Nathan A. Schwadron

IBEX-Hi observed four heliotail lobes above ∼2 keV: the energetic neutral atom (ENA)-suppressed port (P) and starboard (SB) near the ecliptic and the ENA-enhanced north (N) and south (S) at midlatitudes. They trace how the latitude-dependent solar wind (SW) and solar and interstellar magnetic fields shape the downwind heliosheath (HS). Prior work, using five nonoverlapping periods over 11 yr, found cyclic P–SB motion and equatorward N–S motion in phase with the solar cycle. We extend this analysis through 2022 using 14 smoothed yearly maps and add three methods: an intensity-weighted moment tensor (for lobe shapes), spatial and temporal block uncertainty estimates, and elliptical fits to lobe centers (for motion loci). At ∼4.3 keV, downwind-hemispheric intensity drops ∼2.4× from 2009 to 2016, partially recovering by 2022; all four lobes weaken and recover together, lagging the polar coronal hole area by ∼3 yr. P and SB track the reported cyclic motion; SB shows the largest range (∼52° longitude, ∼36° latitude). N and S move equatorward through 2017 (N ∼59° to ∼27°, S ∼−48° to ∼−24°), partially returning by 2022. The N–S great circle lies near an ecliptic meridian ( α ∼ 89 . ° 3), P–SB near the ecliptic ( β ∼ 23 . ° 8), and they are not orthogonal ( γ ∼78 . ° 1), qualitatively consistent with interstellar field draping around the heliopause. Fitted motion loci constrain models: the P-lobe is least elongated, the S-lobe most elongated, the SB-lobe longest, and the N-lobe least reliable owing to ribbon contribution. Results constrain the distant tailward HS and its evolution with the solar cycle, SW, and charge-exchange cooling.

S. T. Bramwell et al. (1998) showed that the fluctuations of turbulence have the same statistical description as those of the order parameter in equilibrium-critical phenomena like the magnetization of ferromagnets. Later, it was shown that various other critical systems exhibit similar fluctuation statistics of the variance κ ₁ of natural time establishing a universal way of defining an order parameter. These systems include avalanches in 3D rice piles, the magnetic flux penetration in type II superconductors, the occurrence of earthquakes in the Earth’s solid crust, acoustic emission before fracture, as well as avalanches generated by the Burridge–Knopoff or Olami–Feder–Christensen models. Here, we show by the most recent measurements of the Interstellar Mapping and Acceleration Probe (IMAP) that the interplanetary magnetic field components under steady solar wind conditions at 1 au when analyzed in natural time share the same statistical description of fluctuations as in critical systems. This property can identify early-warning signatures of episodic events interwoven with the magnetic field.

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.

Energetic neutral atoms (ENAs) created outside the heliospheric termination shock are observed at 1 au by heliospheric observatories IBEX and IMAP. These atoms are subjected to ionization losses underway between their birth and detection sites. Consequently, to infer the ENA flux distribution at their entrance inside the termination shock, the observed maps must be appropriately corrected. Here we provide a pedagogical review of the issue of probabilities of survival of ENAs on their travel to the detector, including presentation of the loss reactions, derivation of survival probabilities from first principles and their dependence on the ENA energy, solar activity phase, viewing geometry, etc. We present results of simulations of survival probabilities of ENAs for the past two solar cycles for the energy range and viewing geometry of IMAP. For the first time, we discuss the probabilities for the lowest energies, below 0.2 keV, and the highest, between ∼4 and 300 keV. We point out correlations introduced to the observed ENA maps both within individual maps and between maps for different energies and elongation angles of the detectors. We also identify the most favorable times during the solar activity cycle and viewing geometries to maximize the likelihood of detection of the lowest-energy ENAs. The paper explains the survival probability product, i.e., L3e data from the IMAP GLOWS experiment.

