J. J. Plaut

Variation of the Martian ionospheric electron density from Mars Express radar soundings

[1]xa0The Mars Advanced Radar for Subsurface and Ionospheric Sounding aboard Mars Express has been in operation for over 2 years. Between 14 August 2005 and 31 July 2007, we obtain 34,492 ionospheric traces, of which 14,060 yield electron density profiles and 12,291 yield acceptable fits to the Chapman ionospheric model. These results are used to study the Martian ionosphere under changing conditions: the presence or absence of solar energetic particles, solar EUV flux, season, solar zenith angle, and latitude. The 2-year average subsolar maximum electron density n0 is 1.62 × 105 cm−3, the average subsolar electron density altitude h0 is 128.2 km, and the average neutral scale height H is 12.9 km. Solar energetic particle events are associated with a 6% increase in n0, a 3 km decrease in h0, and a 0–7 km decrease in H. The value of n0 varies smoothly between 1.4 × 105 and 1.8 × 105 cm−3, yielding d ln n0/d ln F10.7 = 0.30 ± 0.4; h0 varies between 115 and 135 km, while H remains relatively constant with EUV flux and season, in contrast with previous work. The value of h0 decreases toward the terminator at low latitude but increases poleward during summer; H varies from 11 km, for solar zenith angle less than 40°, to between 14 and 17 km near the terminator, depending on season. Near-peak temperatures vary between 220 K and 300 K, less variation than indicated by modeling, probably due to sampling near solar minimum.

Structure and dynamics of the solar wind/ionosphere interface on Mars: MEX‐ASPERA‐3 and MEX‐MARSIS observations

[1]xa0The measurements of the local plasma parameters of the ionospheric and solar wind plasmas and the magnetic field strength carried out by the ASPERA-3 and MARSIS experiments onboard Mars Express (MEX) in the subsolar region of the induced Martian magnetosphere provide us with a first test of the pressure balance across the solar wind/ionosphere interface. The structure of this transition is very dynamic and is controlled by the solar wind. For a broad range of the solar wind dynamic pressures, the magnetic field in the boundary layer raises to the values just sufficient to balance the solar wind pressure. The magnetic field frozen into the electrons is transported across the magnetospheric boundary (MB) where solar wind terminates and the planetary plasma begins to prevail. The dense ionospheric plasma has a sharp outer boundary the position of which is usually a little closer to the planet than the MB. Although the number density reaches on this boundary ∼103 cm−3 the contribution of the ionospheric thermal pressure is rather small and the ionosphere is magnetized. There are also cases when the magnetic field almost does not vary across both boundaries.

Plasma environment of Mars as observed by simultaneous MEX-ASPERA-3 and MEX-MARSIS observations

[1] Simultaneous in situ measurements carried out by the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instruments on board the Mars Express (MEX) spacecraft for the first time provide us with the local parameters of cold ionospheric and hot solar wind plasma components in the different regions of the Martian magnetosphere and ionosphere. On the dayside, plasma of ionospheric and exospheric origin expands to large altitudes and gets in touch with the solar wind plasma. Formation of the magnetic field barrier which terminates the solar wind flow is governed by solar wind. The magnetic field rises up to the value which is just sufficient to balance the solar wind pressure while the position of the magnetospheric boundary varies insignificantly. Although, within the magnetic barrier, solar wind plasma is depleted, the total electron density increases owing to the enhanced contribution of planetary plasma. In some cases, a load caused by a planetaiy plasma becomes so strong that a pileup of the magnetic field occurs in a manner which forms a discontinuity (the magnetic pileup boundary). Generally, the structure of the magnetospheric boundary on the dayside varies considerably, and this variability is probably controlled by the magnetic field orientation. Inside the magnetospheric boundaiy, the electron density continues to increase and forms the photoelectron boundary which sometimes almost coincides with the magnetospheric boundary. The magnetic field strength also increases in this region, implying that the planetary plasma driven into the bulk motion transports the magnetic field inward. A cold and denser ionospheric plasma at lower altitudes reveals a tailward cometary-like expansion. Large-amplitude oscillations in the number density of the ionospheric plasma are another typical feature. Crossings of plasma sheet at low altitudes in the terminator region are characterized by depletions in the density of the ionospheric component. In some cases, density depletions correlate with large vertical components of the crustal magnetic field. Such anticorrelation in the variations of the densities of the cold ionospheric and hot magnetosheath/plasma sheet plasmas is also rather typical for localized aurora-type events on the nightside.

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

We 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 7u2009h and 14u2009h 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 100u2009km with peak electron densities of (1.5 to 2.5)u2009×u2009105u2009cm−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.

