While photoionization rates of Ganymede's dominated atmosphere are well constrained, the contribution of electron-impact ionization is rather uncertain. Previous quantitative estimates have relied on assumptions about densities and energy distributions of precipitating electrons, or on rare spacecraft measurements that cannot be unambiguously mapped to the regions of ionization. In this study, we present a novel approach to quantify electron-impact ionization rates directly through OI 1356 & Aring; emission brightness observations. The analysis of measured cross sections reveals that the ionization-to-excitation ratio is limited to 10-60 over all electron energies, reducing the uncertainty of estimating ionization rates to a factor less than 6. We apply this method to Juno UVS observations of Ganymede's aurora. We find that the OI 1356 & Aring; brightness of the auroral ovals is well described by 3-5 degrees latitude wide Gaussian distributions centered on the open-closed field line-boundary, with an average peak of 120 R. The average brightness outside the ovals in the polar and equatorial background regions is similar to 8 R. From these observations, we derive a global map of electron-impact ionization rates, which are at least an order of magnitude higher than photoionization rates. The estimated total global ionization rate is , with average column rates of c in the ovals and c in the background regions. Comparison of radio occultation measurements with predicted electron densities indicates that transport processes are the dominant loss mechanism in Ganymede's ionosphere. The rate of ionospheric outflow of is or , indicating cm My erosion of Ganymede's surface ice.

We analysed far-ultraviolet (FUV) spectra of Uranus obtained by the HST STIS and COS instruments in 2012 and 2014, respectively, to determine the brightness of Raman-scattered Lyman-alpha (Ly α ) emissions centred at 1280 Å (hereafter, the Raman feature). The Raman feature is unique among the Solar System’s giant planets and forms in Uranus’ atmosphere due to weak vertical mixing of hydrocarbons with H 2 , leading to efficient Rayleigh–Raman scattering. Methane is the dominant hydrocarbon species on Uranus, and since it absorbs FUV radiation, it affects the Rayleigh–Raman scattering of Ly α photons by H 2 and, eventually, the brightness of the Raman feature. We derive a brightness of 20 −6 +1 R from the STIS data, which is similar to the brightness measured by Voyager 2 UVS during the 1986 flyby of Uranus, when considering the suggested recalibration of UVS measurements by a factor of ∼0.5. Based on the observed brightness, we constrain the upper altitude (pressure) level for the abundance of methane in the upper atmosphere using radiative transfer simulations that include resonant scattering by H, Rayleigh–Raman scattering by H 2 , and absorption by CH 4 . We considered the solar Ly α flux as the source of Ly α radiation at Uranus. We find that resonant scattering by H significantly affects Rayleigh–Raman scattering by H 2 and thus the modelled brightness of the Raman feature. We derive methane profiles by obtaining the simultaneous fit to the observed Ly α , as well as the 1280 Å brightness of Uranus. Methane appears to be depleted (number density becomes less than 1 cm −3 ) above the altitude (pressure) range of ∼478–515 km (4 × 10 −3 –2.4 × 10 −3 mbar), while the Ly α absorption optical depth reaches unity for methane in the altitude (pressure) range of ∼237–257 km (2.54 × 10 −1 –1.65 × 10 −1 mbar). When neglecting resonant scattering by H, the methane depletion must be deeper in the atmosphere at an altitude (pressure) of ∼395 km (1.4 × 10 −2 mbar), similar to previous findings based on Voyager 2 observations of the feature. The analysis of the Raman feature provides independent CH 4 constraints in the upper atmosphere for detailed photochemistry modelling and highlights the importance of UV instruments for the future Uranus Orbiter and Probe (UOP) mission.

