Rohini S. Giles

Since 2001, we have used the TEXES high resolution mid-infrared spectrograph at NASA’s Infrared Telescope Facility to study the density of Io’s SO ₂ -dominated atmosphere. The spectra cover several SO ₂ absorption lines near 530 cm ⁻¹ and are used to retrieve the equatorial SO ₂ column density. In addition to covering a long time period, our 191 independent observations also cover the full range of Io central longitudes, allowing us to study the longitudinal variability of Io’s dayside atmosphere in more depth than has previously been possible. By improving our modeling of longitudinal variability and using a seasonal model to apply a temporal normalization to the retrieved SO ₂ densities, we produce two complete longitudinal distributions of Io’s equatorial SO ₂ column abundances, one representing perihelion and one representing aphelion. Both show the same hemispheric asymmetry that has been observed in previous studies, with the highest densities near 200 ^∘ W and the lowest densities near 305 ^∘ W. We find that the contrast between the maximum and minimum densities is a factor of ∼27 at aphelion and a factor of ∼16 at perihelion. Both longitudinal distributions are asymmetric, with a sharper drop-off on Io’s trailing hemisphere than on the leading. We compare these longitudinal distributions to the distributions expected if the spatial variability were primarily driven by volcanic activity, SO ₂ -ice inhomogeneity or the effects of eclipses. We conclude that the effect of eclipses is important but insufficient to explain the shape of the observed longitudinal variability, and frost distribution is likely to also be important.

In October 2024, the Juno spacecraft observed an extreme compression of Jupiter's magnetosphere. This event pushed the magnetosheath into an unprecedented distance of only 30 Jupiter radii from Jupiter. This suggests an enhanced Solar Wind (SW) dynamic pressure (DynP), that was likely produced from a coronal mass ejection after an X1.8 class solar flare. The compression brought the magnetosheath, observed at a high southern latitude, to the closest distance ever observed from Jupiter. Using data from multiple instruments on Juno to identify magnetosheath signatures, it is shown that Juno had many encounters with the magnetosheath as it moved inward from apojove to perijove. This included many short‐lived encounters that were not readily identifiable from a single data set. From model estimates, a SW DynP of around 1.2 is suggested for this event, consistent with previous upper estimates observed in the vicinity of Jupiter's orbital path. Remote imaging suggests the magnetosphere remained compressed for several days after this event. The event raises the possibility that Jupiter's magnetosphere on the dayside could potentially shrink inside of Callisto's orbital path, and that if Callisto happened to be on the dayside of Jupiter at the time (although it was on the nightside during this event), it could find itself outside of the jovian magnetosphere and exposed directly to the SW.

Aims. We analyzed ultraviolet observations of Ganymede recorded on June 7, 2021, by the Juno spacecraft, which observed the aurora at high spatial resolution in order to detect the presence of auroral sub-structures. The emission mainly comes from two oxygen lines at 130.4 and 135.6 nm, which are excited under electron precipitation. Methods. We produced longitude–latitude projections of the oxygen emission lines from Ganymede’s atmosphere to investigate the presence of small-scale auroral features. We adopted a magnetic field model of Ganymede’s magnetosphere to map between the moon surface and the surrounding space, in order to determine the regions magnetically connected to the aurora. Results. We find auroral patches on the leading, downstream side of Ganymede. Their typical size is ∼50 km, and they have a brightness of up to ∼200 Rayleigh. These features map approximately to the downstream reconnection region and seemingly resemble the terrestrial and Jovian mesoscale auroras associated with sub-storms and dawn storms, respectively. This could indicate that magnetospheres of celestial bodies host similar physical process(es) despite their different conditions and dynamics.

