Carl Schmidt

Europa's sodium cloud is thought to be modified by its orbital longitude and by its plasma environment, which modulates with Jovian magnetic geometry. To explore and quantify sodium cloud variability, seven sets of Keck I/HIRES measurements are compared providing comprehensive coverage of orbital and magnetic geometry. The neutral cloud brightness and structure is more stable than past models and measurements indicate, exhibiting little correlation with orbital or magnetic longitude. The sodium cloud's apparent indifference to plasma sheet location may simply be due to smearing, since bright surface reflectance limits measurements to altitudes of several Europa radii and higher, where ballistic transport timescales are comparable to Jupiter's rotation period. The exosphere is more elongated and variable in the dawn‐dusk dimension compared to north‐south. To constrain sodium gas closer to Europa's surface, we searched for electron‐excited emission when Europa is in Jupiter's shadow and bright solar reflectance is absent. Faint emissions observed at 16 ± 11 Rayleighs are still much brighter than expected from electron impact excitation of the column extrapolated from high‐altitude sunlit spectra. This supports the view that Europa hosts a gravitationally bound sodium population close to the surface. Models of Europa's sodium cloud have broad differences in altitude structure depending on interparticle collision rates and it is unclear whether this structure is due to the velocity distribution function of sodium atoms released from the surface ice, collisional damping of hot Na with cold dense O ₂ , or a confluence of both.

Detections of auroral emission from Callisto have been difficult due to the low density of electrons that excite the aurora at its large orbital distance from Jupiter. However, large ground-based telescopes with sensitive spectrographs can detect the faint emissions at optical wavelengths when Callisto is eclipsed by Jupiter. In this study, we analyzed two sets of optical wavelength spectra observed with Keck I/HIRES. We detected the 630.0 nm [O i ] emission line in individual 5 minute integrations and found its disk-averaged brightness decreased with distance from the centrifugal equator of Jupiter’s plasma sheet. The average brightness matched simulations of emission from an atmosphere composed of O, O ₂ , H ₂ O, and CO ₂ using the most up-to-date estimates or upper limits for their individual column densities, supporting the prevailing paradigm that Callisto’s atmosphere may be the densest of the icy Galilean satellites and is composed primarily of O ₂ . We also modeled the emission from the same atmosphere if an ionosphere inhibited excitation by the thermal electron population but found the resulting brightness was an order of magnitude below observations.

JWST imagery of the approaching Kreutz sungrazer C/2026 A1 (MAPS) appears sufficient to distinguish the comet’s nucleus from the surrounding dust. Coma model fitting indicates the morphology is compatible with a ∼0.4 km diameter nucleus—likely larger than those of the minor Kreutz fragments routinely discovered with coronagraphs, and comparable to or slightly smaller than that of C/2011 W3 (Lovejoy). However, since C/2026 A1 is the only Kreutz sungrazer whose nucleus has been directly observed, these comparisons should be treated with caution.

We present the main Alfvén wing (MAW) spots of Io and Europa as observed by the Near‐Infrared Spectrograph onboard the James Webb Space Telescope. These auroral footprint features have been measured previously, but only in emission. Here, the derived ionospheric emission, temperature and column density are reported, as well as spectral radiance. At the Io footprint, temperatures are 670–900 K, excluding a spatially confined cold structure (538 17 K), localized to the MAW spot, with high densities (0.98 0.43 × ). There are suggestions of a similar, less extreme population associated with the Europa footprint. However, temperatures at Io's MAW spot show significant variability within different observational exposures, indicating precipitation energy changes, sampling various regions of the ionosphere's altitudinal temperature profile. This work provides a new window into understanding the auroral processes driven by moon‐magnetosphere interactions in the Jovian system.

