C. Bertucci

The plasma environment of Mars

When the supersonic solar wind reaches the neighborhood of a planetary obstacle it decelerates. The nature of this interaction can be very different, depending upon whether this obstacle has a large-scale planetary magnetic field and/or a well-developed atmosphere/ionosphere. For a number of years significant uncertainties have existed concerning the nature of the solar wind interaction at Mars, because of the lack of relevant plasma and field observations. However, measurements by the Phobos-2 and Mars Global Surveyor (MGS) spacecraft, with different instrument complements and orbital parameters, led to a significant improvement of our knowledge about the regions and boundaries surrounding Mars.

The magnetic memory of Titan's ionized atmosphere

After 3 years and 31 close flybys of Titan by the Cassini Orbiter, Titan was finally observed in the shocked solar wind, outside of Saturns magnetosphere. These observations revealed that Titans flow-induced magnetosphere was populated by “fossil” fields originating from Saturn, to which the satellite was exposed before its excursion through the magnetopause. In addition, strong magnetic shear observed at the edge of Titans induced magnetosphere suggests that reconnection may have been involved in the replacement of the fossil fields by the interplanetary magnetic field.

Titan's near magnetotail from magnetic field and electron plasma observations and modeling: Cassini flybys TA, TB, and T3

[1] The first close Titan encounters TA, TB, and T3 of the Cassini mission at almost the same Saturnian local time � 1030 and in the same spatial region downstream of Titan have enabled us to study the formation of the tail of its induced magnetosphere. The study is based on magnetic field and electron plasma observations as well as threedimensional modeling. Our most important findings are the following: (1) No crossings of a bow shock of Titan were observed, and all encounters occurred at high plasma b > 1 for

Magnetic field draping enhancement at the Martian magnetic pileup boundary from Mars global surveyor observations

[1] The Magnetic Pileup Boundary (MPB) is a sharp and permanent plasma boundary located between the bow shock and the ionospheric boundary, reported so far at Mars and comets. We use Mars Global Surveyor Magnetometer data to do a quantitative analysis of the magnetic field geometry in the surroundings of the Martian MPB. As a result, we report for the first time a dramatic enhancement of the magnetic field draping at this boundary. This new feature, already reported at comets, is independent of the presence of the crustal magnetic sources. Comparisons with similar results across the Martian and cometary magnetotails reveal that the MPB and the magnetotail boundary are connected. Moreover, the study of this feature can help understand the physics of the Venusian magnetic barrier.

Ion cyclotron waves in Saturn's E ring: Initial Cassini observations

The magnetometer onboard the Cassini spacecraft observes ion cyclotron waves produced by both water-group (O + , OH + , H 2 O + , or H 3 O + ) and O + 2 ions at nearly all radial distances and local times within the E ring. These left-hand elliptically polarized waves travel at small angles to the magnetic field and are restricted to near the equatorial plane, where they have their peak amplitudes. They are generated by pickup ions created by the ionization of the neutral exosphere surrounding the E-ring material, and their subsequent acceleration by the electric field associated with the corotating plasma. The energy flux of these waves is proportional to the energy added to the ions when they are accelerated and enables an estimate of the rate of loss of the rings exosphere.

Plasma environment in the wake of Titan from hybrid simulation: A case study

On 26 December 2005, the Cassini spacecraft flew through Titans plasma wake and revealed a complex and dynamic region. Observations suggest a strong asymmetry which seems to be displaced from the ideal position of the wake. Two distinct plasma regions are identified with a significant difference on the electron number density and on the plasma composition. Simulation results using a three-dimensional and multi-species hybrid model, performed in conditions similar to those encountered during the flyby, are presented and compared to the observations. An acceptable agreement is shown between the model predictions and the observations. We suggest that the observed asymmetries, in terms of density and plasma composition, are mainly caused by the a combination of the asymmetry in the ion/electron production rate and the magnetic field morphology, where the first plasma region is connected to the dayside hemisphere of Titans ionosphere while the other is connected to the nightside hemisphere.

Time-dependent global MHD simulations of Cassini T32 flyby: From magnetosphere to magnetosheath

When the Cassini spacecraft flew by Titan on 13 June 2007, at 13.6 Saturn local time, Titan was directly observed to be outside Saturns magnetopause. Cassini observations showed dramatic changes of magnetic field orientation as well as other plasma flow parameters during the inbound and outbound segments. In this paper, we study Titans ionospheric responses to such a sudden change in the upstream plasma conditions using a sophisticated multispecies global MHD model. Simulation results of three different cases (steady state, simple current sheet crossing, and magnetopause crossing) are presented and compared against Cassini Magnetometer, Langmuir Probe, and Cassini Plasma Spectrometer observations. The simulation results provide clear evidence for the existence of a fossil field that was induced in the ionosphere. The main interaction features, as observed by the Cassini spacecraft, are well reproduced by the time-dependent simulation cases. Simulation also reveals how the fossil field was trapped during the interaction and shows the coexistence of two pileup regions with opposite magnetic orientation, as well as the formation of a pair of new Alfven wings and tail disconnection during the magnetopause crossing process.

