Stephen Anthony Fuselier

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 Tandem Reconnection And Cusp Electrodynamics Reconnaissance Satellites (TRACERS) small explorer mission was designed to: Determine whether magnetopause reconnection is primarily spatially or temporally variable for a range of solar wind conditions . Observations from the Analyzers for Cusp Ions (ACIs) on the two TRACERS spacecrafts are used to make this determination. Tandem cusp encounters from November 2025 through February 2026 are used to distinguish cusp ion structures resulting from spatial variability of magnetopause reconnection from those resulting from temporal variability. Preliminary results indicate that magnetopause reconnection is dominantly spatially variable for all interplanetary magnetic field (IMF) orientations and all solar wind dynamic pressures. Spatial variability dominates by at least a factor of 2 for all solar wind conditions. However, almost all encounters categorized as “spatial” display some temporal variability embedded in the larger spatial variability. More tandem cusp encounters are needed to verify these preliminary results and to complete the other science objectives of the mission.

The Juno mission has elucidated magnetosphere processes, structure, and dynamics via multi‐instrument plasma and magnetic field observations. In this study, these instruments are used to identify magnetic reconnection during the Juno extended mission from a survey of 490 magnetopause crossings at the mid‐latitude southern dusk magnetopause. This survey spans distances of 30–99 R _J , 14–20 local time, and 30°–65° in southern latitude. We identify reconnection outflow jets, plasma heating, and Hall magnetic fields as reconnection signatures while independently applying the Swisdak and Walén relations to each magnetopause crossing. Additionally, eastward versus westward interplanetary magnetic field (IMF) conditions are compared in this context. The results show that magnetic reconnection is most likely to occur during eastward IMF at the mid‐latitude dusk magnetopause due to favorable magnetic shear conditions. The identified eastward IMF reconnection events strongly adhere to the Walén relation, showing signatures of Alfvénic flow perturbations.

Context. Cometary isotopes preserve key information about the early Solar System and its presolar heritage. Rosetta’s long-term in situ observations of comet 67P/Churyumov–Gerasimenko offered the first opportunity to examine isotopic variability within a single comet. Past studies on the D/H ratio came to different results, and hence, we have reassessed the entire dataset carefully. Aims. We reassessed the isotopic composition of water in 67P’s coma using a unified Bayesian framework; tested for isotopic heterogeneities across mission phases, latitudes, and cometocentric distances; and evaluated whether hydronium ions (H ₃ O ⁺ ) are present and variable in the Double Focusing Mass Spectrometer’s (DFMS) neutral-mode spectra, particularly during dust-driven activity. Methods. We re-analysed the complete ROSINA/DFMS neutral-mode dataset using Bayesian nested sampling. The model accounts for mass-scale calibration, peak overlaps, and physically motivated priors. Water isotopologues were fitted jointly with potential hydronium contributions at m/z 19. Correlations with gas abundances, cometocentric distance, and dust events were used to identify drivers of variability. Results. More than 7600 D/H measurements yielded a mission-average value of (4.67 ± 0.59) · 10 ⁻⁴ , about three times Earth’s value of 1.56 · 10 ⁻⁴ , largely invariant with activity. Pre-perihelion latitude differences remain within uncertainties. Oxygen isotopes show a ~40% enrichment in ¹⁷ O and a ~25% enrichment in ¹⁸ O relative to Earth, higher than the previously reported ~11% (taken from a subset of the data), without systematic spatial or temporal trends. Hydronium is consistently required to reproduce DFMS neutral-mode spectra at m/z 19 and correlates with NH ₃ and other volatiles, with strong dust-event enhancements. Laboratory experiments indicate that H ₃ O forms mainly inside the DFMS ion source via proton transfer reactions involving H ₂ O and NH ₃ . Conclusions. The isotopic composition of comet 67P reflects stable primordial signatures with little intra-comet variability. Hydronium traces ion–molecule chemistry and precursor availability near the spacecraft (S/C), with variability driven by water abundance, ammonia-mediated proton transfer, and dust-enhanced volatile release.

A key process in solar‐terrestrial interactions is magnetic reconnection at the magnetopause. In addition to in situ observations of plasma entry at the magnetopause, some of the important global aspects of magnetic reconnection are addressed using plasma observations in the Earth's magnetospheric cusp region. Ions injected at the magnetopause are subject to a time‐of‐flight effect, which results in distinctive ion‐energy dispersions observed in the cusp. The shape of these ion dispersions is analyzed and linked with predictions of the magnetopause reconnection location from the maximum magnetic shear model to determine the plasma entry points at the magnetopause. This study determines how those plasma entry locations affect the observed cusp ion dispersion profiles. This study uses first observations from the Analyzers for Cusp Ions (ACIs) onboard the Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites spacecraft.

