MARTIAN ULTRAVIOLET AURORA: RESULTS OF MODEL SIMULATIONS

We present recent modeling results based on observations performed with the UV spectrographs on board the Mars Express and MAVEN missions.Two types of aurora are discussed: the localized and transient discrete aurora and the more stable diffuse aurora observed during periods of active solar periods.CODYMA

Improvement of the MSIS 86 and DTM thermospheric models by investigating the geomagnetic effect

Based on measurements of the CACTUS accelerometer it has been found that during the recovery phase of geomagnetic disturbances the models are unable to describe properly the total density changes in the equatorial thermosphere. Introducing the geomagnetic activity index Dst instead of Kp as a model input parameter gives a much better description of the measurements. The residuals show a diurnal dependence, hinting partly at model errors in the diurnal effect (though MSIS 86 is much better than DTM in this respect), partly at a diurnal term in the geomagnetic effect

Investigation and modelling of an improved geomagnetic term for the CIRA '86 model at low latitudes

Total density data based on CACTUS microaccelerometer measurements — carried out between 230 and 700 km altitudes at low latitudes in the period 1975–1979 — have been compared to corresponding MSIS'86 = CIRA'86 model values. A Dst dependent geomagnetic term is presented with height dependent coefficients (hMSIS) to replace the inadequate Kp dependent term in the otherwise unchanged model. This new geomagnetic term is the generalisation of the previously published function to altitudes below 400 km

3D modeling of accretion disks in close binaries: the precessional spiral density wave

Three-dimensional numerical simulations of gas dynamics are used to study the flow pattern in a close binary system after it has reached the steady-state accretion regime. It is shown that an additional spiral density wave can exist in the inner parts of the cold accretion disk, where gas-dynamical perturbations are negligible. This spiral wave is due to the retrograde precession of the flowlines in the binary system. It is found that shape and position of a substantial part of the disk are specified by a precessional density wave. On timescales comparable to the orbital period, the precessional wave (and hence an appreciable fraction of the disk) will be virtually stationary in the observer’s frame, whereas the positions of other elements of the flow will vary due to the orbital rotation. The periodic variations of the positions of the disk and the bow shock formed when the inner parts of the circumbinary envelope flow around the disk result in variations of both the rate of angular-momentum transfer to the disk and the flow structure near the Lagrange point L3. All these factors lead to a periodic increase of the matter flow into the outer layers of the circumbinary envelope through the vicinity of L3

Kinetic Monte Carlo Model for the Precipitation of High-Energy Protons and Hydrogen Atoms into the Atmosphere of Mars Taking into Account the Measured Magnetic Field

Results of model computations of the interaction of the high-energy protons and hydrogen atoms (H/H+) precipitating into the Martian atmosphere are presented. These computations were performed using a modification of the kinetic Monte Carlo model developed earlier for the analysis of the data from the MEX/ASPERA-3 instrument onboard the Mars Express spacecraft and the MAVEN/SWIA instrument onboard the MAVEN spacecraft. In this modification of the model, an arbitrary (three-dimensional) structure of the magnetic field of Mars is taken into account for the first time. With local measurements of all three components of the magnetic field, not only the flux of protons penetrating into the atmosphere, but also the degradation of the H/H+ flux along the spacecraft orbit and the formation of upward fluxes of protons and hydrogen atoms scattered by the atmosphere, can now be described. A comparison of simulations and measurements of proton fluxes at low altitudes are used to infer the efficiency of charge exchange between the solar wind and the extended Martian hydrogen corona. It was found that the induced magnetic field plays a very important role in the formation of the proton flux back-scattered by the atmosphere and strongly controls its magnitude

The Martian diffuse aurora: a model of ultraviolet and visible emissions

A new type of Martian aurora, characterized by an extended spatial distribution, an altitude lower than the discrete aurora and electron precipitation up to 200 keV has been observed following solar activity on several occasions from the MAVEN spacecraft. We describe the results of Monte Carlo simulations of the production of several ultraviolet and violet auroral emissions for initial electron energies extending from 0.25 to 200 keV. These include the CO2+ ultraviolet doublet (UVD) at 288.3 and 289.6 nm and the Fox–Duffendack–Barker (FDB) bands, CO Cameron and Fourth Positive bands, OI 130.4 and 297.2 nm and CI 156.1 nm and 165.7 nm multiplets. We calculate the nadir and limb production rates of several of these emissions for a unit precipitated energy flux. Our results indicate that electrons in the range 50-200 keV produce maximum CO2+ UVD emission below 75 km, in agreement with the MAVEN observations. We calculate the efficiency of photon production per unit precipitated electron power. The strongest emissions are the CO2+ FDB, UVD and CO Cameron bands and the oxygen mission at 297.2 nm. The metastable a 3Π state which radiates the Cameron bands is deactivated by collisions below about 110 km. As a consequence, we show that the Cameron band emission is expected to peak at a higher altitude than the CO2+ UVD and FDB bands. Collisional quenching also causes the intensity ratio of the CO2+ UVD to CO Cameron bands to increase below ∼100 km in the energetic diffuse aurora.CODYMA

Nonthermal O(1S) and O(1D) populations in cometary atmospheres

Recent developments in the field of cometary science have motivated many studies dealing with the nucleus composition and mineralogy, and also with the photochemistry of the coma. In particular, ground based observations have shown that the visible oxygen emissions at 557.7 and 630 nm, both belonging to the Rosetta-VIRTIS-M passband, present different line profiles, pointing to specific photochemical processes. In this work, we present a Monte Carlo simulation of the O(1D) and O(1S) photochemistry including photodissociation of H2O, CO2 and CO, quenching, collisional thermalization and radiative decay. The model solves Boltzmann's integro differential equation including sources and sinks, as well as a prescribed expansion velocity of the coma. The energy distribution functions (EDF's) of O(1S) and O(1D) are computed at cometocentric distances ranging between 10 and 5000 km. We find that the EDF's of both O(1D) and O(1S) are strongly nonthermal, up to a degree that sharply varies with cometocentric distance, as thermalization is less efficient when the density of the dominant species is reduced. It follows that the Doppler profile of the visible radiations emitted by both species is non-gaussian in a frame of reference moving with the expanding coma. The nonthermal volume emission rate is then integrated along a set of chosen line of sights, accounting for the explicit Doppler profiles derived from the EDF's as well as the expansion motion, and the Doppler profile of the full coma is computed. It appears that most of the line width is due to the expansion motion, although the detailed line shape remains sensitive to the nonthermal nature of the EDF's. Our computation can then be compared with the line profiles observed from the ground with the UVES spectrograph mounted on the ESO-VLT