Horst Fichtner

Physical reasons and consequences of a three-dimensionally structured heliosphere

In this article we have discussed reasons both of solar and of interstellar origin giving rise to a pronounced three-dimensional structure of the expanding solar wind and thus of the global configuration of the heliosphere. Our present observational knowledge on these structurings is reviewed, and all attempts to theoretically model these solar wind structures are critically analysed with respect to their virtues and flaws. It is especially studied here by what mechanisms interstellar imprints on the actual type of solar wind expansion can be envisaged. With concern to this aspect it hereby appears to be of eminent importance that the solar system maintains a relative motion with a submagnetosonic velocity of about 23km/sec with respect to the ambient magnetized interstellar medium corresponding to a magnetosonic Mach number of about 0.5.A heliopause closing the distant heliospheric cavity within a solar distance of about 100AU on the upwind side and opening it into an largely extended tail on the downwind side results as a first consequence from this relative motion. As a second consequence an asymmetric heliospheric shockfront with upwind distances smaller than downwind distances by ratios between 1/3 and 2/3 is most likely provoked which gives rise to at least two important upwind-downwind asymmetric processes influencing the supersonic solar wind expansion downstream from the shock: the anomalous cosmic ray diffusion into the solar wind, and high energetic jet electrons originating at the shock and moving inwards up to an inner critical point at around 20AU. As we shall demonstrate both processes are influencing the solar wind expansion beyond 20AU, however, more efficiently in the upwind hemisphere as compared to the downwind hemisphere. In the region inside 20AU other mechanisms are operating to propagate the interstellar imprint on the solar wind expansion further downstream into the inner heliosphere because here even the original solar wind electrons, in view of the solar wind bulk velocities, behave as a subsonic plasma constituent which can modify the solar wind solutions by means of an appropriate detuning of the circumsolar electric polarisation field. We give quantitative estimates for these effects.What concerns the theory of a solar wind expansion into a counterflowing ambient interstellar medium, some flaws of the present theoretical attempts are identified impeding that the interstellar influence on the actual solar wind solutions can become visible. We thus conclude that there is a clear need for three-dimensional and time-dependent solar wind models with a free outflow geometry taking into account the multisonicity of the solar wind plasma with different eigenmodes for a perturbation propagation.

The influence of the anomalous cosmic-ray component on the dynamics of the solar wind

It is well known that both the galactic and anomalous cosmic rays show positive intensity gradients in the outer heliosphere which are connected with corresponding pressure gradients. Due to an efficient dynamical coupling between the solar wind plasma and these highly energetic media by means of convected MHD turbulences, there exists a mutual interaction between these media. As one consequence of this scenario the enforced pressure gradients influence the distant solar wind expansion. Here we concentrate in our theoretical study on the interaction of the solar wind only with the anomalous cosmic-ray component. We use the standard two-fluid model in which the cosmic-ray fluid modifies the solar wind flow via the cosmic-ray pressure gradient. Then we derive numerical solutions in the following steps: first we calculate an aspherical pressure distribution for the anomalous cosmic rays, describing their diffusion in an unperturbed radial solar wind. Second, we then consider the perturbation of the solar wind flow due to these induced anomalous cosmic-ray pressure gradients. Within this context we especially take account of the action of a non-spherical geometry of the heliospheric shock which may lead to pronounced upwinddownwind asymmetries in the pressures and thereby in the resulting solar wind flows. As we can show in our model, which fits the available observational data, radial decelerations of the distant solar wind by between 5 to 11% are to be expected, however, the deviations of the bulk solar wind flow from the radialdirections are only slightly pronounced.

Exact algebraic dispersion relations for wave propagation in hot magnetized plasmas

A new model is presented for the treatment of wave propagation along an external magnetic field in a hot collisionless plasma. The analysis is based on the so-called polynomial distribution functions along the magnetic field, and takes account of enhanced fractions of high-energy particles, which are characteristic of rarefied and magnetized astrophysical plasmas, in comparison with the bi-Maxwellian distributions. These new distributions permit the derivation of general dispersion relations that are exactly valid for waves with Im (ω) > 0, and represent good approximations for those with Im (ω) > 0. Furthermore, the explicit form of the dispersion relations is shown to be valid for distribution functions of different shapes. Because of their algebraic structure, the solution of the dispersion relations can be shown to be equivalent to the determination of the roots of complex-valued polynomials. The cold plasma, the Maxwellian plasma and the so-called quasi-Maxwellian plasma appear in this formalism as asymptotic and special cases. The reliability of the model is demonstrated with the calculation of dispersion curves, growth and damping rates for several standard modes, and by comparing it with previous calculations carried out using explicit Maxwellian distributions. Finally, the tendency of the solar wind to generate ion-cyclotron waves is investigated as a first, new application.