Since the Voyager mission flybys in 1979, we have known the moon Io to be both volcanically active and the main source of plasma in the vast magnetosphere of Jupiter. Material lost from Io forms neutral clouds, the Io plasma torus and ultimately the extended plasma sheet. This material is supplied from Io’s upper atmosphere and atmospheric loss is likely driven by plasma-interaction effects with possible contributions from thermal escape and photochemistry-driven escape. Direct volcanic escape is negligible. The supply of material to maintain the plasma torus has been estimated from various methods at roughly one ton per second. Most of the time the magnetospheric plasma environment of Io is stable on timescales from days to months. Similarly, Io’s atmosphere was found to have a stable average density on the dayside, although it exhibits lateral (longitudinal and latitudinal) and temporal (both diurnal and seasonal) variations. There is a potential positive feedback in the Io torus supply: collisions of torus plasma with atmospheric neutrals are probably a significant loss process, which increases with torus density. The stability of the torus environment may be maintained by limiting mechanisms of either torus supply from Io or the loss from the torus by centrifugal interchange in the middle magnetosphere. Various observations suggest that occasionally (roughly 1 to 2 detections per decade) the plasma torus undergoes major transient changes over a period of several weeks, apparently overcoming possible stabilizing mechanisms. Such events (as well as more frequent minor changes) are commonly explained by some kind of change in volcanic activity that triggers a chain of reactions which modify the plasma torus state via a net change in supply of new mass. However, it remains unknown what kind of volcanic event (if any) can trigger events in torus and magnetosphere, whether Io’s atmosphere undergoes a general change before or during such events, and what processes could enable such a change in the otherwise stable torus. Alternative explanations, which are not invoking volcanic activity, have not been put forward. We review the current knowledge on Io’s volcanic activity, atmosphere, and the magnetospheric neutral and plasma environment and their roles in mass transfer from Io to the plasma torus and magnetosphere. We provide an overview of the recorded events of transient changes in the torus, address several contradictions and inconsistencies, and point out gaps in our current understanding. Lastly, we provide a list of relevant terms and their definitions.

Jupiter’s moon Europa has a predominantly water-ice surface that is modified by exposure to its space environment. Charged particles break molecular bonds in surface ice, thus dissociating the water to ultimately produce H 2 and O 2 , which provides a potential oxygenation mechanism for Europa’s subsurface ocean. These species are understood to form Europa’s primary atmospheric constituents. Although remote observations provide important global constraints on Europa’s atmosphere, the molecular O 2 abundance has been inferred from atomic O emissions. Europa’s atmospheric composition had never been directly sampled and model-derived oxygen production estimates ranged over several orders of magnitude. Here, we report direct observations of H 2 + and O 2 + pickup ions from the dissociation of Europa’s water-ice surface and confirm these species are primary atmospheric constituents. In contrast to expectations, we find the H 2 neutral atmosphere is dominated by a non-thermal, escaping population. We find 12 ± 6 kg s −1 (2.2 ± 1.2 × 10 26 s −1 ) O 2 are produced within Europa’s surface, less than previously thought, with a narrower range to support habitability in Europa’s ocean. This process is found to be Europa’s dominant exogenic surface erosion mechanism over meteoroid bombardment.

Abstract Juno performed two close flybys of Io and found enhanced field‐aligned proton fluxes are absorbed by Io. These protons are absorbed at mass input rates comparable to previous estimates for hydrogen losses from Io, hence Jupiter is likely the source of hydrogen at Io. The conditions necessary for this to occur are: (a) formation of Alfvén waves at Io, (b) wave‐particle coupling to energize protons, (c) anti‐planetward transport of ions due to the magnetic mirror force and/or parallel acceleration, and (d) strong sub‐Alfvénic interaction slowing the flow connected to Io's fluxtube allowing for sufficient travel time for energized ions to transit to Io. The derived slowdown of ≤12% the upstream value is linked to filamentation within the Alfvén wing. This mechanism is likely operating at all strongly interacting satellites and provides an avenue to transfer material from a planetary body to its satellites, including exoplanets and brown dwarfs.

How much Io's SO2 atmosphere is driven by direct volcanic outgasing or the sublimation of SO2 surface frost is still debated. Since the sublimation supported part of the atmosphere is highly surface temperature dependent, the atmosphere is expected to have a lower SO2 column density on the nightside consistent with observations of a decreased column density in eclipse. Furthermore, the atmosphere is observed to be thicker in equatorial regions compared to the poles and when Jupiter is in Perihelion.To investigate how well observed structures of Io's SO2 distribution can be explained with a purely sublimation driven atmosphere, we developed a time dependent surface temperature model including the effect of thermal inertia. Analyzing the conductive heat transfer from Io's surface towards its interior and vice versa, which is mainly determined by the thermal diffusivity α, allows us to show that many observations can be well explained by assuming a sublimation dominated atmosphere. Simulations show that α=3.1x10-6 m2 / s yields an averaged atmospheric SO2 column density decreasing from 1016 to 2.5x1014 cm-2 from the equator to the poles. In a parameter study regarding the thermal inertia we discuss the influence of different values of the thermal inertia on the diurnal surface temperature and column density variation and find that a thermal diffusivity lower by a factor of 10 results in an atmosphere having both features, a less pronounced latitudinal dependence but a strong day-night asymmetry. Due to Io's inclination, we also find features of the surface temperature and column density that vary seasonally.

