M. Bouhram’s research while affiliated with Max Planck Institute for Physics and other places

A new fluid theory in the guiding-center and gyrotropic approximation derivable from the ensemble-averaged Vlasov-Maxwell equations that included the effect of wave-particle interactions for weakly turbulent, weakly inhomogeneous, nonuniformly magnetized plasma was recently given by Jasperse, Basu, Lund, and Bouhram [Phys. Plasmas 13, 072903 (2006) ]. In that theory, the particles are transported in one spatial dimension (the distance s along the magnetic field) but the turbulence is two-dimensional. In this paper, which is intended as a sequel, the above theory is used for quasisteady conditions to find: (1) a new formula for the perpendicular ion-heating rate per unit volume i⊥(s), where i⊥(s) decreases for large s by what we call the “finite ion gyroradius effect”; and (2) a new formula for the perpendicular ion temperature at low altitudes, Ti⊥(s). Parametrized calculations for Ti⊥(s) are also given.

In this paper, a new fluid theory is given in the guiding-center and gyrotropic approximation which is derivable from the Vlasov-Maxwell equations. The theory includes the effect of wave-particle interactions for the weakly turbulent, weakly inhomogeneous, nonuniformly magnetized plasma, and it is applicable to a variety of space and laboratory plasmas. It is assumed that the turbulence is random and electrostatic, and that the velocity-space Fokker-Planck operator can be used to calculate the correlation functions that describe the wave-particle interactions. Conservation laws are derived that relate the low-order velocity moments of the particle distributions to the turbulence. The theory is based on the work of Hubbard [Proc. R. Soc. London, Ser. A 260, 114 (1961)] and Ichimaru and Rosenbluth [Phys. Fluids 13, 2778 (1970)] . In the work presented here, the idea is proposed that the fluid equations can be solved (1) by using measurements of the turbulence to specify the electric-field fluctuations; and (2) by using measurements of the low-order velocity moments to specify the initial and boundary conditions.

The results of a statistical study of oxygen ion outflow using Cluster data obtained at high altitude above the polar cap is reported. Moment data for both hydrogen ions (H⁺) and oxygen ions (O⁺) from 3 years (2001-2003) of spring orbits (January to May) have been used. The altitudes covered were mainly in the range 5–12 R_E geocentric distance. It was found that O⁺ is significantly transversely energized at high altitudes, indicated both by high perpendicular temperatures for low magnetic field values as well as by a tendency towards higher perpendicular than parallel temperature distributions for the highest observed temperatures. The O⁺ parallel bulk velocity increases with altitude in particular for the lowest observed altitude intervals. O⁺ parallel bulk velocities in excess of 60 km s^-1 were found mainly at higher altitudes corresponding to magnetic field strengths of less than 100 nT. For the highest observed parallel bulk velocities of O⁺ the thermal velocity exceeds the bulk velocity, indicating that the beam-like character of the distribution is lost. The parallel bulk velocity of the H⁺ and O⁺ was found to typically be close to the same throughout the observation interval when the H⁺ bulk velocity was calculated for all pitch-angles. When the H⁺ bulk velocity was calculated for upward moving particles only the H⁺ parallel bulk velocity was typically higher than that of O⁺. The parallel bulk velocity is close to the same for a wide range of relative abundance of the two ion species, including when the O⁺ ions dominates. The thermal velocity of O⁺ was always well below that of H⁺. Thus perpendicular energization that is more effective for O⁺ takes place, but this is not enough to explain the close to similar parallel velocities. Further parallel acceleration must occur. The results presented constrain the models of perpendicular heating and parallel acceleration. In particular centrifugal acceleration of the outflowing ions, which may provide the same parallel velocity increase to the two ion species and a two-stream interaction are discussed in the context of the measurements.

1] The first pass of the Cassini orbiter near the A and B rings of Saturn, formed mainly by H 2 O ice particles, revealed the presence of an ionosphere composed of O + and O 2 + ions. Such a result suggests the existence of an atmospheric halo made up of molecular oxygen surrounding the rings. It is produced by solar UV radiation-induced decomposition of ice releasing molecular oxygen which does not stick on the surface at the relevant temperatures. A Monte Carlo model of the atmosphere/ionosphere ring system that uses test-particles and incorporates chemical processes and transport of both neutrals and plasma ions is developed. Published ion data from the ion mass spectrometer (IMS) experiment were used as constraint and the model provides a very satisfactory fit between simulated O + and O 2 + ion densities and those measured along Cassini trajectory., A test-particle model of the atmosphere/ionosphere system of Saturn's main rings, Geophys. Res. Lett., 33, L05106, doi:10.1029/2005GL025011.