The ion foreshock is the region upstream of Earth's bow shock where magnetic field lines connect to the quasi‐parallel shock surface. Within this region, there exist a variety of backstreaming ion populations from the shock ramp that can generate ultra‐low frequency (ULF) waves through wave‐particle interactions. In this work, we use data from the Magnetospheric Multiscale (MMS) mission to study such ULF waves and backstreaming ions during a prolonged interval of above‐average solar wind helium abundance, embedded in a multi‐day period of strong solar activity driven by a coronal mass ejection. When interplanetary magnetic field orientations positioned MMS within the ion foreshock, the spacecraft captured the complete evolutionary sequence of the backstreaming ion velocity distributions: the initial formation of a reflected ion beam, followed by phase bunching and generation of coherent ULF waves, and eventual thermalization and randomization in velocity space to form diffuse ions. Intervals with elevated energetic He++ flux exhibited broadened and frequency‐downshifted wave spectra, consistent with heavy‐ion cyclotron resonance effects. The unusually rapid beam‐to‐diffuse transitions observed near the foreshock boundary likely result from the combined effects of multi‐species wave‐particle interactions and higher backstreaming ion densities during this active interval. These findings underscore the need for simulations and modeling that incorporate multi‐species effects.

On 10 May 2024, Earth was hit by a CME that drove the largest geomagnetic storm in 20 years. Multi‐spacecraft observations previously showed that the ∼100 nT north‐south IMF variation was driven by Kelvin‐Helmholtz waves with wavelength ∼250 and reconnection jets in the ‐direction (Nykyri, 2024a, https://doi.org/10.1029/2024gl110477 ). Here we show that Magnetospheric Multiscale (MMS), located upstream of the Earth's bow shock at , detected additional reconnection jets not seen by ARTEMIS at lunar orbit ∼60 away, perpendicular to CME propagation. One jet coincided with high‐energy ions and electrons streaming anti‐parallel to the magnetic field. The ions showed dispersion followed by apparent counter‐dispersion, whereas electrons above 100 keV arrived nearly simultaneously. Strong electromagnetic wave activity between these phases is consistent with magnetosonic‐Whistler waves, suggesting that MMS traversed very near the particle acceleration region.

We present observations from two consecutive TRACERS‐2 orbits through the northern low‐altitude cusp. During the first crossing, TRACERS‐2 observed reversed cusp ion dispersion and sunward convection, consistent with magnetopause reconnection tailward of the cusp during this northward IMF interval. Simultaneous THEMIS‐D observations at the equatorial magnetopause show heated magnetosheath plasma captured on closed field lines, with similar particle spectra as in the low‐altitude cusp, indicating that reconnection indeed occurred tailward of the cusp and in both hemispheres. When TRACERS‐2 traversed the northern cusp again, 95 min later, the IMF was dominated by a negative B _X component. Despite the different IMF conditions, TRACERS‐2 recorded nearly the same cusp signatures as before, that is, reversed ion dispersion and sunward convection. The observations indicate that tailward‐of‐cusp reconnection can occur for both northward and B _X ‐dominated IMF and that these distinct IMF geometries can produce remarkably similar plasma and field signatures in the low‐altitude cusp.

Magnetic reconnection is a process that occurs at the boundary between the Earth's magnetic field and the interplanetary magnetic field (IMF) called the magnetopause. Magnetic reconnection allows solar wind plasma to enter Geospace and precipitate along the newly opened geomagnetic field lines into the cusp regions. To fully understand the interaction due to magnetic reconnection between these two disparate plasma regimes, it is important to understand the location along the magnetopause where reconnection occurs. The location of the dayside X‐line at Earth's magnetopause has been well described with the maximum magnetic shear model, which correctly predicts the location within 2 R _E (Petrinec et al., 2014, https://doi.org/10.1002/2014ja020209) for 82% (e.g., Trattner, Burch, et al., 2017, https://doi.org/10.1002/2017ja024488). Events where the model fails are not randomly distributed throughout the parameter space, rather they cluster in clearly defined parameter spaces. One of these event groups are so called “Knee” events, wherein one of the two anti‐parallel reconnection regions along the dayside magnetopause includes a sharp bend (kink or “knee”) such that this region maps through both the northern and southern hemispheres. This scenario causes a deflection of the entire dayside X‐line, which is controlled by the motion of the anchor point (the transition between the component and the anti‐parallel sections along the X‐lines) to where the last draped IMF line crosses the anti‐parallel reconnection region.

The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) mission observes electron energy‐latitude dispersion at the equatorward edge of the magnetospheric cusp, and high‐cadence Analyzer for Cusp Electrons (ACE) measurements resolve the dispersed edge. The inverse velocity dispersion (low energy before high energy) encountered by TRACERS as it travels southward through the northern cusp rules out pure energy‐time dispersion from nearby injections or Alfvén wave‐driven acceleration. TRACERS observes electron dispersion at the equatorward edge of the northern cusp roughly half of the time for southward interplanetary magnetic field (IMF), and almost never for northward IMF. The TRACERS measurements therefore provide strong observational support for the hypothesis that the observed electron dispersion results from dayside magnetic reconnection and plasma convection, much like the ion dispersion that commonly extends across the cusp. Observations of multiple electron dispersions and electron steps suggest fine‐scale spatial and/or temporal variability in magnetic reconnection.