Daniel Brett Reisenfeld

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.

The Halloween solar storms of 2003 and associated coronal mass ejections (CMEs) hurled one of the most intense interplanetary CMEs (ICMEs) during the space age toward Earth. The ICMEs forced many operating spacecraft into “safe mode” and spacecraft that were not “safed” were pushed to their observational limits. In contrast, although the electronics of the NASA Genesis spacecraft were safed, the solar wind (SW) collector array designed to sample CMEs remained deployed. The sample collection arrays were returned to Earth, enabling quantification of the hydrogen and helium particles of the ICMEs by depth profiling. During the entire period of collection by the Genesis spacecraft, the SW was monitored in situ continuously by the Advanced Composition Explorer (ACE). The Genesis results demonstrate that the hydrogen and helium fluences of ICMEs during the Halloween solar storms of 2003 were significantly greater than inferred by the in situ plasma instruments of ACE. In contrast, Genesis and ACE results were shown to be identical within uncertainties except for the period during the Halloween solar storms of 2003. The Genesis returned samples provide a means to investigate intense ICMEs in laboratories using Genesis data as a standard reference.

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.

The Interstellar Boundary Explorer (IBEX) has revealed the existence of the energetic neutral atom (ENA) ribbon, a narrow and nearly circular band of enhanced flux that enables probing plasma and magnetic field conditions outside the heliopause (HP). In this study, we investigate the global structure and temporal variation of the IBEX ribbon using 14 yr of IBEX-Hi ENA observations. We derive geometric parameters of the ribbon: center location, angular radius, eccentricity, and major axis orientation. We find that the ribbon exhibits systematic variations tied to the solar cycle. The ribbon center shows a solar cycle–modulated latitudinal motion at low energies. The angular radius increases with ENA energy and displays a pronounced enlargement between 2013 and 2018, coinciding with changes in high-latitude fast solar wind from polar coronal holes. The eccentricity remains consistently around 0.3 with some temporal evolution, while the major axis rotates gradually over time, becoming increasingly perpendicular to the ecliptic meridional plane. A notable hemispheric asymmetry has emerged over time: the ribbon in the northern hemisphere remains remarkably stable in position, whereas the southern ribbon exhibits temporal migration. By combining these results with solar wind observations, global heliosphere, and ribbon modeling, we suggest that this asymmetry is explained by a coupled effect of magnetic field draping and the latitude-dependent timing of fast solar wind from polar coronal holes. These results establish the most detailed long-term geometric characterization of the IBEX ribbon to date and provide observational support for models of secondary ENA production and magnetic field draping outside the HP.

Saturn's magnetosphere is continuously supplied with neutrals from the Enceladus plume and the icy rings, which undergo ionization and charge‐exchange to form a complex water‐group plasma environment. While the Cassini Plasma Spectrometer (CAPS) instrument has provided extensive compositional information, detailed separation of individual water‐group ion species in time‐of‐flight (TOF) data has not previously been achieved. In this study, we perform forward modeling of CAPS‐IMS energy‐per‐charge (E/Q) and TOF spectra obtained between 2004 and 2012 to resolve O ⁺ , OH ⁺ , H ₂ O ⁺ , and H ₃ O ⁺ and to characterize their plasma properties, including number density, temperature, and thermodynamic κ. Our results demonstrate that O ⁺ is the dominant thermal ion species throughout Saturn's magnetosphere, comprising up to ∼70% of the total ion population beyond ∼5 Saturn radii (R _S ). In contrast, molecular ions such as OH ⁺ , H ₂ O ⁺ , and H ₃ O ⁺ dominate closer to Enceladus but rapidly dissociate into atomic ions between ∼5 and 10 R _S . This radial region is also characterized by the steepest increase in plasma flow speed, which rises from ∼40% to ∼80% of rigid corotation. Simultaneously, ion velocity distributions approach Maxwell–Boltzmann equilibrium, as indicated by high kappa values. These findings provide new constraints on the ion–neutral chemistry that regulates the balance between molecular and atomic ions in Saturn's magnetosphere. They also emphasize the critical role of the 5–10 R _S region as a transition zone for both plasma composition and dynamics. Our results refine previous CAPS‐based studies and underscore the need to incorporate seasonal variability and ionospheric coupling into future global models of Saturn's plasma environment.

