Mars Express Research Articles

Hydraulic modeling of the tributary and the outlet of a Martian paleolake located in the Memnonia quadrangle

AbstractWe present a steady state hydraulic model of the tributary and the outlet of a Martian paleolake located in the Memnonia quadrangle between 167°0'0''W and 167°30'0''W longitude and between 9°25'0''S and 9°45'0''S latitude. The Mars Express High‐resolution stereo camera (HRSC) digital elevation model, H31850000DA4, with a spatial resolution of 75 m is used to describe the geometrical features of the hydraulic system. A steady state hydraulic model, through a Monte Carlo procedure, is adopted to investigate the assumption that the tributary‐lake‐outlet hydraulic system was acting simultaneously during its last water‐related evolutionary phase. Through our analysis, we infer the roughness coefficient and the discharge of both the tributary and the outlet channels. Geomorphic evidence, such as river terraces, were used to constrain the water discharge and the Manning's roughness coefficient, thereby obtaining estimates of the water level in the lake, during the last evolutionary phase of the system. Our analysis of the outlet reveals a median paleodischarge of 7135 m3 s−1 and a median Manning's roughness coefficient of 0.067 m−1/3 s. Tributary analysis provides a median flow and a median roughness coefficient of 6405 m3 s −1 and 0.123 m−1/3 s, respectively. Moreover, the hydraulic analysis suggests that the paleolake water surface level was −1397 m (median value, HRSC elevation), which is consistent with the observed paleoshoreline. The results imply that the tributary, the lake, and the outlet were hydraulically synchronized, therefore confirming the presence of a connected water system. This study demonstrates that hydraulic analysis can provide valuable information regarding ancient Martian water fluxes.

SPICAM observations and modeling of Mars aurorae

Martian 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 determine both the altitudes and the intensities of the auroral emissions. The CO (a3Π–X1Σ) Cameron bands between 190 and 270nm, the CO (A1Π–X1Σ+) Fourth Positive system (CO 4P) between 135 and 170nm, the CO2+ (B2Σu+–X2Πg) doublet at 289nm, the OI at 297.2nm and the 130.4nm 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±27km. 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 40 and 200eV.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 CO and CO2+ emissions is in good agreement with the observations.

Seasonal variation of Martian pick-up ions: Evidence of breathing exosphere

The Mars Express (MEX) Ion Mass Analyser (IMA) found that the detection rate of the ring-like distribution of protons in the solar wind outside of the bow shock to be quite different between Mars orbital summer (around perihelion) and orbital winter (around aphelion) for four Martian years, while the north–south asymmetry is much smaller than the perihelion–aphelion difference. Further analyses using eight years of MEX/IMA solar wind data between 2005 and 2012 has revealed that the detection frequency of the pick-up ions originating from newly ionized exospheric hydrogen with certain flux strongly correlates with the Sun–Mars distance, which changes approximately every two years. Variation due to the solar cycle phase is not distinguishable partly because this effect is masked by the seasonal variation under the MEX capability of plasma measurements. This finding indicates that the variation in solar UV has a major effect on the formation of the pick-up ions, but this is not the only controlling factor.

Indian Mars-Colour-Camera captures far-side of the Deimos: A rarity among contemporary Mars orbiters

Mars and Deimos are tidally locked so one would always get to view the same side of Deimos from Mars. Most of the contemporary functional satellites orbiting Mars viz. Mars Reconnaissance Orbiter , Mars Express, Mars Odyssey, MAVEN etc. are positioned between Mars and Deimos, hence they always view only one side of Deimos i.e the Mars-side, while the far-side is always looking outward with respect to all these satellites. However the maiden Indian Mars Orbiter Mission (MOM) is an exception as it has a very large elliptical orbit and goes behind the orbit of Deimos. This has enabled MOM to view the far-side of Deimos which has not been viewed for last few decades by any Mars orbiter. Mars Colour Camera (MCC), onboard MOM acquired four image-frames of the Deimos, the farthest of two natural satellites of Mars, on October 14, 2014 at around 13:05 UT. These four images of MCC have been used to generate an High Dynamic Range (HDR) product so as to enhance the image details. The two well-known craters of near-side are missing in this HDR image and the shape of far-side, recently updated and reported models of Deimos proposed by other researchers, matches with the shape of Deimos in MCC images thus confirming that the far-side view of Deimos has been captured by MCC of Mars Orbiter Mission. The magnitude of Deimos has also been computed from this far-side image, which varies by a small margin from the known magnitude of Deimos. This could probably be due to possible difference in the near and far side surface characteristics, which needs further detailed investigation and more imaging of far-side in future. However, there could be reasons other than the physical characteristics of the Deimos. Thus far-side sighting has generated a scientific interest to understand it holistically. This note is a brisk or rapid communication regarding sighting of the far-side of the Deimos by MCC after a long spell of more than three decades by any other Mars mission, including the contemporary operational Mars orbiters. It highlights the imaging capabilities of MCC-MOM and possible research to come in future.

