through magnetic reconnection and that this erosion began after a heliospheric distance of ∼0.35 AU from the Sun was reached. This observational study has important implications for understanding the initiation of SBO-CMEs and their interaction with the heliospheric surroundings. 

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3. Galactic Cosmic-Ray Propagation in the Inner Heliosphere: Improved Force-field Model

   https://doi.org/10.3847/1538-4357/ac8cf3  

   Li_李, Jung-Tsung 融宗; Beacom, John F; Peter, Annika_H G (September 2022, The Astrophysical Journal)

   Abstract A key goal of heliophysics is to understand how cosmic rays propagate in the solar system’s complex, dynamic environment. One observable is solar modulation, i.e., how the flux and spectrum of cosmic rays change as they propagate inward. We construct an improved force-field model, taking advantage of new measurements of magnetic power spectral density by Parker Solar Probe to predict solar modulation within the Earth’s orbit. We find that modulation of cosmic rays between the Earth and Sun is modest, at least at solar minimum and in the ecliptic plane. Our results agree much better with the limited data on cosmic-ray radial gradients within Earth’s orbit than past treatments of the force-field model. Our predictions can be tested with forthcoming direct cosmic-ray measurements in the inner heliosphere by Parker Solar Probe and Solar Orbiter. They are also important for interpreting the gamma-ray emission from the Sun due to scattering of cosmic rays with solar matter and photons. 

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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 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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5. Bidirectional anisotropic solar energetic particle events observed by Solar Orbiter

   https://doi.org/10.1051/0004-6361/202556098  

   Ding, Zheyi; Wimmer-Schweingruber, Robert F; Chen, Yu; Zhao, Lingling; Kollhoff, Alexander; Kühl, Patrick; Yang, Liu; Berger, Lars; Heidrich-Meisner, Verena; Rodriguez-Pacheco, Javier; et al (September 2025, Astronomy & Astrophysics)

   Context.Solar energetic particle (SEP) events are critical for understanding particle acceleration and transport in the heliosphere. While most SEP events involve outward streaming particles along open magnetic field lines, bidirectional events characterized by simultaneous sunward and anti-sunward particle flows offer unique insights into magnetic field topology and the interplay of multiple acceleration sources. Aims.We investigate the origin and transport of energetic particles in two rare bidirectional anisotropic SEP events observed by Solar Orbiter, with a particular emphasis on their association with magnetic flux ropes. Methods.Energetic particles, solar wind plasma, magnetic field, and solar radio measurements were analysed. Via the velocity dispersion analysis, we determined release times and path lengths for distinct particle populations. Automated flux rope identification and magnetic helicity diagnostics were used to characterize magnetic flux ropes. Results.Both events showed two clear velocity dispersion signatures with opposite particle anisotropies during their onset phase. The sunward streaming protons, characterized by a delayed release time, a harder spectral index, and higher intensities, may be due to coronal mass ejection-driven shock acceleration, while the promptly released anti-sunward streaming protons are likely linked to flare acceleration. Notably, in both cases, small-scale flux ropes were identified in situ during the time intervals corresponding to the bidirectional particle streaming. Path lengths derived for sunward and anti-sunward injections were substantially greater than nominal values of the Parker field lines, further evidence of the role of the flux rope in shaping particle trajectories. Conclusions.These observations demonstrate that magnetic flux ropes can significantly affect magnetic connectivity to the source region and SEP propagation in the inner heliosphere, and that simultaneous velocity dispersion from two distinct particle sources can be used to place direct constraints on the topology of the flux rope. Our results highlight the value of combining particle anisotropy, release time, source spectra, and magnetic structure diagnostics to unravel SEP transport in complex transient magnetic structures, and also present new challenges for the current SEP transport model. 

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