We observed Io with the James Webb Space Telescope (JWST) while the satellite was in eclipse, and detected thermal emission from several volcanoes. The data were taken as part of our JWST‐ERS program #1373 on 15 November 2022. Kanehekili Fluctus was exceptionally bright, and Loki Patera had most likely entered a new brightening phase. Spectra were taken with NIRSpec/IFU at a resolving power R ≈ 2,700 between 1.65 and 5.3 µm. The spectra were matched by a combination of blackbody curves that showed that the highest temperature, ∼1,200 K, for Kanehekili Fluctus originated from an area ∼0.25 km 2 in size, and for Loki Patera this high temperature was confined to an area of ∼0.06 km 2 . Lower temperatures, down to 300 K, cover areas of ∼2,000 km 2 for Kanehekili Fluctus, and ∼5,000 km 2 for Loki Patera. We further detected the a 1 Δ ⇒ X 3 Σ − 1.707 µm rovibronic forbidden SO emission band complex over the southern hemisphere, which peaked at the location of Kanehekili Fluctus. This is the first time this emission has been seen above an active volcano, and suggests that the origin of such emissions is ejection of SO molecules directly from the vent in an excited state, after having been equilibrated at temperatures of ∼1,500 K below the surface, as was previously hypothesized.

We analyze Hubble Space Telescope observations of Ganymede made with the Space Telescope Imaging Spectrograph between 1998 and 2017 to generate a brightness map of Ganymede's oxygen emission at 1,356 Å. Our Mercator projected map demonstrates that the brightness along Ganymede's northern and southern auroral ovals strongly varies with longitude. To quantify this variation around Ganymede, we investigate the brightness averaged over 36°‐wide longitude corridors centered around the sub‐Jovian (0° W), leading (90° W), anti‐Jovian (180° W), and trailing (270° W) central longitudes. In the northern hemisphere, the brightness of the auroral oval is 3.7 ± 0.4 times lower in the sub‐Jovian and anti‐Jovian corridors compared to the trailing and leading corridors. The southern oval is overall brighter than the northern oval, and only 2.5 ± 0.2 times fainter on the sub‐ and anti‐Jovian corridors compared to the trailing and leading corridors. This demonstrates that Ganymede's auroral ovals are strongly structured in auroral crescents on the leading side (plasma downstream side) and on the trailing side (plasma upstream side). We also find that the brightness is not symmetric with respect to the 270° meridian, but shifted by ∼20° towards the Jovian‐facing hemisphere. Our map will be useful for subsequent studies to understand the processes that generate the aurora in Ganymede's non‐rotationally driven, sub‐Alfvénic magnetosphere.

