Ludivine Libert

Concurrent observations of ultraviolet aurora and energetic electron precipitation with Mars Express

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 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 ASPERA-3/ELS 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 semi open magnetic field lines.

Split-Band Interferometry-Assisted Phase Unwrapping for the Phase Ambiguities Correction

Split-Band Interferometry (SBInSAR) exploits the large range bandwidth of the new generation of synthetic aperture radar (SAR) sensors to process images at subrange bandwidth. Its application to an interferometric pair leads to several lower resolution interferograms of the same scene with slightly shifted central frequencies. When SBInSAR is applied to frequency-persistent scatterers, the linear trend of the phase through the stack of interferograms can be used to perform absolute and spatially independent phase unwrapping. While the height computation has been the main concern of studies on SBInSAR so far, we propose instead to use it to assist conventional phase unwrapping. During phase unwrapping, phase ambiguities are introduced when parts of the interferogram are separately unwrapped. The proposed method reduces the phase ambiguities so that the phase can be connected between separately unwrapped regions. The approach is tested on a pair of TerraSAR-X spotlight images of Copahue volcano, Argentina. In this framework, we propose two new criteria for the frequency-persistent scatterers detection, based respectively on the standard deviation of the slope of the linear regression and on the phase variance stability, and we compare them to the multifrequency phase error. Both new criteria appear to be more suited to our approach than the multifrequency phase error. We validate the SBInSAR-assisted phase unwrapping method by artificially splitting a continuous phase region into disconnected subzones. Despite the decorrelation and the steep topography affecting the volcanic test region, the expected phase ambiguities are successfully recovered whatever the chosen criterion to detect the frequency-persistent scatterers. Comparing the aspect ratio of the distributions of the computed phase ambiguities, the analysis shows that the phase variance stability is the most efficient criterion to select stable targets and the slope standard deviation gives satisfactory results.

Optimum split-band parameters for phase offset retrieval

The potential of Split-Band processing to determine the absolute phase of an interferometric pair of SAR images has been demonstrated in previous studies. In this work we propose a new approach based on the joint use of Split-Band Interferometry and SAR Interferometry for the phase leveling of disconnected regions of an unwrapped interferogram. In this paper a new selector of frequency-persistent scatterers is presented. In order to demonstrate the applicability of the method we propose an artificial disconnection of the regions to validate our results without any ground based measurements. The presented method is tested on TerraSAR-X images acquired with a range bandwidth of 150 MHz over the Nyamuragira volcano in the African region of the Kivu Rift. Several combinations of split parameters are applied and investigated to determine the best configuration for this application.