Solar wind interaction with the Martian upper atmosphere: Crustal field orientation, solar cycle, and seasonal variations

AbstractA comprehensive study of the solar wind interaction with the Martian upper atmosphere is presented. Three global models: the 3‐D Mars multifluid Block Adaptive Tree Solar‐wind Roe Upwind Scheme MHD code (MF‐MHD), the 3‐D Mars Global Ionosphere Thermosphere Model (M‐GITM), and the Mars exosphere Monte Carlo model Adaptive Mesh Particle Simulator (M‐AMPS) were used in this study. These models are one‐way coupled; i.e., the MF‐MHD model uses the 3‐D neutral inputs from M‐GITM and the 3‐D hot oxygen corona distribution from M‐AMPS. By adopting this one‐way coupling approach, the Martian upper atmosphere ion escape rates are investigated in detail with the combined variations of crustal field orientation, solar cycle, and Martian seasonal conditions. The calculated ion escape rates are compared with Mars Express observational data and show reasonable agreement. The variations in solar cycles and seasons can affect the ion loss by a factor of ∼3.3 and ∼1.3, respectively. The crustal magnetic field has a shielding effect to protect Mars from solar wind interaction, and this effect is the strongest for perihelion conditions, with the crustal field facing the Sun. Furthermore, the fraction of cold escaping heavy ionospheric molecular ions [( and/or )/Total] are inversely proportional to the fraction of the escaping (ionospheric and corona) atomic ion [O+/Total], whereas and ion escape fractions show a positive linear correlation since both ion species are ionospheric ions that follow the same escaping path.

MARSIS remote sounding of localized density structures in the dayside Martian ionosphere: A study of controlling parameters

AbstractEnhanced topside electron densities in the dayside Martian ionosphere have been repetitively observed in areas of near‐radial crustal magnetic fields, for periods of tens of days, indicating their long‐term spatial and temporal stability despite changing solar wind conditions. We perform a statistical study of these density structures using the ionospheric mode of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express. We estimate the apparent extents of these structures relative to the altitude of the surrounding ionosphere. The apex of the density structures often lies higher than the surrounding ionosphere (median vertical extent of 18 km), which indicates upwellings. These structures are much wider than they are high, with latitudinal scales of several degrees. The radar reflector regions are observed above both moderate and strong magnetic anomalies, and their precise locations and latitudinal extents match quite well with the locations and latitudinal extents of magnetic structures of given magnetic polarity (oblique to vertical fields), which happen to be regions where the field lines are open part of the time. The majority of the density structures occur in regions where ionospheric plasma is dominant, indicating closed field regions shielded from shocked solar wind plasma.

Cold ion escape from the Martian ionosphere

We here report on new measurements of the escape flux of oxygen ions from Mars by combining the observations of the ASPERA-3 and MARSIS experiments on board the European Mars Express spacecraft. We show that in previous estimates of the total heavy ion escape flow the contribution of the cold ionospheric outflow with energies below 10eV has been underestimated. Both case studies and the derived flow pattern indicate that the cold plasma observed by MARSIS and the superthermal plasma observed by ASPERA-3 move with the same bulk speed in most regions of the Martian tail. We determine maps of the tailside heavy ion flux distribution derived from mean ion velocity distributions sampled over 7 years. If we assume that the superthermal bulk speed derived from these long time averages of the ion distribution function represent the total plasma bulk speed we derive the total tailside plasma flux. Assuming cylindrical symmetry we determine the mean total escape rate for the years 2007–2014 at 2.8±0.4×1025atoms/s which is in good agreement with model estimates. A possible mechanism to generate this flux can be the ionospheric pressure gradient between dayside and nightside.