Pluto exhibits a tenuous, predominantly N-2-CH4 atmosphere, with Titan-like chemistry. Previous observations with ALMA have permitted the detection of CO and HCN at 345.796 and 354.505 GHz in this atmosphere, yielding vertically resolved chemical and thermal information. We report on new observations of Pluto's atmosphere with ALMA, performed in April and July/August 2017 with two main goals: (i) obtaining spatially-resolved measurements (similar to 0.06 '' on the similar to 0.15 '' disk subtended by Pluto and its atmosphere) of CO(3-2) and HCN(4-3) (H) targetting new chemical compounds, primarily hydrogen isocyanide (HNC) . These observations are modeled with radiative transfer codes coupled with inversion methods. The CO line shows an absorption core at beam positions within Pluto's disk, a direct signature of Pluto's cold mesosphere. Analysis of the CO line map provides tentative evidence for a non-uniform temperature field in the lower atmosphere (near 30 km), with summer pole latitudes being 7 +/- 3.5 K warmer than low latitudes. This unexpected result may point to shorter radiative timescales in the atmosphere than previously thought. The HCN emission is considerably more extended than CO, peaking at radial distances beyond Pluto limb, and providing a new method to determine Pluto's HCN vertical profile in 2017. The mean (column-averaged) location of HCN is at 690 +/- 75 km altitude, with an upper atmosphere (> 800 km) mixing ratio of similar to 1.8 x 10(-4). Little or no HCN (<5 x 10(-9) at 65 km) is present in the lower atmosphere, implying undersaturation of HCN there. The HCN emission appears enhanced above the low-latitude limb, but interpretation, in terms of an enhanced HCN abundance or a warmer upper atmosphere there, is uncertain. The first detection of HNC is reported, with a (7.0 +/- 2.1) x 10(12) cm(-2) column density, referred to Pluto surface, and a HNC/HCN ratio of 0.095 +/- 0.026, very similar to their values in Titan's atmosphere. We also obtain upper limits on CH3CN (< 2.6 x 10(13) cm(-2)) and CH3CCH (< 8.5 x 10(14) cm(-2)); the latter value is inconsistent with the reported detection of CH3CCH from New Horizons. These upper limits also point to incomplete resublimation of ice-coated aerosols in the lower atmosphere.

We report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on 7 June 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant timevariability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m(-2) are required for its OI 1,356 angstrom emission making Ganymede a very poor auroral emitter.

In response to ESA’s “Voyage 2050” announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn’s largest moon Titan. Titan, a “world with two oceans”, is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan’s remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a “heavy” drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan’s northern latitudes with an orbiter and in situ element(s) would be highly complementary in terms of timing (with possible mission timing overlap), locations, and science goals with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan’s equatorial regions, in the mid-2030s.

Molecular hydrogen is the third most abundant species in Titan's atmosphere with a tropospheric/lower stratospheric mole fraction of 0.001 derived from Voyager and Cassini measurements and the major species in Saturn's atmosphere. The motivation for this paper is to explore the hypothesis that the corrected H2 density measurements reported by Cassini Ion Neutral Mass Spectrometer (INMS) team were too high by a factor of 2.2, the factor that the INMS team reported in Teolis et al. (2015) as applicable to all species with no exceptions. For Titan, this would lead to the resolution of a possible missing H2 source and eliminate the need for a large downward gas phase H2 flux. For Saturn this would bring the INMS in-situ and UVIS stellar occultation H2 density measurements into agreement in the low latitude thermosphere and attribute the additional mass density derived from Cassini Attitude and Articulation Control Subsystem (AACS) exceeding the reduced INMS H2 mass density to infalling ring particles.

Context. A stellar occultation by Neptune’s main satellite, Triton, was observed on 5 October 2017 from Europe, North Africa, and the USA. We derived 90 light curves from this event, 42 of which yielded a central flash detection. Aims. We aimed at constraining Triton’s atmospheric structure and the seasonal variations of its atmospheric pressure since the Voyager 2 epoch (1989). We also derived the shape of the lower atmosphere from central flash analysis. Methods. We used Abel inversions and direct ray-tracing code to provide the density, pressure, and temperature profiles in the altitude range ~8 km to ~190 km, corresponding to pressure levels from 9 µbar down to a few nanobars. Results. (i) A pressure of 1.18 ± 0.03 µbar is found at a reference radius of 1400 km (47 km altitude). (ii) A new analysis of the Voyager 2 radio science occultation shows that this is consistent with an extrapolation of pressure down to the surface pressure obtained in 1989. (iii) A survey of occultations obtained between 1989 and 2017 suggests that an enhancement in surface pressure as reported during the 1990s might be real, but debatable, due to very few high S/N light curves and data accessible for reanalysis. The volatile transport model analysed supports a moderate increase in surface pressure, with a maximum value around 2005-2015 no higher than 23 µbar. The pressures observed in 1995-1997 and 2017 appear mutually inconsistent with the volatile transport model presented here. (iv) The central flash structure does not show evidence of an atmospheric distortion. We find an upper limit of 0.0011 for the apparent oblateness of the atmosphere near the 8 km altitude.

