Eric Richard Christian

We present multispacecraft observations of a series of ³ He-rich solar energetic particle (SEP) events observed by Solar Orbiter/Solar Isotope Spectrometer (SIS), Advanced Composition Explorer (ACE)/Ultra-Low-Energy Isotope Spectrometer (ULEIS), and the recently launched Interstellar Mapping and Acceleration Probe (IMAP)/Compact Dual Ion Composition Explorer (CoDICE) between 2026 February 5 and 9. The three spacecraft were very closely magnetically aligned, with their magnetic footpoints at 2.5 R _⊙ separated by less than 4°, providing a rare opportunity to investigate SEP evolution with minimal uncertainties due to longitudinal and latitudinal connectivity. Using the February 5 events, we measure the radial evolution of ³ He fluences between 0.57 and 1 au and find a radial scaling of J ( R ) ∝ R ^−2.72 , consistent with a combination of spherical expansion and interplanetary transport effects. The February 8 events are associated with two SEP dropouts. First, ACE/ULEIS observes an SEP dropout coincident with a change in interplanetary magnetic field orientation. CoDICE observations instead reveal a corresponding shift in the incident SEP streaming direction. Second, Solar Orbiter observes a separate dropout at −10 . ° 6 Carrington latitude associated with changing solar wind plasma and magnetic field properties, suggesting a transition between filled and empty magnetic flux tubes. Whether the changing magnetic connectivity is latitude-dependent remains inconclusive. These observations demonstrate the importance of combining multispacecraft measurements with broad angular coverage to distinguish radial transport, magnetic connectivity, and particle anisotropy effects in ³ He-rich SEP populations.

On 2026 February 4, an interplanetary shock crossed the L1 region and was followed about 7.7 hr later by a heliospheric current sheet/heliospheric plasma sheet (HCS/HPS), while energetic-ion intensities continued to evolve throughout the intervening downstream region. The shock was observed by IMAP, Wind, ACE, DSCOVR, and SOLAR-1, providing a multispacecraft view of its geometry. Multispacecraft timing and local shock-normal analyses show that the shock is highly quasiperpendicular but spatially nonplanar across L1. Energetic-ion intensities exhibit a pronounced energy-dependent response. Sub-MeV intensities increase well upstream of the shock and remain enhanced throughout the downstream region until approaching the HCS/HPS, whereas intensities above ∼1 MeV show a much weaker downstream enhancement and tend to peak near the shock. The energy spectra also soften downstream, with the fitted spectral break shifting from several MeV near the shock to lower energies farther downstream. Pitch-angle distributions evolve from pronounced field-aligned streaming upstream to enhanced intensities at oblique-to-perpendicular pitch angles downstream, with the anisotropy persisting through much of the downstream sheath and gradually broadening toward the HCS/HPS. A secondary current sheet embedded within the downstream HPS exhibits signatures consistent with a localized reconnection exhaust. Overall, the observations indicate that the sub-MeV ion population is strongly influenced by the shock, while its subsequent downstream evolution is shaped primarily by energy-dependent transport. Localized energization within the disturbed downstream sheath may provide an additional contribution to the sub-MeV population.

We apply an automated routine to detect and catalog interplanetary shocks observed by the Interstellar Mapping and Acceleration Probe (IMAP). Using this catalog, we derive the upstream diffusion coefficient corresponding to 1 MeV protons along each shock’s normal direction using second-order quasi-linear theory and unified nonlinear transport theory to get the parallel and perpendicular components of the diffusion coefficient tensor, respectively. We show that the magnetic compression, as a proxy for shock strength, of fast forward shocks is statistically correlated with the upstream diffusion coefficient (Pearson correlation of 0.81 ± 0.13 and p -value of 0.014), suggesting that magnetic compression is related to the processes governing particle acceleration and particle transport near interplanetary shocks. This result is consistent with several possible interpretations, including changes in upstream turbulence, particle reflection, acceleration processes whose efficiency depends on shock compression and geometry, or a combination of these effects. We host the interplanetary shock catalog on the IMAP mission’s publicly available webpage and plan to regularly update it for use by the heliophysics community.

Magnetic reconnection at the near-Sun heliospheric current sheet (HCS) dissipates the Parker spiral and converts magnetic energy into plasma kinetic energy. During Encounter 14 at a radial distance of ∼ 16.25 R​ _⊙ , Parker Solar Probe observed an HCS crossing where reconnection-driven acceleration—likely facilitated by merging large-scale flux tubes—energized protons up to ∼400 keV. This energy gain is ≈1000 times greater than the available magnetic energy per particle. We present here a comprehensive analysis of pitch-angle distributions and differential energy spectra for protons and heavy ions (He, O, and Fe) in conjunction with local wave activity during this crossing. Our results provide the first direct in situ observations of simultaneous proton and heavy-ion energization during HCS reconnection. Crucially, we find that heavy-ion power-law spectral indices differ significantly from those of protons, contradicting previous simulations that predict species-independent slopes. We further demonstrate that ion beams and anisotropies produced during reconnection drive waves in the ion cyclotron range of frequencies. Finally, we show that proton pitch-angle scattering is stronger than that of heavy ions, which may account for the flatter spectra or harder spectral indices observed in the heavy-ion populations. These observations provide definitive evidence for in situ reconnection-driven acceleration at the near-Sun HCS and necessitate the inclusion of species-dependent transport and acceleration efficiencies in contemporary reconnection-based particle energization models.