The influence of pick-up ion-induced wave pressures on the dynamics of the mass-loaded solar wind

The solar wind at larger distances is known to be a multicomponent plasma. The different components, solar ions, pick-up ions, and anomalous ions, are without collisional coupling but they are all coupled to the intrinsic wave turbulences by nonlinear wave-particle interactions. Since quite a long time it is not understood why dynamical processes associated with the loading of the primary solar wind by secondary pick-up ions neither lead to a recognizable heating nor to a deceleration of the solar wind at larger distances. While the inefficient heating seems to be explained by the fact that pick-up ions do not assimilate quickly enough to the solar wind distribution function, the unobservable deceleration of the distant solar wind always remained mysterious. Different from all theoretical approaches up to now, here we intend to show that the wave-induced pick-up ion pressure has to be introduced into the equations of motion in an adequate non-polytropic form to correctly describe the multicomponent plasma dynamics. If this is done it becomes clear that the deceleration of the solar wind is considerably reduced or even vanishing.

Cassini as a heliospheric probe - the potential of pick-up ion measurements during its cruise phase

Abstract Measurements of the spatial distributions of pick-up ions can be utilized to infer important information about the structure of the heliosphere. First, a determination of the symmetry axis of flux distributions of pick-up ions not experiencing any significant filtration of their neutral parent atoms in the heliospheric interface represents a new method to derive the orientation of the upwind-downwind axis of the heliosphere from observation. Second, a comparison of pick-up ion flux distributions resulting from neutrals both filtered and not filtered during their passage through the heliospheric interface might provide insight into the geometry of the interface and, subsequently, give information about the strength and orientation of the local interstellar magnetic field. A feasibility study for carrying out such measurements with the Cassini spacecraft is presented, and the most promising pick-up ion candidates are identified.

THREE-DIMENSIONAL STRUCTURING OF THE DISTANT SOLAR WIND BY ANOMALOUS COSMIC RAY PARTICLES

It is well known that both the galactic and the anomalous cosmic rays show positive intensity gradients in the outer heliosphere which are connected with corresponding antiradial pressure gradients. Due to an efficient dynamical coupling between the solar wind plasma and these highly energetic media, by means of a scattering at convected MHD wave turbulences, the latter diffuse through the low energetic solar wind plasma flow, and thus there exists a mutual dynamic interaction of these media. As a prime consequence of this scenario, the diffusion-induced pressure gradients in the cosmic ray distributions influence the distant solar wind expansion. In this paper we concentrate on the interaction of the solar wind with the anomalous cosmic ray component giving a consistent formulation of the system of mutually interacting media. Then we derive numerical solutions in the following steps: First we calculate an aspherical pressure distribution for the anomalous cosmic rays describing their diffusion in an unperturbed radial solar wind. Second we consider the perturbation of the solar wind flow due to these induced anomalous cosmic ray pressure gradients. Within this context we especially take account of the action of an aspherical geometry of the heliospheric shock which may lead to a pronounced upwind/downwind asymmetry in the pressure distribution, and thereby in the resulting solar wind flows. As we can show decelerations of the distant solar wind by between 5 to 11 percent are to be expected, however, deviations of the bulk solar wind flow from the radial directions are only weakly pronounced.

The interstellar wind as a cause for longitudinal solar wind asymmetries

Abstract We show here that the relative motion of the solar system with respect to the ambient interstellar plasma enforces a clearly pronounced upwind/downwind momentum flow asymmetry in the shocked solar wind. With the help of an analytic description of this subsonic flow regime taken from Parker (1963, Interplanetary Dynamical Processes . Interscience, New York) we were able to quantitatively determine the expected momentum flow asymmetries and the forces exerted on closed surfaces from which such a flow is originating. Assuming that the up/down asymmetry can be forwarded from the subsonic to the supersonic solar wind through the heliospheric shock by that wind component behaving subsonically everywhere, i.e. electrons, we arrive at corresponding asymmetric outer boundary conditions which the adapted supersonic solar wind solutions have to match. We shall prove in this paper that via the subsonic solar wind electrons, the solar wind expansion actually realized can be strongly influenced by the asymmetric outside LISM momentum flow conditions. Though this situation cannot be consistently treated with existing solar wind models, we nevertheless want to give an idea of its consequences. With the help of a conventional two fluid solar wind model we produce such solutions here that in a quantitative form show longitudinal solar wind asymmetries even in the region between the coronal basis and the Earths orbit. In order to confirm the existence of such asymmetries we propose to monitor the solar wind momentum flow along the orbit of the Earth. We also calculate the braking rate of the solar system connected with such a momentum flow.

The influence of interstellar medium on subsonic stellar motions

It is not a trivial problem to imagine how a spherical high-pressure balloon with supersonic gas jets leaving from pores densely distributed on its surface can be influenced by an ambient gas flow. The relative motion of such a balloon can be controlled by a corresponding rearrangement of the gas outflow into an aspherical configuration. A similar problem is connected with stars driving a supersonic stellar wind and moving relative to the interstellar medium. As we shall show, the adapted circumstellar flow leads to an upwind-downwind pressure asymmetry balancing the momentum loss that is braking such stars. The opposite process — i.e., acceleration — may occur if luminous stars are closely associated and their wind systems interfere with each other. This should lead to a mutual repulsion.