SPICAV on Venus Express: Three spectrometers to study the global structure and composition of the Venus atmosphere

Spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAV) is a suite of three spectrometers in the UV and IR range with a total mass of 13.9 kg flying on the Venus Express (VEX) orbiter, dedicated to the study of the atmosphere of Venus from ground level to the outermost hydrogen corona at more than 40,000 km. It is derived from the SPICAM instrument already flying on board Mars Express (MEX) with great success, with the addition of a new IR high-resolution spectrometer, solar occultation IR (SOIR), working in the solar occultation mode. The instrument consists of three spectrometers and a simple data processing unit providing the interface of these channels with the spacecraft. A UV spectrometer (118-320 nm, resolution 1. 5 nm) is identical to the MEX version. It is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation. In nadir orientation, SPICAV UV will analyse the albedo spectrum (solar light scattered back from the clouds) to retrieve SO2, and the distribution of the UV-blue absorber (of still unknown origin) on the dayside with implications for cloud structure and atmospheric dynamics. On the nightside, 7 and 6 bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode the UV sensor will measure the vertical profiles of CO2, temperature, SO2, SO, clouds and aerosols. The density/temperature profiles obtained with SPlCAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (similar to 60 km) to 160 km in the atmosphere. This is essential for future missions that would rely on aerocapture and acrobraking. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO+, and CO2+, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere. The SPICAV VIS-IR sensor (0.7-1.7 mu m, resolution 0.5-1.2 nm) employs a pioneering technology: an acousto-optical tunable filter (AOTF). On the nightside, it will study the thermal emission peeping through the clouds, complementing the observations of both VIRTIS and Planetary Fourier Spectrometer (PFS) on VEX. In solar occultation mode this channel will study the vertical structure of H2O, CO2, and aerosols. The SOIR spectrometer is a new solar occultation IR spectrometer in the range lambda=2.2-4.3 mu m, with a spectral resolution lambda/Delta lambda > 15,000, the highest on board VEX. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. The main objective is to measure HDO and H2O in solar occultation, in order to characterize the escape of D atoms from the upper atmosphere and give more insight about the evolution of water on Venus. It will also study isotopes of CO2 and minor species, and provides a sensitive search for new species in the upper atmosphere of Venus. It will attempt to measure also the nightside emission, which would allow a sensitive measurement of HDO in the lower atmosphere, to be compared to the ratio in the upper atmosphere, and possibly discover new minor atmospheric constituents. (C) 2007 Elsevier Ltd. All rights reserved

Noon ionospheric signatures of a sudden commencement following a solar wind pressure pulse

Using experimental data from the Cutlass Super-DARN HF radars and from a subset of ground magnetometers of the IMAGE Scandinavian chain, the response of the ionosphere in the noon sector to a solar wind pressure increase is studied. The emphasis is on the signature of the convection vortices and of the Hall currents that are associated with the pair of opposite parallel currents flowing along the morning and afternoon high-latitude magnetic field lines. We show that the sudden commencement is characterised by an equatorward convection, immediately followed (within less than 3 min) by a strong poleward plasma motion. These results are shown to agree qualitatively with the global model of sudden commencement of Araki (1994).Key words. Ionosphere (plasma convection; electric fields and currents) – Magnetospheric physics (solar wind-magnetosphere interactions

Detection of localized structures from multispacecraft data: Adaptive correlation function

International audienceWhen several spacecraft pass at different distances from a localized structure like a field-aligned current tube, the corresponding magnetic signatures recorded on each spacecraft generally show different timescales, due to the different ``viewpoints'' of the spacecraft. For this reason, it is impossible to identify the presence of a unique structure from these signatures by using the classical correlation functions. In this paper, we present an ``adaptive correlation function'' which is a new analysis tool allowing to look for such ``correlations'' whatever the dilation factor is between two signals

Resonant amplification of magnetosheath MHD fluctuations at the magnetopause

International audienceBroad-band ULF fluctuations are routinely observed upstream of the terrestrial magnetosphere: their amplitude, fairly large in the whole magnetosheath, peaks at the magnetopause, and decays to very small values in the magnetosphere. We test the idea that the source of all these fluctuations is in the magnetosheath and that the high magnetopause level is an amplification effect due to propagation. Starting from given stable magnetosonic waves in the magnetosheath, their behavior is calculated when, during their propagation, they encounter the magnetopause density gradient. For a certain category of incident waves, a 'resonant amplification', occurring inside the gradient layer, is found.Centre d'6tude des Environnements Terrestre et Plan6taires

Looking for SO2: a spectroscopic study of Martian UV albedo using SPICAM

Le fichier pdf est un "extended abstract" de 2 pages en libre acces: http://www-mars.lmd.jussieu.fr/paris2011/abstracts/marcq_paris2011.pdfInternational audiencesan

Noon ionospheric signatures of a sudden commencement following a solar wind pressure pulse

International audienceUsing experimental data from the Cutlass Super-DARN HF radars and from a subset of ground magnetometers of the IMAGE Scandinavian chain, the response of the ionosphere in the noon sector to a solar wind pressure increase is studied. The emphasis is on the signature of the convection vortices and of the Hall currents that are associated with the pair of opposite parallel currents flowing along the morning and afternoon high-latitude magnetic field lines. We show that the sudden commencement is characterised by an equatorward convection, immediately followed (within less than 3 min) by a strong poleward plasma motion. These results are shown to agree qualitatively with the global model of sudden commencement of Araki (1994)

Comparison of UV Albedo of CO₂ and H₂O Ices t North and South Poles Perennial Caps

Perennial polar caps appear very bright in visible light images (Viking, MOC, HRSC). This is also true in the UV range from 200 to 300 nm as seen by SPICAM UV spectrometer, where the albedo is found larger by a factor 3 to 4 higher than surrounding areas outside the caps, both on south and north polar caps. On the south polar cap, which was visited first by Mars Express in January 2004, the UV albedo was found to decrease strongly with wavelength from 300 to 200 nm in the bright regions. Since H2O is not suspected to have such a strong slope in that range, this spectral albedo variation seems to be typical either of CO2 ice or of dust mixed in the ice. A comparison with the north polar cap data, which contain only H2O ice, will be useful to better understand the different spectral signatures of the two types of ices in the UV. Also, it appears that in the visible, the albedo of the south polar cap is distinctly lower in peripheral regions of the perennial cap, interpreted as being the sign of dominating dusty H2O ice. A comparison between visible albedo and UV albedo will be presented on both poles