Thermal electron periodicities at 20RS in Saturn's magnetosphere

Cassini fields and particles observations show clear evidence of periodic phenomena in Saturns magnetosphere. Periodicities have been observed in kilometric radio emissions, total electron density (in the inner magnetosphere), magnetic fields, and energetic particles (in the outer magnetosphere). In this letter the first analysis of periodicities in thermal electron densities in Saturns outer magnetosphere are presented. Plasma sheet electron densities and temperatures at 20 +/- 2 R-S in Saturns magnetosphere are studied and examined as a function of SLS3 longitude. Evidence for a density minimum at 170 degrees is presented which is in excellent agreement with total electron density results in the 3-5 R-S range. The density asymmetry is interpreted as the result of a periodic plasma sheet motion where the northward offset of the plasma sheet varies with longitude hence producing a density modulation in the equatorial plane. The effect of magnetospheric compressions on the dayside density asymmetry are discussed.

Energy deposition processes in titan's upper atmosphere and its induced magnetosphere

Most of Titans atmospheric organic and nitrogen chemistry, aerosol formation, and atmospheric loss are driven from external energy sources such as Solar UV, Saturns magnetosphere, solar wind and galactic cosmic rays. The Solar UV tends to dominate the energy input at lower altitudes of approximately 1100 km but which can extend down to approximately 400 km, while the plasma interaction from Saturns magnetosphere, Saturns magnetosheath or solar wind are more important at higher altitudes of approximately 1400 km, but the heavy ion plasma [O(+)] of approximately 2 keV and energetic ions [H(+)] of approximately 30 keV or higher from Saturns magnetosphere can penetrate below 950km. Cosmic rays with energies of greater than 1 GeV can penetrate much deeper into Titans atmosphere with most of its energy deposited at approximately 100 km altitude. The haze layer tends to dominate between 100 km and 300 km. The induced magnetic field from Titans interaction with the external plasma can be very complex and will tend to channel the flow of energy into Titans upper atmosphere. Cassini observations combined with advanced hybrid simulations of the plasma interaction with Titans upper atmosphere show significant changes in the character of the interaction with Saturn local time at Titans orbit where the magnetosphere displays large and systematic changes with local time. The external solar wind can also drive sub-storms within the magnetosphere which can then modify the magnetospheric interaction with Titan. Another important parameter is solar zenith angle (SZA) with respect to the co-rotation direction of the magnetospheric flow. Titans interaction can contribute to atmospheric loss via pickup ion loss, scavenging of Titans ionospheric plasma, loss of ionospheric plasma down its induced magnetotail via an ionospheric wind, and non-thermal loss of the atmosphere via heating and sputtering induced by the bombardment of magnetospheric keV ions and electrons. This energy input evidently drives the large positive and negative ions observed below approximately 1100 km altitude with ion masses exceeding 10,000 daltons. We refer to these ions as seed particles for the aerosols observed below 300 km altitude. These seed particles can be formed, for example, from the polymerization of acetylene (C2H2) and benzene (C6H6) molecules in Titans upper atmosphere to form polycyclic aromatic hydrocarbons (PAH) and/or fullerenes (C60). In the case of fullerenes, which are hollow spherical carbon shells, magnetospheric keV [O(+)] ions can become trapped inside the fullerenes and eventually find themselves inside the aerosols as free oxygen. The aerosols are then expected to fall to Titans surface as polymerized hydrocarbons with trapped free oxygen where unknown surface chemistry can take place.

Dynamical and magnetic field time constants for Titan's ionosphere: Empirical estimates and comparisons with Venus

plasma flow speed relative to the neutral gas speed is approximately 1 m s −1 near an altitude of 1000 km and 200 m s −1 at 1500 km. For comparison, the thermospheric neutral wind speed is about 100 m s −1 . The ionospheric plasma is strongly coupled to the neutrals below an altitude of about 1300 km. Transport, vertical or horizontal, becomes more important than chemistry in controlling ionospheric densities above about 1200–1500 km, depending on the ion species. Empirical estimates are used to demonstrate that the structure of the ionospheric magnetic field is determined by plasma transport (including neutral wind effects) for altitudes above about 1000 km and by magnetic diffusion at lower altitudes. The paper suggests that a velocity shear layer near 1300 km could exist at some locations and could affect the structure of the magnetic field. Both Hall and polarization electric field terms in the magnetic induction equation are shown to be locally important in controlling the structure of Titan’s ionospheric magnetic field. Comparisons are made between the ionospheric dynamics at Titan and at Venus.