Context . The interaction between Io and the Jovian magnetosphere produces the auroral “Io footprint”, which can be observed in the ultraviolet thanks to the deexcitation of atmospheric H and H ₂ under electron precipitation. Since 2016, Jupiter has been explored with Juno’s Ultraviolet Spectrograph, which monitors the emission from 68 to 210 nm. Aims . We used Juno observations of the Io footprint near the planetary limb to determine its vertical ultraviolet emission profile in the northern hemisphere. We simulated emission as a function of the altitude and the mean energy of the precipitating electrons, and we used the results to determine the energy spectrum associated with the footprint profile. Methods . We estimated the source location of the Io footprint UV emission, and we extracted its vertical profile. We analyzed the variability of the emission altitude, and we derived the corresponding energy spectrum of precipitating electrons using TransPlanet. The results were compared with the in situ measurements from Juno’s particle detectors. Results . The main spot emission peaks around ~500 km, with a variability of ~200 km correlated with longitude. The emission of the footprint tail within 20° from the main spot peaks around ~500±300 km, and it is moderately correlated with the magnitude of the magnetic field near Jupiter. The trans-hemispheric electron beam spot is located at ~300 km, and the associated precipitation appears depleted at low energy. The retrieved energy spectrum of the precipitation shows remarkable agreement with the particle measurements. Conclusions . The longitudinal modulation of the Io footprint altitude suggests that the morphology of the torus affects the transmission of the Alfvén waves responsible for the acceleration of auroral electrons. The dependency of the footprint tail altitude on the magnetic field strength indicates that the magnetic field also plays a role in the acceleration mechanism.

Context . Ganymede’s UV aurorae, observed by HST and Juno/UVS, trace interactions between its atmosphere and Jupiter’s magneto-sphere. These emissions, dominated by O I lines at 130.4 and 135.6 nm, are driven by electron impact on species such as H ₂ O, O, and O ₂ , and yet the properties of the precipitating electrons remain poorly constrained. Aims . Our aim was to retrieve the energy and flux of precipitating electrons using UV observations from Juno/UVS during PJ34 and to assess the dominant atmospheric species producing the observed emissions. Methods . Using the TransPlanet electron transport model and a non-local thermodynamic equilibrium (non-LTE) radiative transfer module, we simulated O I emissions for 17 auroral subregions, testing both monoenergetic and kappa-type electron distributions. The I (135.6 nm)/ I (130.4 nm) line ratio was used as a diagnostic, with values varying by target species. Results . Monoenergetic distributions fit most regions better, with mean energies of 17–300 eV and fluxes up to 2 mW m ⁻² . Kappa and Maxwellian distributions yielded higher fluxes, but poorer spectral fits. Poor fits in some regions reflect low S/N or non-ideal electron populations. Conclusions . Our results suggest that Ganymede’s UV aurorae are mainly driven by low- to intermediate-energy electrons. Upcoming high-resolution observations and in situ data from Juice and Europa Clipper will be key to refining these diagnostics.

Context . Sulfur dioxide and water are two key minor species of Venus’ atmosphere which drive its chemical evolution. However, the long-term variations in the SO ₂ abundance at the cloud top, measured since 1978, remain unexplained. Aims . In order to address this question, since 2012, we have performed a ground-based campaign to monitor the SO ₂ and H ₂ O abundances in the region of the Venus upper cloud, using the TEXES (Texas Echelon Cross-Echelle Spectrograph) imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). Methods . Observations were recorded in three spectral ranges: (1) the 1342–1348 cm ⁻¹ (7.4 µm) spectral range, where SO ₂ , CO ₂ and HDO (used as a proxy for H ₂ O) transitions are observed at an altitude of about 62 km, defined in our model as the cloud top; (2) the 529–530 cm ⁻¹ range (18.9 µm), where SO ₂ and CO ₂ are probed within the clouds a few kilometers below the cloud top; (3) the 1160–1165 cm ⁻¹ range (8.6 µm) where the weak SO ₂ v ₁ band is used to probe a few kilometers above the cloud top. Results . We present here the data recorded from July 2023 to February 2025. As was reported in our previous analyses, the SO ₂ maps show evidence for the formation of SO ₂ plumes, mostly located around the equator, with a typical lifetime of a few hours; large variations in the SO ₂ disk-integrated abundance also appear on a timescale of a few months. In contrast, the H ₂ O abundance is remarkably uniform over the disk and shows moderate variations as a function of time. The present dataset shows for the first time the detection of the SO ₂ v ₁ band above the cloud top. The simultaneous analysis of the three SO ₂ bands allows us to constrain the SO ₂ volume mixing ratio within and above the cloud. In addition, we have re-analyzed the long-term evolution of the SO ₂ and H ₂ O mixing ratios at the cloud top. Between 2012 and 2023, the SO ₂ abundance is clearly anti-correlated with the solar activity, which suggests that photochemistry, activated by the solar UV flux, is the main mechanism driving the SO ₂ abundance. After 2023, SO ₂ exhibits strong variations with a timescale as short as two months, which suggest that other mechanisms are involved. Finally, we have re-analyzed the distribution of SO ₂ at the cloud top as a function of the local time, using three different data subsets (2012–2017, 2021–2022, 2023–2025). In spite of the differences, there is a general trend toward a minimum value around 12:00 and maxima around 5:00 and 17:00. Conclusions . Our data suggest that the solar cycle plays an important role in the long-term evolution of SO ₂ and H ₂ O at the cloud top. In addition, in some cases, other mechanisms are probably also involved, possibly associated with dynamical motions, implying sublimation/condensation processes and/or gas-aerosol conversion.

