JUNO/JIRAM's view of Jupiter's H3+ emissions.

The instrument JIRAM (Jovian Infrared Auroral Mapper), on board the NASA spacecraft Juno, is both an imager and a spectrometer. Two distinct detectors are used for imaging and spectroscopy. The imager acquires Jupiter images in two bands, one of which (L band, 3.3-3.6 µm) is devoted to monitor the H3+ emission. The spectrometer covers the spectral region from 2 to 5 µm (average spectral resolution 9 nm) with a 256 pixels slit that can observe the same scene of the L band imager with some delay. JIRAM scientific goals are the exploration of the Jovian aurorae and the planet's atmospheric structure, dynamics and composition. Starting early July 2016 Juno is orbiting around Jupiter. Since then, JIRAM has provided an unprecedented amount of measurements, monitoring both Jupiter's atmosphere and aurorae. In particular, the camera has monitored Jupiter's poles with very high spatial resolution, providing new insights in both its aurorae and the polar dynamic. The main findings obtained by the L imager are detailed pictures of Jupiter's aurorae showing an extremely complex morphology of the H3+ distribution in the main oval and in the moon's footprints. The spectrometer has enabled the measure the distribution of both H3+ concentration and temperature. The analysis of the north auroral region limb observations shows that the peak density of H3+ is above 750 km and that often it is anticorrelated to the temperature, confirming the infrared cooling effect of H3+. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

Juno observations of spot structures and a split tail in Io-induced aurorae on Jupiter.

Jupiter's aurorae are produced in its upper atmosphere when incoming high-energy electrons precipitate along the planet's magnetic field lines. A northern and a southern main auroral oval are visible, surrounded by small emission features associated with the Galilean moons. We present infrared observations, obtained with the Juno spacecraft, showing that in the case of Io, this emission exhibits a swirling pattern that is similar in appearance to a von Kármán vortex street. Well downstream of the main auroral spots, the extended tail is split in two. Both of Ganymede's footprints also appear as a pair of emission features, which may provide a remote measure of Ganymede's magnetosphere. These features suggest that the magnetohydrodynamic interaction between Jupiter and its moon is more complex than previously anticipated.

Clusters of cyclones encircling Jupiter's poles.

The familiar axisymmetric zones and belts that characterize Jupiter's weather system at lower latitudes give way to pervasive cyclonic activity at higher latitudes. Two-dimensional turbulence in combination with the Coriolis ß-effect (that is, the large meridionally varying Coriolis force on the giant planets of the Solar System) produces alternating zonal flows. The zonal flows weaken with rising latitude so that a transition between equatorial jets and polar turbulence on Jupiter can occur. Simulations with shallow-water models of giant planets support this transition by producing both alternating flows near the equator and circumpolar cyclones near the poles. Jovian polar regions are not visible from Earth owing to Jupiter's low axial tilt, and were poorly characterized by previous missions because the trajectories of these missions did not venture far from Jupiter's equatorial plane. Here we report that visible and infrared images obtained from above each pole by the Juno spacecraft during its first five orbits reveal persistent polygonal patterns of large cyclones. In the north, eight circumpolar cyclones are observed about a single polar cyclone; in the south, one polar cyclone is encircled by five circumpolar cyclones. Cyclonic circulation is established via time-lapse imagery obtained over intervals ranging from 20 minutes to 4 hours. Although migration of cyclones towards the pole might be expected as a consequence of the Coriolis ß-effect, by which cyclonic vortices naturally drift towards the rotational pole, the configuration of the cyclones is without precedent on other planets (including Saturn's polar hexagonal features). The manner in which the cyclones persist without merging and the process by which they evolve to their current configuration are unknown.

Retrieving cloud geometrical extents from MIPAS/ENVISAT measurements with a 2-D tomographic approach.