Europa’s auroras result from interactions between its tenuous atmosphere and the exogenous electron population trapped within Jupiter’s magnetosphere. In this study we analyzed spectra of Europa’s optical wavelength aurora taken during eclipse by Jupiter over 10 nights between 2021 and 2025 using Keck I/HIRES. We found that the bright 630.0 nm [O i ] auroral emission correlated strongly with the expected change in ambient magnetospheric electron density owing to Europa’s changing distance from the centrifugal equator of Jupiter’s magnetic field, suggesting that the optical aurora brightnesses can be used as a proxy for measuring electron density variations when assuming a stable atmosphere composition and density. We derived a typical azimuthal heterogeneity in electron densities with System iii longitude of about ±11% and a long-term secular variability in the azimuthal average of about ±20% over the five years of observations, consistent with in situ measurements of plasma density at Europa’s orbit during the Galileo mission. Finally, we determined that any local sublimated water atmosphere present during daytime must collapse to below 2 × 10 ¹⁸ m ⁻² within the first few minutes of eclipse ingress.

Planetary upper atmospheres couple the deep atmosphere to the space environment. The dynamics and energetics of this rarefied, partially ionized region govern atmospheric evolution. At Jupiter, decades of past plasma measurements have revealed a variable and enigmatic ionosphere inconsistent with photochemical predictions and unusual global structures imprinted by the planet’s powerful magnetic field. Upper-atmospheric temperatures have been measured sporadically and thus are unable to fully characterize the energy transfer mechanism responsible for its unexpectedly hot thermosphere. Observations to date have been too limited spatially, or too insensitive, to uncover the driving mechanisms behind the strong variability and magnetically organized features found in Jupiter’s upper atmosphere. Here, we present high spatial resolution global maps of ion densities and temperatures constructed from >175,000 Keck/NIRSPEC spectra collected over 4 yr. Irregular ionospheric emission features, first seen more than 25 yr ago, are shown to be persistent and due to local ion density modifications. Global temperatures decrease steadily from auroral to equatorial latitudes and are remarkably stable, with equatorial deviations of <10% night to night. Thus, despite appearing stochastic in previous observations, Jupiter’s upper atmosphere exhibits predominantly spatial rather than temporal variability, yielding a steady global structure generated by local plasma dynamics. These results illustrate how neutral winds and magnetic fields can create globally persistent plasma structures on hydrogen-dominated worlds and provide an explanation for decades of puzzling Jovian upper-atmospheric observations.

Observations of continuous vertical electron and ion distributions at Jupiter were obtained simultaneously for the first time on 2023 September 7 UTC. On that date, Juno performed a radio occultation experiment and, in parallel, JWST and Keck scanned Jupiter’s limb, focused on characterizing H 3 + . Here, using Keck infrared spectra, we derive constraints on H 3 + densities and temperatures. While on-disk column densities are roughly constant with latitude, with a mean value of (5.7 ± 2.1) × 10 ¹⁵ m ⁻² , temperatures decrease smoothly from 774 ± 50 K at high latitude to 683 ± 28 K at low latitude. Vertical H 3 + temperatures are comparable to their on-disk values and mostly isothermal. Keck spectra sampled Jupiter’s dawn limb, finding H 3 + volumetric number densities of (1–40) × 10 ⁸ m ⁻³ with peak altitudes near 1500 km. Comparison with the Juno electron altitude density profile confirms Jupiter’s ionosphere is proton dominated. Ionospheric model simulations that can broadly replicate observed plasma densities indicate that field-aligned vertical transport driven by neutral winds is primarily responsible for the observed high-altitude peak. We use model outputs, validated by comparison with plasma observations, to demonstrate that H 3 + is in photochemical equilibrium (PCE) below ∼1600 km altitude in Jupiter’s ionosphere. PCE analysis further reveals that the diurnal peak in N H 3 + can occur before noon, that peak electron densities should generally be 11 m ⁻³ , and that rapidly varying auroral H 3 + emissions can be explained by narrow ( 3 + just above the homopause produced by energetic particle precipitation.