NASA’s Interstellar Mapping and Acceleration Probe (IMAP) mission is providing comprehensive observations of both the in situ magnetic field and charged particle environment as well as remotely exploring energetic neutral atoms (ENAs), dust particles, and ultraviolet photons that originate from the outer heliosphere and beyond. Collectively, these measurements will advance our understanding of particle acceleration throughout the solar system and how the solar wind and interstellar medium interacts with the boundary of our heliosphere. Here, we focus on heliospheric observations from the IMAP-Ultra (Ultra) instrument, which is one of three ENA imagers on IMAP. Ultra measures ~3 – 100 keV neutral atoms over its large ~96° × 120° field of view (FoV), achieving angular resolutions ≤ 6° above 10 keV for H. Since IMAP’s launch on September 24, 2025, both Ultra sensors have been fully commissioned and are collecting new and exciting observations of the heliosphere. In this presentation, we highlight the global heliospheric configuration of > 3.7 keV H ENAs with detailed investigations into its spectral variations, evolving heliotail structure as a function of energy, and high angular resolution structures (up to 2° for energies > 30 keV).

We present new, multipoint observations of interplanetary shocks observed by six observatories around the first Sun-Earth Lagrange (L1) point. With IMAP, SWFO-L1, ACE, Wind, DSCOVR, and Aditya-L1, we investigate multiple interplanetary shocks associated with the passage of two coronal mass ejections (CMEs) on 11-12 November 2025. Inter-spacecraft separations range from 10s of Earth radii (Re) to ~200 Re, offering a number of different baselines and geometries for multipoint analysis. We analyze shock orientations and speeds and diagnose the interaction between the two CMEs. The 6-point observatories showcase how the CME shocks and sheaths are structured and allow for analysis of the interface between ejecta and ambient solar wind plasmas. We also show that spatial gradients of the structures can be resolved by combining different sets of tetrahedra within the constellation and discuss options for full volume reconstruction of the time-dependent plasma dynamics.

Magnetic reconnection at the near-Sun heliospheric current sheet (HCS) dissipates the Parker spiral and converts magnetic energy into kinetic energy of the plasma constituents. Observations at a radial distance of ~16.25 Rs by Parker Solar Probe associated with the encounter 14 (E14) HCS crossing have shown that reconnection-driven particle acceleration mechanisms, likely facilitated by the merging of large-scale flux tubes, are able to accelerate protons up to ~400 keV, which is ≈1000 times greater than the available magnetic energy per particle during this crossing (Desai et al. 2025; Phan et al. 2024). In this paper, we present a detailed analysis of the pitch-angle distributions, differential energy spectra, and maximum energies and spectra of protons and heavy ions (He, O, and Fe) in conjunction with observations of local wave activity during the E14 HCS crossing. Our results show the following: 1) First direct observations of the energization of protons and heavy ions during reconnection. 2) First direct observations that the power-law spectral slopes of heavy ions differ from that of protons, which contradicts previous simulation results where the spectral indices of all ion species are essentially identical. 3) First demonstration that the anisotropies and beams of ions produced during reconnection drive waves in the ion cyclotron range of frequencies. 4) First evidence that the pitch angle scattering of protons is stronger than that of the other ion species and that this might be responsible for the harder spectral slopes of the heavy ions compared with protons. In summary, PSP observations during the E14 HCS crossing provide strong evidence for in-situ reconnection-driven acceleration of protons and heavy ions at the near-Sun HCS that will need to be fully accounted for by contemporary reconnection-driven energization models.

Occasionally, solar energetic particle (SEP) events associated with fast, wide coronal mass ejections (CMEs) driving interplanetary shocks exhibit Fe/O abundance ratios that are significantly higher than the nominal value of 0.134. The cause of this enrichment in Fe remains unclear, but there are two prevailing theories: contribution from flare-accelerated material, and reacceleration of suprathermal remnant flare material by a quasiperpendicular shock. The 2024 July 23 SEP event was observed by four spacecraft spread over ∼160° of solar longitude and 0.4 au in radial distance. We examine the H, He, O, and Fe spectra and He/H and Fe/O abundance ratios observed at each spacecraft, and find that three measured Fe/O ratios >0.6 were relatively independent of energy. The fourth spacecraft measured strongly energy-dependent Fe/O ratios reaching very low values of 0.02 at a few MeV nuc ⁻¹ . In an effort to evaluate the plausibility of the two explanations for Fe-rich SEP events, we studied the magnetic connection of the spacecraft to the solar source and the evolving CME; the velocity dispersion of the first arriving particles; and the time evolution of the Fe/O ratios. We also noted the pre-event CME and flare activity. We find that neither scenario is clearly consistent with the compositional variations between the four spacecraft, but more detailed modeling of the magnetic connections between the spacecraft and the solar source and the propagating shock may be beneficial.