The secondary energetic neutral atom (ENA) mechanism proposed to explain the ribbon-like enhancement of ENA emission observed by Interplanetary Boundary Explorer (IBEX) involves a pickup ion (PUI) ring-beam population beyond the heliopause. The stability of this highly anisotropic PUI distribution is crucial for determining the physical process responsible for the ribbon formation. It has been proposed that PUIs with pitch angles near 90° can become trapped via the magnetic mirroring process in the turbulent magnetic field of the local interstellar medium. However, the turbulent mirroring scenario typically neglects self-generated waves excited by the kinetic instabilities of the ring-beam PUIs, which can efficiently scatter PUIs in pitch angle. Using a magnetohydrodynamics–particle-in-cell approach implemented in ATHENA++ , we simulate the evolution of the ring-beam PUIs, incorporating both large-scale interstellar magnetic fluctuations and microscale self-excited instabilities. We examine the competition between turbulent mirroring and pitch-angle scattering by the self-generated waves. Our results show that the PUIs can be rapidly scattered toward a nearly isotropic distribution on the order of one day, much shorter than their characteristic lifetime of a couple of years. This suggests that theoretical difficulties remain for maintaining an anisotropic PUI distribution in physical models of the IBEX ribbon.

In this study, we analyze an important property of the very local interstellar medium, i.e., turbulence that affects the intensity and shape of the Interstellar Boundary Explorer (IBEX) ribbon. Specifically, we simulate the propagation of the ribbon’s parent ions before they become secondary energetic neutral atoms that can be observed at 1 au by IBEX. We then test how different slab/2D turbulence fractions affect the intensity and shape of the modeled ribbon and compare to the latest IBEX ribbon-separated data. We compare 1D cuts across the modeled and observed ribbon as a function of angle away from the ribbon center. We find that the intensities of the modeled ribbon are larger than most of the data, except near the ecliptic plane. However, the model intensities are sensitive to how we model the neutral solar wind, which forms the source ion population for the ribbon. We then compare the model and data by normalizing the fluxes to their respective peak intensities and find the peaks’ angular distances from the ribbon center. We find that most of the model peaks lie within 3° of the data peaks, which we consider our 1σ uncertainty (half the size of an IBEX pixel). By averaging the instances where the model peaks match the data peaks (within 3°), we find that the mean is slab-50%, with a standard deviation of ±28% and standard error of ±5%.

We present a comprehensive statistical analysis of ion-scale waves including dual-band ion-scale waves (DBIWs) observed by the Parker Solar Probe in the solar wind. DBIWs are characterized by the simultaneous occurrence of distinct left- and right-handed polarized wave packets at higher and lower frequencies, respectively, in the spacecraft frame, implying the presence of bidirectionally propagating L-mode waves. We identify and compare DBIWs to single-band ion-scale waves (SBIWs) and investigate their spatial distributions, background magnetic field conditions, and associated inertial range turbulence characteristics. We find that DBIWs exhibit a strong preferential occurrence at heliocentric distances between 0.2 and 0.25 au (equivalently 43 and 54 R _S ) in contrast to SBIWs, the occurrence rate of which peaks closer to the Sun and decreases with the heliocentric distance. DBIWs are also preferentially observed under conditions of strong Alfvénic turbulence, characterized by low magnetic compressibility and perpendicular-dominant fluctuations. These findings support a scenario in which stochastic ion heating via sufficiently large perpendicular Alfvénic fluctuation generates the ion temperature anisotropies required to excite bidirectional ion cyclotron waves. This work advances our understanding of kinetic wave generation in the solar wind and its connection to large-scale turbulence.

The heliospheric energetic neutral atoms (ENAs) are products of charge exchange between solar wind and pick-up ions and interstellar neutral atoms. They are created in different regions of the heliosphere and its boundary region with the interstellar medium, constituting different ENA populations, and they carry information about their parent populations and production processes. Thus, ENAs enable mapping of the global structure of the heliosphere and the processes within and at its edge. Three instruments have provided sky maps of the heliospheric ENAs from 200 eV up to 44 keV over the solar activity cycle. The IBEX-Lo and IBEX-Hi instruments on board the Interstellar Boundary Explorer (IBEX) have provided ENA sky maps from 200 eV to 4.3 keV (central energy) from 2009 throughout Solar Cycle 24. The Ion and Neutral Camera (INCA) on board the Cassini–Huygens mission provided sky maps of the ENAs from 8–44 keV (central energy) on the spacecraft route to and in orbit around Saturn through Solar Cycles 23 and 24. We compare large-scale structures of ENA enhancements present across the sky maps in a wide energy range based on IBEX-Lo, IBEX-Hi, and INCA. They include Ribbon, heliotail lobes, and upwind ENA enhancement. We report on similarities and differences observed, including the evolution of the Ribbon from low to higher energies, and the presence of confined north and south heliotail lobes up to 44 keV.