We observed Io with the James Webb Space Telescope (JWST) NIRSpec/Integral Field Unit (1.0–5.3 μm, ) in August 2023 while the satellite was in eclipse. Thermal emission from Kanehekili Fluctus is consistent with the cooling of lava flows after a vigorous eruption in November 2022. At Loki Patera, after a new brightening event was detected in November 2022, the lava lake was in a quiescent state, as expected from previous analyses. We mapped the SO emission band at 1.707 μm, and detected, for the first time, [SI] emission lines at 1.082 and 1.131 μm. The SO emissions are concentrated above Kanehekili Fluctus, and in two regions in the northern hemisphere. The disk‐averaged brightness is 14.5 kR. The emissions are sourced from SO molecules ejected from 1,500 to 1,700 K vents in an excited state, with a typical SO column density above the northern hemisphere of . Sulfur emissions are distributed homogeneously across a band in the northern hemisphere. The disk‐averaged total brightness is 5.6 kR, versus 9.65 kR in the north. The emissions are produced through direct electron impact excitation by 4 eV electrons in the torus (density 2,500 ) penetrating the atmosphere, and require the atmosphere to be hot (1,700 K) to populate the upper levels before excitation. The sulfur column density over the northern hemisphere is . These same parameters can explain recent 0.7725‐m observations, as well as the 147.9‐nm multiplet emissions observed with HST‐STIS 20 years earlier. This suggests a quite stable system over decades‐long timescales.

We observed Io’s optical aurora in eclipse on six nights between 2022 and 2024 using Keck I/HIRES. Spectra revealed 13 new auroral emissions not identified previously, tripling the total number of optical emissions lines detected at Io. These included the O i lines at 777.4 and 844.6 nm, the Na i lines at 818.3 and 819.5 nm, the [S i] lines at 458.9 and 772.5 nm, the S i triplet at 922.3 nm, the [O ii] lines at 732.0 and 733.0 nm, and the [S ii] lines at 406.9, 407.6, 671.6, and 673.1 nm. We leveraged these new detections by comparing with imaging data from the 2001 Cassini flyby to better understand the distribution of atmospheric species and their contribution to the observed auroral brightnesses. Our auroral emission model showed that the observed 557.7, 777.4, and 844.6 nm oxygen emission-line brightnesses could be explained by excitation by electron impact of canonical 5 eV torus electrons on an atmosphere composed of O, SO₂, and an isoelectronic proxy for SO. The SO₂ emission did not decrease immediately after eclipse ingress, suggesting that the emitting column may be restricted to higher altitudes. The derived O/SO₂ mixing ratio was typically about 10%, but it also exhibited order-of-magnitude variance during some observations. Io’s 630.0 nm [O i] brightness did not strongly vary with plasma sheet distance, suggesting that electron flux at Io varies substantially beyond model predictions.