Energetic neutral atom (ENA) flux time series measured by the Interstellar Boundary Explorer (IBEX) from the north and south polar regions have recently revealed persistence, which is in accord with the persistence in the solar wind (SW) flow pressure at 1 au. This is not unexpected since the SW is involved in the generation of ENAs. Here, we further explore the origins of this similarity by studying for the first time the multifractal character inherent to these time series. We show that even when removing periodic trends from the time series, the remaining fluctuations exhibit rich multifractality. The multifractality reflects the existence of multiple long-range correlations in the ENA fluctuations. The north polar region presents wider multifractality than the south, exhibiting higher complexity. This is compatible with the fact that the northern region is much more affected than the southern from the IBEX Ribbon, an unanticipated ENA source discovered by IBEX. The ENA flux time series multifractal spectra (MFS) show a clear tendency (>70%) to be right-skewed. SW flow pressure MFS is also right-skewed, indicating the influence of the SW pressure on the ENAs. The analysis relies on two modern techniques commonly used in fractal analysis: the multifractal detrended fluctuation analysis (MFDFA) and the Multiscale Multifractal Analysis (MMA), which is a generalization of MFDFA, both of order m = 3 (determined in our previous study as optimal for detrending). Finally, the MMA of ENA flux time series indicates persistence in 97% of the cases studied, generalizing our previous results.

Dust near the Sun is implicated in the production of an unexpected “inner source” of pickup ions (PUIs). Multiple mechanisms related to dust have been proposed to explain this phenomenon, and all depend on the amount, along with the spatial and size distributions, of zodiacal dust near the Sun. Leveraging new insight showing the zodiacal cloud is highly spatially structured, we investigate the implications of spatial fluctuations of the zodiacal cloud on temporal variability in the inner source of PUIs. Temporal variability of dust near the Sun is inherited from spatial fluctuations upstream at larger distances. Modulation of the inner source due to dust variability can occur on timescales of days to decades, depending on the relevant dust size responsible for generating the inner source and the spatial characteristics of zodiacal abundance fluctuations. Cometary disruptions occurring every few days are also expected to directly modulate the inner source. Similar variability is expected for the production of nanograins incorporated into the solar wind. These results present a new mechanism for days-to-decades variability in the inner-source abundance and provide a quantitative relationship between inner-source variability and zodiacal fluctuations.

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.

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.

In SEP events, velocity dispersion (VD) is characterized by the earlier arrival of faster, higher-energy particles relative to slower ones, assuming negligible acceleration time and transport effects. The "Labor Day event" at Parker Solar Probe (PSP) on 2022 September 5 provided a unique arrival profile, in which the medium energy (~ few MeV) particles arrive earlier than both lower and higher energy particles. This created a so-called "nose" structure in the intensity spectrogram formed by measurements from the two energetic particle instruments, EPI-Lo and EPI-Hi, of the Integrated Science Investigation of the Sun (ISOIS) suite. Unlike typical VD, the delayed arrival of higher energy particles compared to medium energy particles, i.e., the "inverse velocity arrival" (IVA), could be caused by various acceleration, transport, and instrumental effects, including shock acceleration. By applying a new method based on the contour-line of the intensity, we found 14 IVA events in the ISOIS observations up to the end of 2024. Several parameters that may modify velocity dispersion characteristics are further explored including the spacecraft radial distance, the speed of corresponding CMEs and shocks, the angle between the shock normal and the upstream magnetic field, and the spacecraft magnetic footpoint longitudinal separation from the flare location. The energy of the early arriving particles, i.e., the nose energy, can be grouped into low (L, 5 MeV) categories. Most (11/14) of the IVA events have medium nose energies. This SEP list provides ingredients for examination of shock acceleration in the inner heliosphere, and the existence of IVA events sheds new light on the acceleration and propagation of SEPs.

Quasi-separatrix layers (QSLs) at the Sun are created in regions where channels of open magnetic flux have footpoints near regions of large-scale closed magnetic flux. These regions show rapid changes in curvature and field strength. Numerical simulations of a relaxed coronal magnetic field and solar wind using the Magnetohydrodynamic Algorithm outside a Sphere model coupled to the Energetic Particle Radiation Environment Module model indicate common sources of energetic particles over broad longitudinal distributions in the background solar wind. These regions accelerate energetic particles from QSLs and current sheets. Here, we develop an analytical framework to describe the acceleration of energetic particles due to the magnetic field changes within and near separatrix layers. The reduced field strength near the separatrix layer drives magnetic field magnitude changes that accelerate energetic particles in the presence of plasma flow along the structure. Separatrix layers are prone to magnetic reconnection, creating fluctuations in the field that propagate out from the Sun, and release material previously contained within closed magnetic field structures, which are often rich in heavy ions and ³ He ions. Thus, we present a model of energetic particles accelerated from separatrix layers in the corona. Our results provide a plausible source for seed populations near the Sun.