Bruno P. Besser

A global MHD solar wind model with WKB Alfvén waves: Comparison with Ulysses data

We use a steady state global axisymmetric MHD model to reproduce quantitatively the Ulysses observations during its first fast latitude traversal in 1994–1995. In particular, we are able to account for the transformation of a surface dipole magnetic field near the Sun into the configuration observed at large heliocentric distances. The MHD equations are solved by combining a time relaxation numerical technique with a marching-along-radius method. We assume that Alfven waves, propagating outward from the Sun, provide additional heating and acceleration to the flow. Only solutions with waves reproduce the plasma parameters observed in the high-latitude fast solar wind. We show that the meridional distribution of solar wind plasma and magnetic field parameters is dominated by two processes. First, inside ∼24 R⊙ both the plasma velocity and magnetic field relax toward a latitude-independent profile outside the equatorial current sheet (where magnetic forces dominate over thermal and wave gradient forces). Second, outside ∼24 R⊙ there is another meridional redistribution due to a poleward thermal pressure gradient that produces a slight poleward gradient in the radial velocity and an equatorward gradient in the radial component of the magnetic field. We reproduce the observed bimodal structure and morphology of both fast and slow wind and show that computed parameters are generally in agreement with both in situ data and conditions inferred to be characteristic of the solar corona.

Time-dependent localized reconnection of skewed magnetic fields

We describe and analyze a model for time-varying, localized reconnection in a current sheet with skewed magnetic field orientations on opposite sides. As in Petscheks description, disruption is initiated in a localized part of the current sheet known as the diffusion region, and the disturbances are subsequently propagated into the system at large through magnetohydrodynamic (MHD) waves. The MHD waves therefore play the dominant role in energy conversion, and collectively they form an outflow for plasma streaming toward the current sheet and a field reversal region joining magnetic field lines from opposite sides. We restrict the analysis to an incompressible plasma, in which case the Alfven wave and the slow shock merge to form shocks bounding the field reversal or outflow region, and to the case of weak reconnection, which implies that the reconnection electric field is much smaller than the product of the characteristic values of the external field strength and Alfven speed. It is then possible to perform a perturbation analysis of the MHD equations which govern the plasma and field behavior. The analysis can be formulated as a mixture of three well-known problems. The problem of determining the appropriate combination of MHD waves corresponds to the Riemann problem, which also specifies the tangential field and flow components in the field reversal region. These results, it is important to note, are not sensitive to variations in the reconnection rate. Reconnection also acts as a source of surface waves, and their analysis determines the behavior of the perpendicular field and flow components and the shape of the shocks. Lastly, the field reversal region can be considered as a thin boundary layer in our treatment, and the external disturbances can therefore be solved in a way similar to the flow around a thin aerofoil. The model presented here can be applied to the Earths magnetopause, where reconnection is considered to be the dominant process coupling the solar wind and the magnetosphere. In particular, the results can be used to interpret different manifestations of reconnection such as accelerated plasma flows along the magnetopause and flux transfer events.

A study of the propagation of electromagnetic waves in Titan's atmosphere with the TLM numerical method

Abstract A numerical modeling of the electromagnetic characteristics of Titan’s atmosphere is carried out by means of the TLM numerical method, with the aim of calculating the Schumann resonant frequencies of Saturn’s satellite. The detection and measurement of these resonances by the Huygens probe, which will enter Titan’s atmosphere at the beginning of 2005, is expected to show the existence of electric activity with lightning discharges in the atmosphere of this satellite. As happens with the Schumann frequencies on Earth, losses associated with electric conductivity will make these frequencies lower than theoretically expected, the fundamental frequency being located between 11 and 15 Hz. This numerical study also shows that the strong losses associated to the high conductivity make it impossible for an electromagnetic wave with a frequency of 10 MHz or lower, generated near the surface, to reach the outer part of Titan’s atmosphere.

Model computations of Schumann resonance on Titan

Abstract We model the global electromagnetic (Schumann) resonance in the atmosphere of Titan. Parameters of conductivity of the lower ionosphere were implemented taken from existing aeronomic models of Titans atmosphere. Two exponential conductivity profiles were constructed: one of them suggests favorable conditions for Schumann resonance and the other models considerable attenuation in the ionospheric plasma. Peak frequencies and Q-factors of resonance were computed as well as resonance spectra for the signals arriving from individual vertical lightning discharges and from strokes uniformly distributed over the planet. The models show that detection of Schumann resonance on Titan is feasible, especially in favorable conditions. Possible applications of Schumann resonance in the studies of Titans lightning activity are outlined.

