O. Witasse

Martian dayglow as seen by the SPICAM UV spectrograph on Mars Express

[1]xa0In this paper we describe measurements of the Martian dayglow obtained by SPICAM UV spectrograph on board Mars Express between October 2004 and March 2005. Typical spectra (of more than 24,000 individual spectra) display the main features of the dayglow already reported more than 30 years ago (Barth et al., 1971, 1972). The variations with altitude, solar zenith angle, and aerocentric longitude of the main emissions, namely, the CO Cameron band emissions (a3Π – X1Σ+) from 180–260 nm, the CO2+ ultraviolet doublet (B2Σ+ - X2Π) emission near 289 nm, the O resonance line at 130.4 nm, and the H Lyman alpha line at 121.6 nm, are detailed. The average temperature deduced from the CO2+ (B2Σ+ - X2Π) emission scale height between 150 and 190 km is T∞ = 201 ± 10 K. The altitude of the peak of the dayglow emission varies from 120 km at low solar zenith angle (∼30°) up to 132 km at high solar zenith angle (∼75°). We also find 24 orbits in which the individual derived exospheric temperatures do not show significant variations with respect to solar zenith angle, aerocentric longitude, or longitude. The presence of crustal magnetic fields appears to correlate with a significant (∼100 K) increase in exospheric temperature. We report also the first observations of emissions associated with the N2 Vegard Kaplan band system in the Martian upper atmosphere.

Modeling the OI 630.0 and 557.7 nm thermospheric dayglow during EISCAT‐WINDII coordinated measurements

The 630.0 and 557.7 nm thermospheric dayglow was modeled at high-latitude using an eight-moment fluid model, from measurements coordinated between the European Incoherent Scatter (EISCAT) radar and the Wind Imaging Interferometer (WINDII). The emission computation in particular included the electron (suprathermal and thermal) impact whose cross sections have been updated, the dissociative recombination of O2+ ion described for the first time by a theoretical rate coefficient, the photodissociation of molecular oxygen, and some relevant chemical reactions. The neutral atmosphere was adjusted by calibrating the ionospheric model outputs to EISCAT data. Slight adjustments were needed in order to reach a good agreement. The results were successfully compared to WINDII observations. Our present study shows that simultaneous EISCAT-WINDII measurements can be used to reduce uncertainties due to the neutral composition and that new observations of the EUV solar spectrum are still needed.

Observations of the nightside ionosphere of Mars by the Mars Express Radio Science Experiment (MaRS)

[1]xa0The vertical structure of the nightside ionosphere of Mars and its dependence on solar zenith angle are currently poorly determined, as is the importance of two key sources of nightside plasma, electron precipitation and transport of dayside plasma. We examined 37 electron density profiles of the ionosphere of Mars at solar zenith angles of 101°–123° obtained by the Mars Express Radio Science Experiment (MaRS) between 18 August and 1 October 2005. In general, solar activity was low during this period, although several solar energetic particle events did occur. The results show that (1) trends in peak electron density and altitude with solar zenith angle are consistent with transport of dayside plasma as an important plasma source up to 115°, but not higher; (2) peak altitudes of around 150 km observed at larger (>115°) solar zenith angles are consistent with simulated plasma production by electron precipitation; and (3) peak altitudes observed during solar energetic particle events are at 90 km, consistent with accepted models. Solar energetic particle events can be the main source of nightside plasma. These results challenge current models of the nightside ionosphere, including their implications for plasma sources. The total electron content is correlated with peak electron density, requiring explanation. Due to the geographical distribution of this data set (latitudes poleward of 38°N), we do not explore the influence of crustal field strength and direction on the nightside ionosphere.

A clear view of the multifaceted dayside ionosphere of Mars

[1]xa0By examining electron density profiles from the Mars Express Radio Science Experiment MaRS, we show that the vertical structure of the dayside ionosphere of Mars is more variable and more complex than previously thought. The top of the ionosphere can be below 250xa0km (25% occurrence rate) or above 650xa0km (1%); the topside ionosphere can be well-described by a single scale height (10%) or two/three regions with distinct scale heights (25% or 10%), where those scale heights range between tens and hundreds of kilometers; the main layer of the ionosphere can have a sharply pointed (5%), flat-topped (6%), or wavy (8%) shape, in contrast to its usual Chapman-like shape; a broad increase in electron density is detected at 160–180xa0km (10%); a narrow increase in electron density is sometimes found in strongly-magnetized regions; and an additional layer is present between the M1 and M2 layers (3%).

Three‐dimensional Martian ionosphere model: II. Effect of transport processes due to pressure gradients

To study the transport of the ionospheric plasma on Mars, we have included a 3D multifluid dynamical core in a Martian General Circulation Model (GCM). Vertical transport modifies the ion density above ~160u2009km on the dayside, especially the ions produced at high altitudes like O+, N+ and C+. Near the exobase, the dayside to nightside flow velocity reaches few hundreds of m/s, due to a large horizontal pressure gradient. Comparison with MEX/ASPERA-3 measurements between 290 - 500u2009km, suggests this flow could account for at least 20% of the flow produced by the solar wind. This flow is not sufficient to populate substantially the nightside ionosphere at high altitudes, in agreement with recent observations, because of a strong nightside downward flow produced by vertical pressure gradient. The O2+ and NO+ ion densities on the nightside at low altitudes (~130u2009km) are modified by this downward flow, compared to simulated densities without ion dynamics, while other ions are lost by chemical reactions. Variability at different time scales (diurnal, seasonal and solar cycle) are studied. We simulate diurnal and seasonal variations of the ionospheric composition due to the variability of the neutral atmosphere and solar flux at the top of the atmosphere. The ionospheric dynamics are not strongly affected by seasons and solar cycles and the retroaction of the ionosphere on the neutral atmosphere temperature and velocity is negligible compared to other physical processes below the exobase.