Oxygen foreshock of Mars

Mars Express (MEX) has operated for more than 10 years in the environment of Mars, providing solar wind ion observations from the Analyzer of Space Plasmas and Energetic Atoms experiment's Ion Mass Analyser (IMA). On 21 September 2008, MEX/IMA detected foreshock-like discrete distributions of oxygen ions at around 1keV in the solar wind attached to the bow shock and this distribution was observed continuously up to more than 2000km from the bow shock. Foreshock-like protons are also observed but at a shifted location from the oxygen by about 1000km, at a slightly higher energy, and flowing in a slightly different direction than the oxygen ions. Both protons and oxygen ions are flowing anti-sunward at different angles with respect to the solar wind direction. This is the first time that a substantial amount of planetary oxygen is observed upstream of the bow shock. Although rare, this is not the only IMA observation of foreshock-like oxygen: oxygen ions are sometimes observed for a short period of time (<5min) inside the foreshock region. These observations suggest a new escape channel for planetary ions through the acceleration in the bow shock–magnetosheath region.

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

AbstractThe 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.

Astrometric observations of Phobos with the SRC on Mars Express

Aims. From April 2008 to August 2011 Mars Express carried out 74 Phobos flybys at distances between 669 and 5579 km. Images taken with the Super Resolution Channel (SRC) were used to determine the spacecraft-centered right ascension and declination of this Martian moon.Methods. Image positions of Phobos were measured using the limb-fit and control-point measurement techniques. Camera pointing and pointing drift were controlled by means of background star observations that were compared to corresponding positions from reference catalogs. Blurred and noisy images were restored by applying an image-based point spread function in a Richardson-Lucy deconvolution.Results. Here, we report on a set of 158 Phobos astrometric observations with estimated accuracies between 0.224 and 3.405 km circular w.r.t. the line of sight to the satellite. Control point measurements yield slightly more accurate results than the limb fit ones. Our observations are in good agreement with the current Phobos ephemerides by the Jet Propulsion Laboratory (JPL) and the Royal Observatory of Belgium (ROB) with mean offsets of up to 335 m. Our data can be used for the maintenance and update of these models.

The Martian atmospheric ion escape rate dependence on solar wind and solar EUV conditions: 1. Seven years of Mars Express observations

More than 7 years of ion flux measurements in the energy range 10 eV–15 keV have allowed the ASPERA-3/IMA (Analyzer of Space Plasmas and Energetic Ions/Ion Mass Analyzer) instrument on Mars Express to collect a large database of ion measurements in the Mars environment, over a wide range of upstream solar wind (density and velocity) and radiation (solar EUV intensity) conditions. We investigate the influence of these parameters on the Martian atmospheric ion escape rate by integrating IMA heavy ion flux measurements taken in the Martian tail at similar (binned) solar wind density (nsw), velocity (vsw), and solar EUV intensity (IEUV) conditions. For the same solar wind velocity and EUV intensity ranges (vsw and Is constrained), we find a statistically significant decrease of up to a factor of 3 in the atmospheric ion escape rate with increased average solar wind density (5.6 × 1024s−1 to 1.9 × 1024s−1 for 0.4 cm−3 and 1.4 cm−3, respectively). For low solar wind density (0.1–0.5 cm−3) and low EUV intensity, the escape rate increases with increasing solar wind velocity from 2.4 × 1024s−1 to 5.6 × 1024s−1. During high solar EUV intensities the escape fluxes are highly variable, leading to large uncertainties in the estimated escape rates; however, a statistically significant increase in the escape rate is found between low/high EUV for similar solar wind conditions. Empirical-analytical models for atmospheric escape are developed by fitting calculated escape rates to all sufficiently sampled upstream conditions.