Time-dependent reconnection in a current sheet with velocity shear

Abstract Magnetic field reconnection is a macroscopic energy-conversion and transport process which operates in current sheets. Here we investigate analytically a physico-mathematical model of reconnection in a 2-D configuration consisting of a current sheet separating two different plasma regions with antiparallel magnetic field orientations. Reconnection is initiated in a localized region of the current sheet known as the diffusion region. The disruption of the current sheet generates MHD waves, which propagate the local disturbances into the system at large. We apply the MHD equations to describe and analyse the macroscopic response of the plasma and magnetic field to an imposed reconnection rate, which is generally a function of time. The dissipative process leading to disruption is not specified, and instead an arbitrary reconnection rate is used as an initial-boundary condition for solving the MHD equations. Analysis is restricted to an incompressible plasma, and to the case of weak reconnection, which implies that the magnetic field component perpendicular to the current sheet remains small relative to the field strength specified in the initial configuration. Time dependency and the inclusion of different plasma parameter values and field strengths on opposite sides of the current sheet lead to new features which are not evident in Petscheks analysis of steady-state reconnection in a symmetric current sheet configuration. These features include an asymmetric evolution of the outflow region on opposite sides of the current sheet, and a shift of the reconnection line into the region of higher field strength. The outflow region is partially bounded by a tangential discontinuity as a result of the different propagation speeds of the shocks and the surface waves. Once the reconnection rate drops to zero, i.e. reconnection is no longer active, the outflow region splits at the reconnection line and propagates like two solitary waves moving in opposite directions along the current sheet. The previous shift of the reconnection line now corresponds to a displacement of the current sheet downstream of the outflow region. Although no more flux is reconnected at this stage, the outflow region continues to grow in size and gathers up an increasingly large volume of reconnected plasma. The growth is eventually confined to a stretching of the outflow region along the current sheet. With velocity shear there is an additional asymmetry in the evolution on opposite sides of the reconnection line, and the conditions for the appearance and outward propagation of MHD waves are shown to be related to the criterion for Kelvin-Helmholtz stability of the current sheet.

The effective polytropic index in a magnetized plasma

The effective polytropic index γeƒƒ in a magnetized anisotropic plasma is obtained in the frame of one-fluid, double-adiabatic magnetohydrodynamics. The value of γeƒƒ is found to depend on the characteristics of the plasma flow, and on the temperature anisotropy. It is shown that the most probable value of γeƒƒ is 1.4–1.9 at the bow shock, and it may be less than unity in the magnetopause region.

Magnetosheath model in the Chew–Goldberger–Low approximation

Parameters of the solar wind plasma and magnetic field in the magnetosheath are calculated for an anisotropic plasma model in the Chew–Goldberger–Low approximation. It is shown that in the case when the energy transfer between the perpendicular and parallel (with respect to the magnetic field) degrees of freedom is absent, the resulting temperature anisotropy may significantly affect the plasma density and magnetic field intensity profiles across the magnetosheath. However, in this case, the value of the temperature anisotropy (the ratio of the perpendicular to the parallel component of the temperature with respect to the magnetic field, T⊥/T‖) becomes unrealistic high. To bring agreement between the model values of the temperature anisotropy and experimental data, the existence of an intensive proton pitch-angle diffusion is assumed. In the case when the temperature anisotropy relaxation time is much smaller than the time taken by the solar wind plasma to move from the bow shock to the magnetopause, one ...

Electromagnetic wave propagation in the surface-ionosphere cavity of Venus

The propagation of extremely low frequency (ELF) waves in the Earth surface-ionosphere cavity and the properties of the related Schumann resonances have been extensively studied in order to explain their relation with atmospheric electric phenomena. A similar approach can be used to understand the electric environment of Venus and, more importantly, search for the evidence of possible atmospheric lightning activity, which remains a controversial issue. We revisit the available models for ELF propagation in the cavity of Venus, recapitulate the similarities and differences with other planets, and present a full wave propagation finite element model with improved parameterization. The new model introduces corrections for refraction phenomena in the atmosphere; it takes into account the day-night asymmetry of the cavity and calculates the resulting eigenfrequency line splitting. The analytical and numerical approaches are validated against the very low frequency electric field data collected by Venera 11 and 12 during their descents through the atmosphere of Venus. Instrumentation suitable for the measurement of ELF waves in planetary atmospheres is briefly addressed.

Shumann resonances and electromagnetic transparence in the atmosphere of Titan

Abstract Among the multiple questions that the CASSINI/HUYGENS mission tries to answer is the likelihood of electric discharges in Titans atmosphere. The instruments “Huygens Atmospheric Structure Instrument” and “Radio and Plasma Wave Science” will probe the electromagnetic emissions during the Huygens descent and Cassini flybys, respectively. Although no lightning was observed during Voyagers encounters with Titan in 1980 and 1981, this does not exclude the existence of lightning phenomena. Recent investigations show that lightning discharges could occur in the lower atmosphere, such as the detection of methane condensation clouds in the troposphere and the theoretical prediction of an electric field that would be sufficient enough to cause lightning. We present a numerical model of Titans atmosphere with the aim of calculating the resonance frequencies and the atmospheric transparency to electromagnetic waves. The detection and measurement of these resonances, Schumann frequencies, by the Huygens probe, would show the existence of electric activity connected with lightning discharges in the atmosphere. As it happens with the Schumann frequencies of Earth, losses associated with the electric conductivity will make these frequencies to be lower than the theoretically predicted, the fundamental one being located between 11 and 15 Hz. An analytical study shows that the strong losses associated with the high conductivity make it impossible that an electromagnetic wave generated near the surface with a frequency of 10 MHz or lower reaches the outer part of Titans atmosphere. Therefore the detection of electromagnetic waves coming from Titans lower atmosphere by the RPWS instrument is very unlikely.

MHD-modelling of the magnetosheath

Abstract A short discussion of some problems of magnetosheath physics is presented. In particular, anisotropic MHD models of the magnetosheath are discussed. A method to estimate the value of the characteristic relaxation time (τ) of the proton temperature anisotropy from experimental data is proposed. Another problem considered in the review concerns the conditions of formation of a magnetic barrier within the magnetosheath. The existing controversy in this question is explained in the authors’ opinion by different definitions of the term “magnetic barrier” used in papers by Pudovkin et al. J. Geophys. Res., 87 (1982) 8131; Ann. Geophys. 13 (1995) 828) and Phan et al. J. Geophys. Res. 99 (1994) 121). Experimental data on the magnetic barrier dependence on the IMF orientation are discussed.