Measurements of nitryl chloride (ClNO2) and its precursors (O3, NO2, particulate chloride) were made in 2014–2016 at three contrasting locations in the United Kingdom: Leicester, Penlee Point and Weybourne. ClNO2 was observed at all sites and in every season, with the highest concentrations between 00:00 and 04:00 GMT. The median nocturnal concentration of ClNO2 ranged between the detection limit (4.2 ppt) and 139 ppt. A clear seasonal cycle, with maxima in spring and winter, and significant differences between locations in the same season were observed. The main source of particulate chloride was sea salt aerosol (including at Leicester, ∼200 km from the coast). In general, ClNO2 levels were controlled by the concentrations of O3 and NO2, rather than by the uptake and reaction of N2O5 with particulate chloride. Under these conditions, the seasonality and geographical distribution of ClNO2 can be explained in terms of O3‐limited and NO2‐limited regimes affecting the formation of the N2O5 precursor. A global version of the GEOS‐Chem model at medium resolution (2° × 2.5°) was not able to fully capture the observed seasonality of ClNO2, mostly because the model overestimated the concentrations of the precursors, particularly of nocturnal O3. A higher‐resolution (0.25° × 0.3125°) version of GEOS‐Chem showed better agreement with the observations, although it still overestimated ClNO2 concentrations during summer.

We have discovered pulsating emission within Jupiter's main auroral oval, providing evidence of the auroral signature of Jovian ULF wave processes. The form comprises a 1° × 2° spot located directly on the main emission, whose intensity oscillates with a period of ∼10 min throughout the 45 min observation. The feature appears on the duskward edge of the discontinuity, maps to ∼13–14 h LT and ∼20–50 RJ, and rotates at around a half of rigid corotation. We show that the period of the oscillation is similar to the expected Alfvén travel time between the ionosphere and the upper edge of the equatorial plasma sheet in the middle magnetosphere, and we thus suggest that the pulsating aurora is driven by a mode confined to the low‐density region outside the plasma sheet. This significant new observation shows that Jupiter's auroras present an important remote sensing window on Jovian magnetospheric wave processes.

The upper atmosphere of Uranus has been observed to be slowly cooling between 1993 and 2011. New analysis of near-infrared observations of emission from H 3 + obtained between 2012 and 2018 reveals that this cooling trend has continued, showing that the upper atmosphere has cooled for 27 years, longer than the length of a nominal season of 21 years. The new observations have offered greater spatial resolution and higher sensitivity than previous ones, enabling the characterization of the H 3 + intensity as a function of local time. These profiles peak between 13 and 15 h local time, later than models suggest. The NASA Infrared Telescope Facility iSHELL instrument also provides the detection of a bright H 3 + signal on 16 October 2016, rotating into view from the dawn sector. This feature is consistent with an auroral signal, but is the only of its kind present in this comprehensive dataset. This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H 3 + , H 5 + and beyond’.

We present a detailed study of the H3+ auroral emissions at Jupiter, using data taken on 31 December 2012 with the long‐slit Echelle spectrometer CRIRES (ESO‐VLT). From this data set the rotational temperature of the H3+ ions in Jupiter's upper atmosphere was calculated using the ratio of the ν2 Q(1,0−) and ν2 Q(3,0−) fundamental emission lines. The entire northern auroral region was observed, providing a highly detailed view of ionospheric temperatures, which were mapped onto polar projections. The temperature range we derive in the northern auroral region is ~750–1000 K, which is consistent with past studies, although the temperature structure differs. We identify two broad regions which exhibit temperature changes over a short period of time (~80 minutes). We propose that the changes in temperature could be due to a local time change in particle precipitation energy, or they could be caused by dynamic temperature changes generated in the neutral thermosphere due to the magnetospheric response to a transient enhancement of solar wind dynamic pressure, as predicted by models. By comparing the H3+ temperature, column density, total emission, and line‐of‐sight velocity, we were unable to identify a single dominant mechanism responsible for the energetics in Jupiter's northern auroral region. The comparison reveals that there is complex interplay between heating by impact from particle precipitation and Joule heating, as well as cooling by the H3+ thermostat effect.

The Cassini spacecraft revealed that Saturn's magnetic field displayed oscillations at a period originally thought to match the planetary rotation rate but later found not to. One of many proposed theories predicts that a polar twin‐cell neutral weather system drives this variation, producing observable differences in flows within Saturn's ionosphere. Here, using spectral observations of auroral H3+ ${\mathrm{H}}_{3}^{+}$ emission lines taken by the Keck Observatory's Near Infrared Echelle Spectrograph (Keck‐NIRSPEC) in 2017, we derive ion line‐of‐sight velocity maps after grouping spectra into rotational quadrants matching phases of the planetary magnetic field. We measure 0.5 km s−1 wind systems in the ionosphere consistent with predicted neutral twin‐vortex flow patterns. These findings demonstrate that neutral winds in Saturn's polar regions cause the rotational period, as determined via the magnetic field, to exhibit differences and time variabilities relative to the planet's true period of rotation in a process never before seen within planetary atmospheres.