Seasonal variation of the HDO/H2O ratio in the atmosphere of Mars at the middle of northern spring and beginning of northern summer

We present the seasonal variation of the HDO/H2O ratio caused by sublimation–condensation processes in a global view of the martian water cycle. The HDO/H2O ratio was retrieved from ground-based observations using high-dispersion echelle spectroscopy of the Infrared Camera and Spectrograph (IRCS) of the Subaru telescope. Coordinated joint observations were made by the Planetary Fourier Spectrometer (PFS) onboard Mars Express (MEX). The observations were performed during the middle of northern spring (Ls=52°) and at the beginning of summer (Ls=96°) in Mars Year 31. The retrieved latitudinal mean HDO/H2O ratios are 4.1±1.4 (Ls=52°) and 4.4±1.0 (Ls=96°) times larger than the terrestrial Vienna Standard Mean Ocean Water (VSMOW). The HDO/H2O ratio shows a large seasonal variation at high latitudes. The HDO/H2O ratio significantly increases from 2.4±0.6wrt VSMOW at Ls=52° to 5.5±1.1wrt VSMOW at Ls=96° over the latitude range between 70°N and 80°N. This can be explained by preferential condensation of HDO vapor during the northern fall, winter, and spring and sublimation of the seasonal polar cap in the northern summer. In addition, we investigated the geographical distribution of the HDO/H2O ratio over low latitudes at the northern spring in the longitudinal range between 220°W and 360°W, including different local times from 10h to 17h. We found the HDO/H2O ratio has no significant variation (5.1±1.2wrt VSMOW) over the entire range. Our observations suggest that the HDO/H2O distribution in the northern spring and summer seasons is mainly controlled by condensation-induced fractionation between the seasonal northern polar cap and the atmosphere.

Probing of meteor showers at Mars during the encounter of comet C/2013 A1: predictions for the arrival of MAVEN/Mangalyaan

We have estimated (1) production rates, (2) ion and electron densities of meteor ablation and (3) ionization for different masses and velocities of meteoroids when comet C/2013 A1 crossed the orbit of Mars on 19 October, 2014 at 18:27 UT. Meteor ablations of small masses < 10−4 g have created a broad layer between altitude ~ 90 km and 110 km. The meteoroids of large masses ≥ 10−4 g are burnt at around 60–90 km well below the main ionization peak at altitude ~160 km produced in the nighttime by solar wind particle impact. The production rates and densities of 15 metallic ions (Mg+, Fe+, Si+, MgO+, FeO+, SiO+, MgCO2 +, MgO2 +, FeCO2 +, FeO2 +, SiCO2 +, SiO2 +, MgN2 +, FeN2 +, and SiN2 +) have been computed self-consistently between altitudes 50 km and 150 km. The twelve major peaks in the Ion Mass Spectra (IMS) are predicted by our model calculations. Our predicted ion and electron density profiles of metals provide benchmark values that can be observed by plasma probes onboard Mars Express (MEX), Mars Atmosphere and Volatile Evolution (MAVEN) and Mangalyaan.

An ionized layer in the upper atmosphere of Mars caused by dust impacts from comet Siding Spring

AbstractWe report the detection of a dense ionized layer in the upper atmosphere of Mars caused by the impact of dust from comet Siding Spring. The observations were made by the ionospheric radar sounder on the Mars Express spacecraft during two low‐altitude passes approximately 7 h and 14 h after closest approach of the comet to Mars. During these passes an unusual transient layer of ionization was detected at altitudes of about 80 to 100 km with peak electron densities of (1.5 to 2.5) × 105 cm−3, much higher than normally observed in the Martian ionosphere. From comparisons to previously observed ionization produced by meteors at Earth and Mars, we conclude that the layer was produced by dust from the comet impacting and ionizing the upper atmosphere of Mars.

