The database of the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument between late January 2004 and Mars 2014 has been searched to identify signatures of CO Cameron and CO2+ doublet ultraviolet auroral emissions. This study has almost doubled the number of auroral detections based on SPICAM spectra. Auroral emissions are located in the vicinity of the statistical boundary between open and closed field lines. From a total of 113 nightside orbits with SPICAM pointing to the nadir in the region of residual magnetic field, only nine nightside orbits show confirmed auroral signatures, some with multiple detections along the orbital track, leading to a total of 16 detections. The mean energy of the electron energy spectra measured during concurrent Analyzer of Space Plasma and Energetic Atoms/Electron Spectrometer observations ranges from 150 to 280 eV. The ultraviolet aurora may be displaced poleward or equatorward of the region of enhanced downward electron energy flux by several tens of seconds and shows no proportionality with the electron flux at the spacecraft altitude. The absence of further UV auroral detection in regions located along crustal magnetic field structures where occasional aurora has been observed indicates that the Mars aurora is a time‐dependent feature. These results are consistent with the scenario of acceleration of electrons by transient parallel electric field along semiopen magnetic field lines.

International audienceMartian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to quantify both the altitudes and the intensities of the auroral emissions. The CO (a³Π - X¹Σ) Cameron bands between 190 and 270 nm, the CO (A¹Π - X¹Σ⁺) Fourth Positive system (CO 4P) between 135 and 170 nm, the CO₂ (B²Σ_u⁺ - X²Π_g) doublet at 289 nm, the OI at 297.2 nm and the 130.4 nm OI triplet emissions have been identified in the spectra and in the time variations of the signals. The intensities of these auroral emissions have been quantified and the altitude of the strongest emission of the CO Cameron bands has been estimated to be 137±27 km. The locations of these auroral events have also been determined and correspond to the statistical boundary of open-closed magnetic field lines, in cusp-like structures. The observed altitudes of the auroral emissions are reproduced by a Monte-Carlo model of electron transport in the Martian thermosphere for mono-energetic electrons between 60 and 200 eV.No correlation between electron fluxes measured in the upper thermosphere and nadir auroral intensity has been found. Here, we simulate auroral emissions observed both at the limb and at the nadir using electron energy spectra simultaneously measured with the ASPERA-3/ELS instrument. The simulated altitudes are in very good agreement with the observations. We find that predicted vertically integrated intensities for the various auroral emissions are overestimated, probably as a consequence of the inclination and curvature of the magnetic field line threading the aurora. However, the relative brightness of the CO₂⁺emissions is in good agreement with the observations

Abstract. Synthetic aperture radar interferometry (InSAR) is an efficient technique for mapping the surface elevation and its temporal change over glaciers and ice sheets. However, due to the penetration of the SAR signal into snow and ice, the apparent elevation in uncorrected InSAR digital elevation models (DEMs) is displaced versus the actual surface. We studied relations between interferometric radar signals and physical snow properties and tested procedures for correcting the elevation bias. The work is based on satellite and in situ data over Union Glacier in the Ellsworth Mountains, West Antarctica, including interferometric data of the TanDEM-X mission, topographic data from optical satellite sensors and field measurements on snow structure, and stratigraphy undertaken in December 2016. The study area comprises ice-free surfaces, bare ice, dry snow and firn with a variety of structural features related to local differences in wind exposure and snow accumulation. Time series of laser measurements of NASA's Ice, Cloud and land Elevation Satellite (ICESat) and ICESat-2 show steady-state surface topography. For area-wide elevation reference we use the Reference Elevation Model of Antarctica (REMA). The different elevation data are vertically co-registered on a blue ice area that is not affected by radar signal penetration. Backscatter simulations with a multilayer radiative transfer model show large variations for scattering of individual snow layers, but the vertical backscatter distribution can be approximated by an exponential function representing uniform absorption and scattering properties. We obtain estimates of the elevation bias by inverting the interferometric volume correlation coefficient (coherence), applying a uniform volume model for describing the vertical loss function. Whereas the mean values of the computed elevation bias and the elevation difference between the TanDEM-X DEMs and the REMA show good agreement, a trend towards overestimation of penetration is evident for heavily wind-exposed areas with low accumulation and towards underestimation for areas with higher accumulation rates. In both cases deviations from the uniform volume structure are the main reason. In the first case the dense sequence of horizontal structures related to internal wind crust, ice layers and density stratification causes increased scattering in near-surface layers. In the second case the small grain size of the top snow layers causes a downward shift in the scattering phase centre.

No abstract

Abstract. Synthetic aperture radar interferometry (InSAR) is an efficient technique for mapping the surface elevation and its temporal change over glaciers and ice sheets. However, due to the penetration of the SAR signal into snow and ice the apparent elevation in uncorrected InSAR digital elevation models (DEMs) is displaced versus the actual surface. We studied relations between interferometric radar signals and physical snow properties and tested procedures for correcting the elevation bias. The work is based on satellite and in-situ data over Union Glacier in the Ellsworth Mountains, West Antarctica, including interferometric data of the TanDEM-X mission, topographic data from optical satellite sensors and field measurements on snow structure and stratigraphy undertaken in December 2016. The study area comprises ice-free surfaces, bare ice, dry snow and firn with a variety of structural features related to local differences in wind exposure and snow accumulation. Time series of laser measurements of NASA’s Ice, Cloud and land Elevation Satellite (ICESat) and ICESat-2 show steady state surface topography. For area-wide elevation reference we use the Reference Elevation Model of Antarctica (REMA). The different elevation data are vertically co-registered on a blue ice area and an ice-free slope, surfaces not affected by radar signal penetration. The backscatter simulations with a multi-layer radiative transfer model show large variations for scattering of individual snow layers due to different size and structure of the scattering elements. The average depth-dependent backscatter contributions can be approximated by an exponential function. We obtain estimates of the elevation bias by inverting the interferometric volume correlation coefficient (coherence) applying a uniform volume model for describing the vertical loss function. Whereas the mean values of the computed elevation bias and the elevation difference between the TDM DEMs and the REMA show good agreement, a trend towards overestimation of penetration is evident for heavily wind-exposed areas and towards underestimation for areas with higher accumulation rates. The angular gradients of the backscatter intensity show also distinct differences between these two domains. This behaviour can be attributed to the anisotropy of the snow/firn volume structure showing differences in the size and shape of the scattering elements and in stratification related to snow accumulation and wind-driven erosion and deposition.