HF radio wave attenuation due to a meteoric layer in the atmosphere of Mars

In the atmosphere of Mars, the effect of an ionospheric layer of meteoric origin on an HF radio wave propagation is investigated. We consider the putative magnesium ion layer which results from the ablation of sporadic meteors from a work recently published. To account for day-night variability, two electron profiles are considered in an altitude range around 80 km. First we argue that there may be some observational evidence of this magnesium layer in early Mars observations that were not noticed before. Then we study the effect of this ionospheric layer on the attenuation of HF waves. For a propagation path through the ionosphere as envisaged for HF subsurface sounding on future Mars missions, the one-way attenuation may range from up to 360 dBs at 1.8 MHz and up to 18 dBs at 9 MHz.

Optical estimation of auroral ion upflow: 2. A case study

[1]xa0This work presents a detailed multi-instrument and modeling case study of an ion upflow event occurring over the Sondrestrom incoherent scatter radar on 17 February 2001. This case study is used to demonstrate the effectiveness of a previously developed optical estimator of ion upflow. The optical estimator of ion upflow is applied in a two step process: (1) near-infrared (NIR) optical observations in the 700–850 nm wavelength range are inverted to reconstruct the energy distribution of precipitating electrons then (2) the reconstructed precipitation is applied to a combined fluid-kinetic model of the ionosphere (TRANSCAR) to estimate the plasma response to the auroral particles. Plasma parameters estimated by this procedure are compared with plasma parameters derived from incoherent scatter radar data recorded during the event and are shown to be in reasonably good agreement. However, this study highlights an important limitation of the estimation technique, the lack of a direct measure of the ambient ionospheric state (nighttime plasma density). Means for addressing this limitation are discussed and may eventually allow for estimation of ionospheric state parameters under certain conditions by using optical sensors.

Characterization of zonal winds in the stratosphere of Titan with UVES: 2. Observations coordinated with the Huygens Probe entry

[1]xa0The Huygens Probe has successfully entered Titans atmosphere and landed on its surface on 14 January 2005. With the aim of characterizing the zonal wind flow in Titans stratosphere close to the time of entry, coordinated observations were carried out at the Very Large Telescope on the nights of 7, 12, 14, and 15 January. As in our previous investigation (Luz et al., 2005), we used the UVES instrument, mounted on the Kueyen-UT2 telescope, simultaneously achieving high spectral resolving power and high spatial resolution. The field has been derotated in order to align the 0.3-arcsec aperture perpendicularly to Titans rotation axis. In this configuration, spatial information in the east-west direction is preserved in a set of spectra in the direction perpendicular to dispersion. We present measurements of zonal winds obtained with the technique of absolute accelerometry. The observations were made in the wavelength range 4200–6200 A, probing between 115 and 280 km, with peak contributions at 200 and 170 km for the lower and upper parts of the domain. We detect prograde zonal winds with lower limits 46 and 53 ms−1 at these altitudes. These values are close to our previous measurements.

Sodium recycling at Europa: what do we learn from the sodium cloud variability?

[1]xa0We study the ejection of sodium atoms from Europas surface by both magnetospheric ion and electron sputtering and desorption stimulated by UV solar photons. The depletion of the surface by ejection and its enrichment by redeposition of sodium atoms are described. The redistribution of sodium atoms at the surface induced by photo-stimulated desorption from the dayside and by sputtering ejection from the trailing hemisphere cannot explain the observed variation of the Na emission brightness. However, a transient increase of the sputtering rate due to a plasma injection may explain such an increase. The relationship between the sodium surface content and the sodium exosphere are also discussed.

Limb imaging of the Venus O2 visible nightglow with the Venus Monitoring Camera

[1]xa0We investigated the Venus O2 visible nightglow with imagery from the Venus Monitoring Camera on Venus Express. Drawing from data collected between April 2007 and January 2011, we study the global distribution of this emission, discovered in the late 1970s by the Venera 9 and 10 missions. The inferred limb-viewing intensities are on the order of 150 kR at the lower latitudes and seem to drop somewhat toward the poles. The emission is generally stable, although there are episodes when the intensities rise up to 500 kR. We compare a set of Venus Monitoring Camera observations with coincident measurements of the O2 nightglow at 1.27u2009µm made with the Visible and Infrared Thermal Imaging Spectrometer, also on Venus Express. From the evidence gathered in this and past works, we suggest a direct correlation between the instantaneous emissions from the two O2 nightglow systems. Possible implications regarding the uncertain origin of the atomic oxygen green line at 557.7u2009nm are noted.