Clouds represent a critical factor in regulating the Earth's atmosphere and its energy balance. Satellite instruments can measure the energy balance and global atmospheric properties only through an accurate knowledge of the vertical profile of cloudiness, which is as yet one of the key shortages in atmospheric science. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on-board the ENVISAT satellite, designed to infer the amount of atmospheric trace-gases, demonstrated also sensitivity to the radiation emitted from clouds. In order to model the effect of the geometrical extent of a cloud on MIPAS measurements, we developed a retrieval model capable to simulate cloud effects on broad spectral intervals accounting for the two-dimensional (2-D) variability of the atmosphere in the satellite orbit plane. The 2-D analysis revealed a sensitivity of MIPAS spectra to both the vertical and horizontal extents and the position of clouds along the instrument line of sight. One-dimensional models were found to underestimate Cloud Top Height (CTH) by approximating clouds as an infinite horizontal layer with a finite vertical extents. With the 2-D approach, we showed it is possible, for optically thin Polar Stratospheric Clouds (PSCs), to retrieve both CTH and horizontal dimension by analyzing simultaneously all the limb observations that come across the cloud with their field of view. For a selected case study we found a very good agreement for both PSC CTH and horizontal extents retrieved from MIPAS measurements and those retrieved from coincident CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarisation) measurements.

Assessment of the horizontal resolution of retrieval products derived from MIPAS observations.

We report the results of a study aimed at the assessment of the trade-off between precision and horizontal resolution of the retrieval products of MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) operating onboard the ENVIronmental SATellite. By exploiting different observation setups we could perform the study by acting on both the retrieval and the sampling grids. Our results are compared with those previously obtained on simulated observations [Appl. Opt. 43, 1-11 (2004)]. We show that the horizontal sampling of the atmosphere operated by the spectrometer cannot be pushed beyond some limits without inducing unacceptable correlations among the retrieved profiles. These correlations show-up only when using a two-dimensional retrieval algorithm and can be evaluated through the instabilities that they trigger in the horizontal distribution of the retrieval products. In order to reduce these instabilities we compare the strategy of degrading the retrieval grid with the strategy of applying horizontal regularization. We discuss the different trade-off between precision and spatial resolution connected with the two strategies. The method adopted in this study, is applicable to any orbiting limb sounder measuring along the orbit track.

Discrete representation and resampling in limb-sounding measurements.

The processes of discretization, interpolation, and resampling are frequently used in data analysis. Here the formalism of functional spaces is used as a framework for the description and characterization of both the measurement operation and these subsequent processes. The tools provided by this formalism are applied to the problem of resampling of atmospheric volume mixing ratio vertical profiles obtained with limb-sounding measurements. In particular, a resampling method that uses the conservation of the vertical column as a constraint is presented and compared with other methods. The effects of the resampling process in terms of error propagation and loss of vertical resolution are also evaluated.

Geo-fit Approach to the Analysis of Limb-Scanning Satellite Measurements.

We propose a new approach to the analysis of limb-scanning measurements of the atmosphere that are continually recorded from an orbiting platform. The retrieval is based on the simultaneous analysis of observations taken along the whole orbit. This approach accounts for the horizontal variability of the atmosphere, hence avoiding the errors caused by the assumption of horizontal homogeneity along the line of sight of the observations. A computer program that implements the proposed approach has been designed; its performance is shown with a simulated retrieval analysis based on a satellite experiment planned to fly during 2001. This program has also been used for determining the size and the character of the errors that are associated with the assumption of horizontal homogeneity. A computational strategy that reduces the large number of computer resources apparently demanded by the proposed inversion algorithm is described.

Optimized forward model and retrieval scheme for MIPAS near-real-time data processing.

An optimized code to perform the near-real-time retrieval of profiles of pressure, temperature, and volume mixing ratio (VMR) of five key species (O(3), H(2)O, HNO(3), CH(4), and N(2)O) from infrared limb spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) experiment on board the European Space Agency (ESA) Environmental Satellite ENVISAT-1 was developed as part of a ESA-supported study. The implementation uses the global fit approach on selected narrow spectral intervals (microwindows) to retrieve each profile in sequence. The trade-off between run time and accuracy of the retrieval was optimized from both the physical and the mathematical points of view, with optimizations in the program structure, in the radiative transfer model, and in the computation of the retrieval Jacobian. The attained performances of the retrieval code are noise error on temperature <2 K at all the altitudes covered by the typical MIPAS scan (8-53 km with 3-km resolution), noise error on tangent pressure <3%, and noise error on VMR of the target species <5% at most of the altitudes covered by the standard MIPAS scan, with a total run time of less than 1 min on a modern workstation.