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Astron. Astrophys. 357, 268-282 (2000)

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A 5-fluid hydrodynamic approach to model the solar system-interstellar medium interaction

H.J. Fahr, T. Kausch and H. Scherer

Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received 25 October 1999 / Accepted 3 February 2000


Since about several decades now it is clearly recognized that the interaction of the solar system with the ambient interstellar medium flow primarily occurs on the basis of a hydrodynamic adaptation of two counterflowing fluids, the solar wind and the interstellar plasmas. The hydrodynamic nature of the interaction unavoidably invokes structures like an inner solar wind termination shock (TS), the heliopause and an outer interstellar bow shock with plasma sheath regions in between. Though the main outlines of this interaction scenario were established long ago, some debates about the location and geometry of these structures still continue up to the present. Fundamentally new aspects of this interaction problem have only quite recently appeared calling for new and more sophisticated calculations. The revisions of the earlier one-fluid interaction concept starts with the consideration of the neutral LISM H-atom gas passing through the solar system. At the occasion of ionizations of this neutral component a medium-energetic plasma component in form of keV-energetic pick-up ions (PUI`s) is emerging. This component changes the distant solar wind properties by mass-, momentum-, and energy-loading, as well as by wave generation and lowering the solar wind Mach numbers. Also it has to be taken into account that PUI`s serve as seed particles for a high-energetic plasma population with energies between 10 and 100 MeV/nuc called anomalous cosmic rays (ACR`s). This latter component together with galactic cosmic rays (GCR`s) not only modify the supersonic solar wind by means of their pressure gradients upstream and downstream of the termination shock but also, connected with the ACR shock acceleration process, modulate the shock structure itself. Since all these five fluid components are dynamically coupled, the interaction modes are strongly enriched with respect to what was taken into account in earlier approaches. In the numerical simulations of this paper it is shown how the characteristic features of this complicated 5-fluids interaction scenario look like and which differences occur with respect to the earlier mono- or two-fluid views. As evident the influence of H-atoms and PUI`s systematically reduces TS- shock and heliopause locations to smaller scales, whereas ACR`s mainly reduce the TS-shock compression ratios. GCR`s dependent on their spatial diffusion coefficients more or less blow up the sheath region between the heliopause and the LISM bow shock leaving the rest essentially unchanged.

Key words: hydrodynamics – acceleration of particles – shock waves – Sun: solar wind – inteplanetary medium

Send offprint requests to: H.J. Fahr (hfahr@astro.uni-bonn.de)

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000