Ronan Modolo

MAVEN observations of the response of Mars to an interplanetary coronal mass ejection

Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

The spatial distribution of planetary ion fluxes near Mars observed by MAVEN

We present the results of an initial effort to statistically map the fluxes of planetary ions on a closed surface around Mars. Choosing a spherical shell ~1000 km above the planet, we map both outgoing and incoming ion fluxes (with energies >25 eV) over a 4 month period. The results show net escape of planetary ions behind Mars and strong fluxes of escaping ions from the northern hemisphere with respect to the solar wind convection electric field. Planetary ions also travel toward the planet, and return fluxes are particularly strong in the southern electric field hemisphere. We obtain a lower bound estimate for planetary ion escape of ~3 × 1024 s−1, accounting for the ~10% of ions that return toward the planet and assuming that the ~70% of the surface covered so far is representative of the regions not yet visited by Mars Atmosphere and Volatile EvolutioN (MAVEN).

Ionospheric storms on Mars: Impact of the corotating interaction region

Measurements made by the ASPERA-3 and MARSIS experiments on Mars Express have shown, for the first time, that space weather effects related to the impact of a dense and high pressure solar wind (corotating interaction region) on Mars cause strong perturbations in the martian induced magnetosphere and ionosphere. The magnetic barrier formed by pile-up of the draped interplanetary magnetic field ceases to be a shield for the incoming solar wind. Large blobs of solar wind plasma penetrate to the magnetosphere and sweep out dense plasma from the ionosphere. The topside martian ionosphere becomes very fragmented consisting of intermittent cold/low energy and energized plasmas. The scavenging effect caused by the intrusions of solar wind plasma clouds enhances significantly (by a factor of ≥10) the losses of volatile material from Mars.

Early MAVEN Deep Dip campaign reveals thermosphere and ionosphere variability

The Mars Atmosphere and Volatile Evolution (MAVEN) mission, during the second of its Deep Dip campaigns, made comprehensive measurements of martian thermosphere and ionosphere composition, structure, and variability at altitudes down to ~130 kilometers in the subsolar region. This altitude range contains the diffusively separated upper atmosphere just above the well-mixed atmosphere, the layer of peak extreme ultraviolet heating and primary reservoir for atmospheric escape. In situ measurements of the upper atmosphere reveal previously unmeasured populations of neutral and charged particles, the homopause altitude at approximately 130 kilometers, and an unexpected level of variability both on an orbit-to-orbit basis and within individual orbits. These observations help constrain volatile escape processes controlled by thermosphere and ionosphere structure and variability.

A global hybrid model for Titan's interaction with the Kronian plasma: Application to the Cassini Ta flyby

The interaction between the corotating magnetospheric plasma of Saturn and the exosphere of Titan is investigated by means of a three-dimensional and multispecies hybrid simulation model coupling charged and neutral species via three ionizing mechanisms: the absorption of extreme ultraviolet solar photons, the impacts of magnetospheric electrons, and the charge exchange reactions between ions and neutral atoms or molecules. The simulation model includes the low and energetic components of the magnetospheric plasma, the main exospheric neutral species (molecular hydrogen and nitrogen and methane), and the atmospheric slowing down of charged particles penetrating below the exobase. Ionization rates of the exospheric species are computed as consistently as possible for each of the three ionizing mechanisms by making use of the relevant local number densities and cross sections or ionization frequencies. This model is thus able to provide a priori estimates of the escaping fluxes of exospheric ionic species and to separate for the contributions of the different ionization sources. A simulation run has been made for the conditions encountered by spacecraft Cassini during flyby Ta of Titan on 26 October 2004. Results are presented to characterize the main features of the simulated plasma environment of Titan: the induced magnetic tail and the flow of magnetospheric plasma around Titan, as well as the wake and the acceleration of the planetary plasma. Considering the coarse spatial resolution of the present simulation, these features are in reasonable agreement with in situ plasma measurements made by spacecraft Cassini.

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.

Electron density and temperature measurements in the cold plasma environment of Titan : Implications for atmospheric escape

We present electron temperature and density measurements of Titans cold ionospheric plasma from the Langmuir probe instrument on Cassini from 52 flybys. An expression of the density as a function of temperature is presented for altitudes below two Titan radii. The density falls off exponentially with increased temperature as log(ne) = −2.0log(Te) + 0.6 on average around Titan. We show that this relation varies with location around Titan as well as with the solar illumination direction. Significant heating of the electrons appears to take place on the night/wake side of Titan as the density-temperature relation is less steep there. Furthermore, we show that the magnetospheric ram pressure is not balanced by the thermal and magnetic pressure in the topside ionosphere and discuss its implications for plasma escape. The cold ionospheric plasma of Titan extends to higher altitudes in the wake region, indicating the loss of atmosphere down the induced magnetospheric tail.

Ionization processes in the atmosphere of Titan - II. Electron precipitation along magnetic field lines

Context. The Cassini probe regularly passes the vicinity of Titan, providing new insights into particle precipitation by use of its electron and ion spectrometers. A discrepancy between precipitation models and observations of electron fluxes has been found. This discrepancy was suspected to be caused by the geometry of the magnetic field. Aims. In this article, we compute the electron impact ionization in the nightside ionosphere of Titan, assuming non-trivial geometry for the magnetic field lines. Methods. We use the TransTitan model, modified to take into account the magnetic field line geometry in the nightside, and we compare these results with the electron flux measurements during the T5 fly-by of Cassini. We use several magnetic field line geometries, including one produced by hybrid simulations. Results. The geometry of the lines implies a longer path of the electron inside the atmosphere of Titan. The electron fluxes are therefore modified considerably compared to the vertical precipitation hypothesis. At an altitude of 1200 km, the electron flux can be divided up to ten times with a field line resulting from hybrid simulation. Thanks to the use of more accurate field lines, the model reproduces the experiment well without any further adjustment of the precipitated measured electron flux. Conclusions. Several hypothesis had been suggested to explain the discrepancies between the different models and the observation of the electron flux during the T5 fly-by of Cassini. Our approach shows that the most probable explanation is the magnetic field line geometry. This work shows that the computation of ion production by electron impact in the atmosphere of Titan needs the consideration of both magnetic field and the input electron fluxes. Based on these considerations, our model can compute the conditions for future fly-by, and could be used to compare models with experiments.

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 ~160 km 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 - 500 km, 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 (~130 km) 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.

Cold ionospheric plasma in Titan's magnetotail

The interaction between Titan and the corotating Saturnian plasma forms an induced magnetosphere with an elongated Alfven-wing-style magnetotail. On 26 December 2005, the Cassini spacecraft flew th ...