The highly elliptical polar orbit of the Juno mission provides a unique opportunity to simultaneously measure the compression state of Jupiter's magnetosphere and the total power emitted by the planet's ultraviolet aurora, using a single spacecraft. This allows us to study how Jupiter's aurora respond to a compression event. In this paper, we present a case study of an extreme compression event that occurred on December 6–7 2022 when Juno was a distance of 70 R_J from Jupiter. This extreme compression was accompanied by a very large increase in the ultraviolet auroral emissions to 12 TW, a factor of six higher than the baseline level. This event coincided with the predicted arrival of a powerful interplanetary shock, which was expected to cause the largest increase in the solar wind dynamic pressure seen thus far during the Juno mission. The simultaneous occurrence of the interplanetary shock, the extreme compression and the bright ultraviolet aurora suggests that in this case, the auroral brightening was caused by the solar wind shock compressing the magnetosphere.

The highly elliptical polar orbit of the Juno mission provides a unique opportunity to simultaneously measure the compression state of Jupiter's magnetosphere and the total power emitted by the planet's ultraviolet aurora, using a single spacecraft. This allows us to study how Jupiter's aurora respond to a compression event. In this paper, we present a case study of an extreme compression event that occurred on December 6-7 2022 when Juno was a distance of 70 R$_{J}$ from Jupiter. This extreme compression was accompanied by a very large increase in the ultraviolet auroral emissions to 12 TW, a factor of six higher than the baseline level. This event coincided with the predicted arrival of a powerful interplanetary shock, which was expected to cause the largest increase in the solar wind dynamic pressure seen thus far during the Juno mission. The simultaneous occurrence of the interplanetary shock, the extreme compression and the bright ultraviolet aurora suggests that in this case, the auroral brightening was caused by the solar wind shock compressing the magnetosphere.

We present radiative transfer analyses of IRTF-TEXES and SOFIA-EXES mid-infrared spectra of Jupiter's mid- to high latitudes recorded between 2019 April 16 and 2023 July 20. The spectra were inverted across a photochemical model grid of varying eddy diffusion coefficient profiles, and the quality of fit of the synthetic spectra to the observed was used to constrain the CH₄ homopause level. For a subset of latitudes/dates, we find that the CH₄ homopause level is elevated in the region enclosed inside of, or magnetospherically poleward of, the northern ultraviolet main auroral emissions (MAEs) in comparison to the region outside or equatorward of the MAE. For example, using SOFIA-EXES results on 2021 June 10, we derived a CH₄ homopause level of log(p _H(nbar)) = 1.54 − 0.69 + 0.51 or z _H = 453 − 76 + 128 km above 1 bar poleward of the northern MAE at 68^∘N compared to a lower limit of log(p _H) > 2.43 and upper limit of z _H < 322 km derived equatorward of the northern MAE. We therefore conclude that the region poleward of the northern MAE is, at times, subject to enhanced vertical transport resulting from auroral energy deposition. The exact mechanisms responsible for the enhanced vertical transport in Jupiter's auroral regions are uncertain: time-dependent circulation modeling of Jupiter's polar atmosphere is required to better understand this phenomenon. Poleward of the southern MAE, derived homopause levels agreed within uncertainty with those at equatorward locations. However, we consider this result a spatial sampling artifact rather than concluding that the southern auroral region is not subject to enhanced vertical transport.