Morphometry of volcanic cones on Mars in perspective of Astrobiological Research

AbstractThe permanently frozen volcanic sediment is one of the most promising geological objects for searching life on Mars. On Earth, volcanic intrusions into permafrost result in formation of the unique microbial communities. We propose several terrestrial analogues of Martian polar volcanoes, such as the permanently frozen volcanic sediments on the Kamchatka peninsula and in Antarctica. The present study shows applicability of the morphometric analysis for demonstration of the morphological similarity between the terrestrial and Martian cinder cones. In the present work, the morphometric analysis of young Martian landforms is based on the assumption that the conical structures identified on digital terrain model (DTM) are volcanic cinder cones. Morphometric analysis of the studied cones showed a range of degradation. The extent of degradation may be an indicator of age based on comparison with volcanic cinder cones on Earth. A morphometric analysis of potentially young volcanic cones in the North Polar Region of Mars was performed to estimate their relative age. The 14 potential cinder cones were identified using the DTM provided by Mars Express High Resolution Stereo Camera (HRSC), allowing for the basic morphometric calculations. The majority of the cinder cones are localized in the Chasma Boreale region within the area 79°–81°N and 261°–295°E. The calculated morphometric parameters showed that the cone average steepness varied from 3.4° to 11.8°, cone height-to-width ratio varied from 0.025 to 0.12, and the ratio between surface and basal area of the cone varied from 1.005 to 1.131. The studied cinder cones were classified with respect to the morphometric ratios assuming that larger values correspond to the younger structures. Employing the terrestrial analogy of morphometric ratios as a proxy for relative geological age, we suggest that existing microorganisms may be found in permafrost of young Martian cinder cones.

The equivalent slab thickness of Mars' ionosphere: Implications for thermospheric temperature

AbstractThe total electron content (TEC) of a planetary ionosphere is dominated by plasma near and above the height of maximum electron density (Nmax). The ratio TEC/Nmax represents the thickness (τ) of a TEC slab of uniform density (Nmax). For a photochemical ionosphere, τ relates to the scale height (H = kT/mg) of the ionized neutral gas as τ ~ 4 × H. Derived temperatures refer to ~160 km in thermosphere height—below the asymptotic temperature of the exosphere. The MARSIS instrument on Mars Express has produced data sets of TEC and Nmax. We used them to form τ patterns versus solar zenith angle and solar cycle phase. For daytime (SZA &lt; 90°) conditions, &lt;τ &gt; day ~ 50 km, decreasing rapidly for solar zenith angle (SZA) &gt; 90° to &lt; τ &gt; night ~ 25 km. These correspond to Tn values of 250°K and 125°K. Using Mars Global Surveyor data, τ patterns show a mild dependence upon the solar EUV flux proxy F10.7, with ΔTn(°K) ~ 0.3° per unit change in F10.7 at Mars.

Prediction of blue, red and green aurorae at Mars

The upper atmosphere of Mars is a laboratory for better understanding the planetary atmosphere evolution, and is an example of the interaction of the solar wind with an unmagnetized planet that has only patches of crustal magnetic field. In that context, several space missions were launched to study the Martian environment and its aurorae, notably ESA's Mars Express discovered the first aurora-like structures, and more recently NASA's MAVEN, which is dedicated to understand the atmospheric escape. However, none of the existing missions have spectrometers in the visible spectral range for the observation of the upper atmosphere and the aurorae, but there are UV spectrometer which can be used to infer visible aurora emission.The UV aurorae on Mars have a counterpart in the visible spectral range which should be detectable under the right conditions. We discuss what are the most favorable conditions to observe these aurorae discernible with the naked eye. In this paper, we simulate the Martian aurora in the visible spectral range both with an experimental setup (the Planeterrella, which we use to measure intensity with respect to the naked eye) and with a numerical ionosphere simulation model (Trans⁎/Aeroplanets). We show that the electron impact on CO2 produces strong emissions at 412nm and 434nm, i.e., in the blue part of the visible spectrum which are due to the CO2+(A) Fox–Duffendack–Barker bands. The modeling of the electron transport at Mars shows that these blue emissions as well as the emissions of the 630nm (red) and 557.7nm (green) lines of atomic oxygen may be observable several times during a solar cycle during strong solar events.The absence of visible spectrometers dedicated to these observations onboard existing space missions and the location of the different Martian rovers, far from the vertically aligned crustal magnetic field lines of Mars, have prevented so far the observations of such an aurora. In the foreseeable future, two missions may help observing these aurorae: the exo-Mars/Trace Gas Orbiter mission will carry a visible spectrometer which could be used to detect these events in the visible spectral range. NOMAD (Nadir and Occultation for Mars Discovery) will carry a UV-visible spectrometer in the 200–650nm range.