Observations during the New Horizons (NH) spacecraft flyby of Pluto in July 2015 revealed that Pluto's atmosphere supports an extensive circumplanetary haze with embedded layers, suggesting several possible microphysical and/or dynamical excitation processes. The purpose of this paper is to build upon existing observations and analyses of Pluto's atmosphere-specifically of the complex haze layer structures-to identify wave structure in Pluto's atmosphere. Here three NH/Long Range Reconnaissance Imager (LORRI) image sequences from the flyby at high phase angles (148 degrees-169 degrees) and three different resolutions (0.093 km/pix, 0.96 km/pix, and 3.86 km/pix) are analyzed. Several haze layer characteristics were extracted, namely-slope, amplitude, waveform, and the associated power spectral densities (PSDs); and their variations with local geography. These are then explored in the context of possible wave types in Pluto's atmosphere, such as tidal and orographically driven inertia-gravity (buoyancy) waves. PSD peaks at 8-10 km and 18-22 km vertical wavelength are found in NH images, which is consistent with the perturbations seen in Earth-based stellar occultations of Pluto's atmosphere. The 8-10 km signals are localized to low-latitudes and equatorial regions and the 18-22 km signals are more globally distributed. Haze layer background relative amplitudes were found to be around 0.01-0.04. Slopes of layers were found to be correlated with the emergence and disappearance of a 25 km layer around 30 degrees N. An amplitude increase of oscillations below 30 km altitude exists in the high-resolution image sequence. These findings indicate the possibility of waves in Pluto's atmosphere and motivate further studies of wave dynamics combining NH data with state-of-the-art models of Pluto's atmosphere. These results are important because they can provide strong constraints to models and to the type of waves that can be present in Pluto's atmosphere.

The Alice spectrograph on New Horizons performed several far-ultraviolet (FUV) airglow observations during the July 2015 flyby of Pluto. One of these observations, named PColor2, was a short (226 s) scan across the dayside disk of Pluto from a range of similar to 34,000 km, at about 40 minutes prior to closest approach. The brightest observed FUV airglow signal at Pluto is the Lyman alpha (Ly alpha) emission line of atomic hydrogen, which arises primarily through the resonant scattering of solar Ly alpha by H atoms in the upper atmosphere, with a brightness of about 30 Rayleigh. Pluto appears dark against the much brighter (similar to 100 Rayleigh) sky background; this sky background is likewise the result of resonantly scattered solar Ly alpha, in this case by H atoms in the interplanetary medium (IPM). Here we use an updated photochemical model and a resonance line radiative transfer model to perform detailed simulations of the Ly alpha emissions observed in the Alice PColor2 scan. The photochemical models show that H and CH4 abundances in Pluto's upper atmosphere are a very strong function of the near-surface mixing ratio of CH4, and could provide a useful way to remotely monitor seasonal climate variations in Pluto's lower atmosphere. The morphology of the PColor2 Ly alpha emissions provides constraints on the current abundance profiles of H atoms and CH4 molecules in Pluto's atmosphere, and indicate that the globally averaged near-surface mixing ratio of CH4 is currently close to 0.4%. This new result thus provides independent confirmation of one of the primary results from the solar occultation, also observed with the New Horizons Alice ultraviolet spectrograph.

The radio thermal emission from Pluto was observed from the New Horizons spacecraft at a wavelength of 4.2 cm along two scans across the planetary disk shortly after closest approach to Pluto on 14 July 2015. The measurements were performed as part of the New Horizons Radio Science Experiment (REX) using the 2.1 m High Gain Antenna (HGA) and the spacecraft's X-Band receiver. The HGA boresight first scanned along a diametric chord across the Pluto disk and then reversed direction to traverse a chord that crossed close to Pluto's winter pole. The diametric scan reveals a "hot spot" on the Pluto nightside associated with an optically bright region centered roughly at the planetocentric coordinates 280 degrees E, 55 degrees S, imaged in 2002-03 with the Hubble Space Telescope. The nightside was also found to be warmer than the dayside during the polar scan. The highest emission was not observed at the maximum southern latitude, however, but rather near the outbound Pluto limb at lower latitude. The REX emission profile from the polar scan is qualitatively consistent with a bright U-shaped polar cap observed on Pluto's Charon-facing hemisphere during the recurring Pluto/Charon mutual events in the late 1980's. The REX radiometer measurements show distinct variations in microwave brightness that constrain volatile transport models and provide unique information on the thermal structure and composition on the regions in winter night during the New Horizons encounter at Pluto.