During its insertion on an orbit around Saturn, the Cassini orbiter flew above the A and B rings and observations have revealed the presence of an ionosphere composed of O+ and O2+ ions. These observations can be explained by the presence around the rings of an atmospheric halo made up of molecular oxygen produced by the photolytic decomposition of the ice which constitutes the grains. At the temperature of the ring particles, oxygen molecules do not stick on the ice and thus are not lost during collisions with grains. A model of the formation of the ionospheric halo is presented based on a test-particle approach to describe the transport and the physical and chemical mechanisms that play an essential role. This model provides a very satisfactory fit with the data. To cite this article: M. Bouhram et al., C. R. Physique 7 (2006).

Molecular oxygen produced by the decomposition of icy surfaces is ubiquitous in Saturn's magnetosphere. A model is described for the toroidal O2 atmosphere indicated by the detection of and O+ over the main rings. The O2 ring atmosphere is produced primarily by UV photon-induced decomposition of ice on the sunlit side of the ring. Because O2 has a long lifetime and interacts frequently with the ring particles, equivalent columns of O2 exist above and below the ring plane with the scale height determined by the local ring temperature. Energetic particles also decompose ice, but estimates of their contribution over the main rings appear to be very low. In steady state, the O2 column density over the rings also depends on the relative efficiency of hydrogen to oxygen loss from the ring/atmosphere system with oxygen being recycled on the grain surfaces. Unlike the neutral density, the ion densities can differ on the sunlit and shaded sides due to differences in the ionization rate, the quenching of ions by the interaction with the ring particles, and the northward shift of the magnetic equator relative to the ring plane. Although O+ is produced with a significant excess energy, is not. Therefore, should mirror well below those altitudes at which ions were detected. However, scattering by ion–molecule collisions results in much larger mirror altitudes, in ion temperatures that go through a minimum over the B-ring, and in the redistribution of both molecular hydrogen and oxygen throughout the magnetosphere. The proposed model is used to describe the measured oxygen ion densities in Saturn's toroidal ring atmosphere and its hydrogen content. The oxygen ion densities over the B-ring appear to require either significant levels of UV light scattering or ion transmission through the ring plane.

The first pass of the Cassini probe in the vicinity of Saturn, above the E-ring, put in evidence a plasma consisting of water group ions with a small nitrogen bearing ion component (3%). Using a simple model for the transport of ions along Saturn's magnetospheric field lines, we show that these ions can be traced back to the Enceladus torus. Such a result can be explained by the existence in this icy satellite, which was observed to be geologically active during a close fly-by, of volatile components such as ammonia NH3, or by the previous implantation of Nitrogen of external origin on its surface.

Plasma measurements during Cassini's orbit insertion on July 1, 2004 revealed the + presence of an ionosphere above the A and B rings with O2 and O+ as major ions, suggesting the existence of a toroidal atmosphere made up of molecular oxygen. It is likely produced by radiation-induced decomposition of ice releasing both H2 and O2 molecules which do not stick on the surface at the relevant temperatures (80-100 K). Here, we present results of a study based on a hybrid model of the neutral and plasma tori that uses a test-particle approach for ions and neutrals. The model makes use of Monte Carlo techniques and takes into account chemical processes that lead to plasma creation such as photo-ionization and charge exchange. Key features of the plasma transport along Saturn's corotating magnetic field lines include the impact/transmission probability for plasma particles to pass through the rings, taking into account the electrostatic charging of ring particles, and the ambipolar electric field, which preserves electric neutrality above the ring plane. We will account for escape of oxygen and hydrogen from the ring system and chemical recombination of the ring particle surfaces. Published plasma data from CAPS are used as constraints or inputs to the model.

Recently we have developed a fluid theory in the guiding center and gyrotropic approximation that includes the effect of wave-particle interactions for the turbulent weakly inhomogeneous non-uniformly magnetized plasma and is applicable to the field-aligned Birkeland current system of the earth s magnetosphere We assume that the turbulence is random and electrostatic and that we can calculate the correlation functions that describe the wave-particle interactions from the velocity-space Fokker-Planck operator From FAST satellite data of the electric field fluctuations we can determine the momentum and energy transfer rates in the fluid equations due to the plasma turbulence This model also uses satellite data as a boundary condition we solve the fluid equations downward to the estimated double layer altitude and upward into the magnetosphere The results of this calculation depend to a significant extent on how the system of equations is closed We seek to verify the closure relations in our model by comparing the high-altitude predictions of a run based on FAST data at sim 4000 km with measurements from Polar at sim 8 R E on nearly the same flux tube We can run our model with different possible closures in order to see which one best predicts the high-altitude observations

The first pass of the Cassini probe in the vicinity of Saturn, above the E-ring, demonstrated a plasma consisting of water group ions (H+, O+, OH+, H2O+) with a small N+ ion component (3%). Using a simple model for the transport of magnetospheric ions, we show that the N+ ions can be traced back to the Enceladus satellite. Such a result can be explained by the existence in this icy satellite, supposed to be still geologically active, of volatile components such as ammonia NH3, or by the previous implantation of N+ ions of external origin on its surface. To cite this article: M. Bouhram et al., C. R. Physique 6 (2005).