Local-time averaged maps of H3+ emission, temperature and ion winds.

We present Keck-NIRSPEC observations of Saturn's [Formula: see text] aurora taken over a period of a month, in support of the Cassini mission's 'Grand Finale'. These observations produce two-dimensional maps of Saturn's [Formula: see text] temperature and ion winds for the first time. These maps show surprising complexity, with different morphologies seen in each night. The [Formula: see text] ion winds reveal multiple arcs of 0.5-1 km s-1 ion flows inside the main auroral emission. Although these arcs of flow occur in different locations each night, they show intricate structures, including mirrored flows on the dawn and dusk of the planet. These flows do not match with the predicted flows from models of either axisymmetric currents driven by the Solar Wind or outer magnetosphere, or the planetary periodic currents associated with Saturn's variable rotation rate. The average of the ion wind flows across all the nights reveals a single narrow and focused approximately 0.3 km s-1 flow on the dawn side and broader and more extensive 1-2 km s-1 sub-corotation, spilt into multiple arcs, on the dusk side. The temperature maps reveal sharp gradients in ionospheric temperatures, varying between 300 and 600 K across the auroral region. These temperature changes are localized, resulting in hot and cold spots across the auroral region. These appear to be somewhat stable over several nights, but change significantly over longer periods. The position of these temperature extremes is not well organized by the planetary period and there is no evidence for a thermospheric driver of the planetary period current system. Since no past magnetospheric or thermospheric models explain the rich complexity observed here, these measurements represent a fantastic new resource, revealing the complexity of the interaction between Saturn's thermosphere, ionosphere and magnetosphere. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

The Great Cold Spot in Jupiter's upper atmosphere.

Past observations and modeling of Jupiter's thermosphere have, due to their limited resolution, suggested that heat generated by the aurora near the poles results in a smooth thermal gradient away from these aurorae, indicating a quiescent and diffuse flow of energy within the subauroral thermosphere. Here we discuss Very Large Telescope-Cryogenic High-Resolution IR Echelle Spectrometer observations that reveal a small-scale localized cooling of ~200 K within the nonauroral thermosphere. Using Infrared Telescope Facility NSFCam images, this feature is revealed to be quasi-stable over at least a 15 year period, fixed in magnetic latitude and longitude. The size and shape of this "Great Cold Spot" vary significantly with time, strongly suggesting that it is produced by an aurorally generated weather system: the first direct evidence of a long-term thermospheric vortex in the solar system. We discuss the implications of this spot, comparing it with short-term temperature and density variations at Earth.

Cooling by H3(+) emission.

Emission by the H3(+) molecular ion may be important in determining the energy balance in astrophysical situations, such as in (exo)planetary atmospheres. Here we report the calculation of a new cooling function, based on refitted partition functions and a recalculation of the total energy emitted by the molecule. This new function gives significantly increased cooling at higher temperatures, typical of those found in the atmospheres of gas giants. It is shown that nonthermal effects also need to be considered. A link to a web-based code to calculate radiative cooling in H2/H3(+) gas mixtures, including the effects of departures from equilibrium, is provided.

Temperature changes and energy inputs in giant planet atmospheres: what we are learning from H3+.

Since its discovery at Jupiter in 1988, emission from H(3)(+) has been used as a valuable diagnostic tool in our understanding of the upper atmospheres of the giant planets. One of the lasting questions we have about the giant planets is why the measured upper atmosphere temperatures are always consistently hotter than the temperatures expected from solar heating alone. Here, we describe how H(3)(+) forms across each of the planetary disks of Jupiter, Saturn and Uranus, presenting the first observations of equatorial H(3)(+) at Saturn and the first profile of H(3)(+) emission at Uranus not significantly distorted by the effects of the Earth's atmosphere. We also review past observations of variations in temperature measured at Uranus and Jupiter over a wide variety of time scales. To this, we add new observations of temperature changes at Saturn, using observations by Cassini. We conclude that the causes of the significant level of thermal variability observed over all three planets is not only an important question in itself, but that explaining these variations could be the key to answering the more general question of why giant planet upper atmospheres are so hot.

H3+ cooling in planetary atmospheres.

We review the role of H3+ in planetary atmospheres, with a particular emphasis on its effect in cooling and stabilising, an effect that has been termed the "H3+ thermostat" (see Miller et al., Philos. Trans. R. Soc. London, Ser. A, 2000, 58, 2485). In the course of our analysis of this effect, we found that cooling functions that make use of the partition function, Q(T) based on the calculated H3+ energy levels of Neale and Tennyson (Astrophys. J., 1995, 454, L169) may underestimate just how much energy this ion is radiating to space. So we present a new fit to the calculated values of Q(T) that is accurate to within 2% for the range 100 K to 10 000 K, a very significant improvement on the fit originally provided by Neale and Tennyson themselves. We also present a fit to Q(T) calculated from only those values Neale and Tennyson computed from first principles, which may be more appropriate for planetary scientists wishing to calculate the amount of atmospheric cooling from the H3+ ion.

Complex structure within Saturn's infrared aurora.

The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission, and ionize the hydrogen, leading to H(3)(+) infrared emission. Although the morphology of these aurorae is affected by changes in the solar wind, the source of the currents which produce them is a matter of debate. Recent models predict only weak emission away from the main auroral oval. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn's magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown.

Jovian-like aurorae on Saturn.

Planetary aurorae are formed by energetic charged particles streaming along the planet's magnetic field lines into the upper atmosphere from the surrounding space environment. Earth's main auroral oval is formed through interactions with the solar wind, whereas that at Jupiter is formed through interactions with plasma from the moon Io inside its magnetic field (although other processes form aurorae at both planets). At Saturn, only the main auroral oval has previously been observed and there remains much debate over its origin. Here we report the discovery of a secondary oval at Saturn that is approximately 25 per cent as bright as the main oval, and we show this to be caused by interaction with the middle magnetosphere around the planet. This is a weak equivalent of Jupiter's main oval, its relative dimness being due to the lack of as large a source of ions as Jupiter's volcanic moon Io. This result suggests that differences seen in the auroral emissions from Saturn and Jupiter are due to scaling differences in the conditions at each of these two planets, whereas the underlying formation processes are the same.

H3+: the driver of giant planet atmospheres.

We present a review of recent developments in the use of H3+ molecular ion as a probe of physics and chemistry of the upper atmospheres of giant planets. This ion is shown to be a good tracer of energy inputs into Jupiter (J), Saturn (S) and Uranus (U). It also acts as a 'thermostat', offsetting increases in the energy inputs owing to particle precipitation via cooling to space (J and U). Computer models have established that H3+ is also the main contributor to ionospheric conductivity. The coupling of electric and magnetic fields in the auroral polar regions leads to ion winds, which, in turn, drive neutral circulation systems (J and S). These latter two effects, dependent on H3+, also result in very large heating terms, approximately 5 x 10(12) W for Saturn and greater than 10(14) W for Jupiter, planet-wide; these terms compare with approximately 2.5 x 10(11) W of solar extreme UV absorbed at Saturn and 10(12) W at Jupiter. Thus, H3+ is shown to play a major role in explaining why the temperatures of the giant planets are much greater (by hundreds of kelvin) at the top of the atmosphere than solar inputs alone can account for.