Comets are among the most primitive bodies in the Solar System, preserving primordial materials from the early formation of the Solar System. Among various species observed in cometary comae, sodium is relatively lesser understood due to the high environmental temperature required for its sublimation, which restricts the observable samples to near-Sun comets. Here we present an analysis of five comets using a combination of narrowband imaging and high-resolution spectroscopy. Coronagraph ImagingIn early July 2020, comet C/2020 F3 (NEOWISE) approached the Sun for the first time in nearly 4,500 years. Comet NEOWISE was observed by the Planetary Science Institute’s Io Input/Output Facility (IoIO; Morgenthaler et al., 2019) 35-cm coronagraph from July 7-16. Narrowband filters were used to isolate the dust and sodium gas emissions in the comae, and this apparition at only 0.33 AU from the Sun allowed some of the best quality images of a cometary sodium tail taken to date, as seen in Figure 1. For inner solar system comets, sodium is often the brightest emission line available to ground-based telescopes thanks to its efficient resonant scattering of sunlight near the maximum of the solar irradiance spectrum. Strong radiation pressure from resonant scattering shapes sodium gas into a vast anti-sunward tail.Figure 1: Narrowband filtered images of continuum dust reflectance and sodium D line emission in C/2020 F3 (NEOWISE). At 86° phase angle, the sodium tail here appears nearly orthogonal to the line of sight.Structure in cometary sodium emissions is shaped by the collisional entrainment in the bulk outflow from the nucleus, additional dusty sources, the temperature of the atoms, absorption of intervening sunlight in the dense coma regions, and radiation pressure. At a certain radius from the nucleus, Na atoms collisionally decouple from the bulk outflow velocity with a thermal distribution. While it is challenging to uniquely parameterize the collisional radius, outflow velocity, and gas temperature, additional insight can be gained from emission line profiles at ample spectral resolutions of R >100,000 thanks to Doppler broadening. Resolved Linewidth MeasurementsDuring the IoIO imaging campaign, C/2020 F3 (NEOWISE) was also observed with the Lowell Discovery Telescope’s Extreme Precision Spectrometer (EXPRES; Jurgenson et al., 2016), at a resolving power of R = 150,000. At a phase angle of 108° and heliocentric distance of 0.46 AU, forward modeling of the line profiles that accounts for hyperfine structure retrieves effective temperatures ranging 1750 to 1950 K. The RMS velocity of this Doppler broadening is equivalent to 1.13 to 1.19 km/s, which suggests that collisional coupling to the bulk water outflow velocity is the dominant component in the emission line shape.Figure 2: Outflow velocity at five comets with varying heliocentric distances as derived from resolved Na D emissions. Comets near 90° phase angle roughly follow the r-0.5 relationship outlined by Budzien et al. (1994). C/2023 A3 was measured at both ~0.4 and 0.85 AU and shows variation consistent with this trend. Comets with phase angle >20° from quadrature are shown as outlines.The water outflow velocity in comets is well known to vary with heliocentric distance. The often-cited relationship of vH2O = 0.85r-0.5 applied by Budzien et al. (1994) is a compromise between hydrodynamical models, which suggest 0.7r-0.5 (Gombosi et al., 1986), and radio observations of OH and HCN at 1P/Halley, which suggest 1.1r-0.5 (Combi, 1989). By comparing Na linewidths in a survey of comets at varying heliocentric distance, we find that the thermal velocity derived from Na linewidths produces a similar relationship. Figure 2 shows linewidth-derived outflow velocity for five comets at a range of heliocentric distances. All spectra were obtained with EXPRES, with the exception of the C/2023 A3 data point at 0.85 AU, which was obtained using the Large Binocular Telescope’s Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI; Strassmeier et al., 2015). Na D2 speeds appear systematically higher, implying that the line core may exceed unity optical depth. Comets 96P, 2P, and C/2023 A3 were observed at phase angles >20° from quadrature, which caused distortion of the line shape due to projection effects along the tail, resulting in outflow speeds higher than the overall trend. Monte-Carlo ModelingFigure 3: Preliminary results of Monte-Carlo modeling sodium gas in the coma of C/2020 F3 (NEOWISE), assuming here a collisional radius of 10,000 km, bulk outflow speed of 1.5 km/s and gas temperature of 150 K.Phase angle distortion of the emission line profiles can be accounted for with Monte-Carlo modeling, and with nearly concurrent imaging and spectroscopy in the case of C/2020 F3 (NEOWISE), this approach has good potential to uniquely quantify the parameters that control sodium dynamics in its coma. We adapt the Monte-Carlo model of C/2012 S1 (ISON) by Schmidt et al. (2015), and this presentation will describe attempts to self-consistently simulate both spatial structure in the tail and Doppler broadening near the optocenter, using C/2020 F3 (NEOWISE) as a case study. A preliminary model run is shown in Figure 3, though it does not yet include emission from within the collisional radius and emission from dusty sources, which will broaden the tail. Work thus far suggests that spectrally resolved sodium emissions could offer a proxy for the water outflow velocity, enabling a new method at optical wavelengths.