Since 2007 the Alice spectrograph on the New Horizons (NH) spacecraft has been used to periodically observe the Lyman- α (Ly α ) emissions of the interplanetary medium (IPM), which mostly result from resonant scattering of solar Ly α emissions by interstellar hydrogen atoms passing through the solar system. Three observations of IPM Ly α along a single great circle were made during the NH cruise to Pluto, and these have been supplemented by observations along six great circles (spread over the sky at 30° intervals), acquired one month before and one day after the NH flyby of Pluto, and on a further five occasions since then, out to just over 47 au from the Sun. These data indicate a distant Ly α background of 43 ± 3 Rayleigh brightness (equivalent to 56 ± 4 nW m −2 sr −1 ), which is present in all directions (i.e., not only in the upstream direction, as previously reported). This result is found independently by: (1) the falloff with distance from the Sun of the IPM Ly α brightness observed by NH–Alice in several directions on the sky, and (2) the residual between the observed brightness and a model brightness accounting for the resonantly scattered solar Ly α component alone. The repeated observations show that this distant Ly α background is constant and uniform over the sky, and represents the local Galactic Ly α background. The observations show no strong correlation with the cloud structure of the local IPM. The observed brightness constrains the absorption coefficient of interstellar dust at Ly α to 0.2 ± 0.01 kpc −1 .

Suprathermal ions form from interstellar gas that is first ionized into pickup ions and then accelerated to tens and hundreds of keV in energy. The resulting suprathermal ion spectra with hundreds of keV have been previously observed throughout the heliosphere; however, measurements at lower energies, around the pickup ion cutoff energy where they are accelerated from, were limited to <10 au. Here we present a statistical study of suprathermal ions in the keV to hundred keV energy range. We use the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on the New Horizons spacecraft, which recorded observations at a wide range of heliocentric distances, and compare these measurements to charge energy mass spectrometer (CHEMS) observations on Cassini , which cruised to and remained at Saturn. We find that the power-law exponents of suprathermal ion intensity over energy are between −1 and −2, change abruptly close to discontinuities that are likely corotating merged interaction regions, correlate with the solar wind bulk speed, and show a long-term evolution on the timescale of the solar cycle. The independent measurements from New Horizons and Cassini are consistent, confirming the first fully calibrated measurements from the New Horizons /PEPSSI instrument.

Context. Observations of auroral emissions are powerful means to remotely sense the space plasma environment around planetary bodies and ultracool dwarfs. Therefore successful searches and characterization of aurorae outside the Solar System will open new avenues in the area of extrasolar space physics. Aims. We aim to demonstrate that brown dwarfs are ideal objects to search for UV aurora outside the Solar System. We specifically search for UV aurora on the late-type T6.5 brown dwarf 2MASS J12373919+6526148 (in the following 2MASS J1237+6526). Methods. Introducing a parameter referred to as auroral power potential, we derive scaling models for auroral powers for rotationally driven aurora applicable to a broad range of wavelengths. We also analyze Hubble Space Telescope observations obtained with the STIS camera at near-UV, far-UV, and Ly-α wavelengths of 2MASS J1237+6526. Results. We show that brown dwarfs, due to their typically strong surface magnetic fields and fast rotation, can produce auroral UV powers on the order of 1019 watt or more. Considering their negligible thermal UV emission, their potentially powerful auroral emissions make brown dwarfs ideal candidates for detecting extrasolar aurorae. We find possible emission from 2MASS J1237+6526, but cannot conclusively attribute it to the brown dwarf due to low signal-to-noise values in combination with nonsystematic trends in the background fluxes. The observations provide upper limits for the emission at various UV wavelength bands. The upper limits for the emission correspond to a UV luminosity of ~1 × 1019 watt, which lies in the range of the theoretically expected values. Conclusions. The possible auroral emission from the dwarf could be produced by a close-in companion and/or magnetospheric transport processes.