Long-term variability of CO2and O in the Mars upper atmosphere from MRO radio science data

We estimate the annual variability of CO2 and O partial density using approximately 6 years of Mars Reconnaissance Orbiter (MRO) radio science data from August 2006 to January 2012, which cover three full Martian years (from the northern hemisphere summer of 28 to the northern hemisphere summer of 31). These two elements are the dominant species at the MRO periapsis altitude, constituting about 70–80% of the total density. We report the recovered annual cycle of CO2 and the annual and seasonal cycle of O in the upper atmosphere. Although no other observations are available at those altitudes, our results are in good agreement with the density measurements of the Mars Express Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars, which uses stellar occultations between 60 and 130 km to determine the CO2 variability, and with the Mars Global Reference Atmospheric Model 2010 for the O annual and seasonal variabilities. Furthermore, the updated model provides more reasonable MRO drag coefficients (CD), which are estimated to absorb mismodeling in the atmospheric density prediction. The higher content of dust in the atmosphere due to dust storms increases the density, so the CDs should compensate for this effect. The correlation between the drag coefficient and the dust optical depth, measured by the Mars Odyssey Thermal Emission Imaging System (THEMIS) instrument, increases from 0.4 to 0.8 with the a priori and adjusted models, respectively. The trend of CDs not only confirms a substantial improvement in the prediction of the atmospheric density with the updated model but also provides useful information for local dust storms, near MRO periapsis, that cannot be measured by the opacity level since THEMIS does not always sample the southern hemisphere evenly.

Intensity of nightside MARSIS AIS surface reflections and implications for low‐altitude ionospheric densities

AbstractSpacecraft radar sounding signals at frequencies higher than the ionospheric peak plasma frequency are not reflected by the ionosphere. Instead, they make it to the ground where they are reflected by the planetary surface. We analyze the intensity of the surface reflections measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) ionospheric radar sounder on board the Mars Express spacecraft. Apart from the surface reflectivity and the spacecraft altitude, the detected intensity of surface reflections is controlled primarily by the signal attenuation during the ionospheric propagation. We focus on the nightside region, where the ionospheric densities in the main layer are too low to cause a significant attenuation and allow sampling of the surface reflections at frequencies down to 3 MHz. The attenuation occurs mainly at altitudes below 100 km, where the electron‐neutral collision frequency is a maximum. The intensity of surface reflections can thus serve as a proxy for electron densities at low altitudes not accessible by the direct ionospheric radar sounding. We analyze the intensity of surface reflections as a function of relevant controlling parameters. The intensity of surface reflections is lower at higher solar zenith angles on the nightside and during the periods of larger solar activity. Moreover, it exhibits a seasonal variation that is related to the dust storm occurrence. The intensity of surface reflections is lower in areas of closed magnetic field lines, suggesting that nightside electron densities behave rather differently at low altitudes than at higher altitudes. This is confirmed by comparison with simultaneous observations of the main ionospheric layer.

Control of the topside Martian ionosphere by crustal magnetic fields

AbstractWe present observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument onboard Mars Express of the thermal electron plasma density of the Martian ionosphere and investigate the extent to which it is influenced by the presence of Mars's remnant crustal magnetic fields. We use locally measured electron densities, derived when MARSIS is operating in active ionospheric sounding (AIS) mode, covering an altitude range from ∼300 km to ∼1200 km. We compare these measured densities to an empirical model of the dayside ionospheric plasma density in this diffusive transport‐dominated regime. We show that small spatial‐scale departures from the averaged values are strongly correlated with the pattern of the crustal fields. Persistently elevated densities are seen in regions of relatively stronger crustal fields across the whole altitude range. Comparing these results with measurements of the (scalar) magnetic field also obtained by MARSIS/AIS, we characterize the dayside strength of the draped magnetic fields in the same regions. Finally, we provide a revised empirical model of the plasma density in the Martian ionosphere, including parameterizations for both the crustal field‐dominated and draping‐dominated regimes.