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS): concept and performance

Describes the key features and expected performance of a new radar sounder instrument currently under development by a team of Italian and US researchers and industrial partners, selected to fly with the ESA Mars Express orbiter scheduled for launch to Mars late in 2003. Very low transmitted frequency (1-5 MHz), large instantaneous bandwidth and coherent on-board processing techniques will make it possible to acquire a large amount of science-relevant data about the Mars interior, surface and atmosphere ensuring global coverage at all latitudes while respecting the Mars Express mission constraints.

Interplanetary coronal mass ejection observed at STEREO-A, Mars, comet 67P/Churyumov-Gerasimenko, Saturn, and New Horizons en route to Pluto: Comparison of its Forbush decreases at 1.4, 3.1, and 9.9 AU

We discuss observations of the journey throughout the Solar System of a large interplanetary coronal mass ejection (ICME) that was ejected at the Sun on 14 October 2014. The ICME hit Mars on 17 October, as observed by the Mars Express, MAVEN, Mars Odyssey and MSL missions, 44 hours before the encounter of the planet with the Siding-Spring comet, for which the space weather context is provided. It reached comet 67P/Churyumov-Gerasimenko, which was perfectly aligned with the Sun and Mars at 3.1 AU, as observed by Rosetta on 22 October. The ICME was also detected by STEREO-A on 16 October at 1 AU, and by Cassini in the solar wind around Saturn on the 12 November at 9.9 AU. Fortuitously, the New Horizons spacecraft was also aligned with the direction of the ICME at 31.6 AU. We investigate whether this ICME has a non-ambiguous signature at New Horizons. A potential detection of this ICME by Voyager-2 at 110-111 AU is also discussed. The multi-spacecraft observations allow the derivation of certain properties of the ICME, such as its large angular extension of at least 116°, its speed as a function of distance, and its magnetic field structure at four locations from 1 to 10 AU. Observations of the speed data allow two different solar wind propagation models to be validated. Finally, we compare the Forbush decreases (transient decreases followed by gradual recoveries in the galactic cosmic ray intensity) due to the passage of this ICME at Mars, comet 67P and Saturn.

Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS): subsurface performances evaluation

According to the Mars Express mission, the MARSIS primary scientific objectives are to map the distribution of water, both liquid and solid, in the upper pot-lions of the crust of Mars. Three secondary objectives are also defined subsurface geologic probing, surface characterization, and ionosphere sounding. In order to obtain the primary objectives the Radar Sounder design was based on the Ice/water interface and Dry/ice interface scenario: defining the material composition of the first layers and porosity and the pore filling materials. Concerning the surface, we have characterized the geometric structure in terms of a large-scale morphology, on which a small-scale geometric structure, due to rocks, is superimposed, taking into account also that recently the structure of the planets surface was described by means of fractals and in particular the new MARS surface models obtained by processing of the MOLA data. According to these models, this paper provides a description of the operational planning approach and expected performances of MARSIS.

MARSIS data inversion approach: Preliminary results

An approach to the inversion of the data available from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express is described. The data inversion gives an estimation of the materials composing the different detected interfaces, including the impurity (inclusion) of the first layer, if any, and its percentage, by the evaluation of the values of the permittivity that would generate the observed radio echoes. The data inversion method is based on the analysis of the surface to subsurface power ratio and the relative time delay as measured by MARSIS. The constraints, due to the known geological history of the surface, the local temperature and the thermal condition of the observed zones and the results of other instruments on Mars Express and other missions to Mars, have to be considered to improve the validity of the utilized models and the obtained results that are given in parametric way.

Detecting sub‐glacial aquifers in the north polar layered deposits with Mars Express/MARSIS

[1]xa0The penetration of the MARSIS radar signal into the polar ice mass is modeled to determine the capability of the instrument to locate sub-glacial aquifers. As a ground penetrating radar, the orbiting MARSIS transmits a signal >1 W between 1–5 MHz. In this work we will investigate the effect of ice reflective and conductive losses on the radar-detection of subsurface aquifers using an MGS MOC profile of the ice layering. We find that a basal lake located ∼2.5 km below the surface is at the limit of detectability.

Mars ionosphere data inversion by MARSIS surface and subsurface signals analysis

According to the Mars Express mission , the MARSIS primary scientific objectives are to map the distribution of water, both liquid and solid, in the upper portions of the crust of Mars. Moreover three secondary objectives are defined for the MARSIS experiment: subsurface geologic probing, surface characterization, and ionosphere sounding.