Journal of Geophysical Research, Vol. 104, NO. A3, PAGES 4731-4741, MARCH 1, 1999

Available formats: PostScript

## Filtration of interstellar hydrogen in the two-shock heliospheric interface: Inferences on the local interstellar cloud electron density

V.V. Izmodenov (1,2,3), J. Geiss (1), R. Lallement (2), G. Gloeckler (4), V. B. Baranov (5) and Yu. G. Malama (5)

(1) International Space Science Institute, Bern, Switzerland
(2) Service d'Aeronomie, CNRS, F-91371 Verrieres le Buisson, France (Vlad.Izmodenov; Rosine.Lallement@aerov.jussieu.fr)
(3) Departament of Aeromechanics and Gas Dynamics, Faculty of Mechanics and Mathematics, Moscow State University (izmod@ipmnet.ru).
(4) Department of Physics, University of Maryland, College Park
(5) Institute for Problems in Mechanics, Moscow, Russia

Abstract

The solar system is moving through the partially ionized local interstellar cloud (LIC). The ionized matter of the LIC interacts with the expanding solar wind forming the heliospheric interface. The neutral component (interstellar atoms) penetrates through the heliospheric interface into the heliosphere,  where it is measured directly  "in situ"  as pick-up ions and neutral atoms (and as anomalous cosmic rays)  or indirectly through resonant scattering of solar Ly  $\alpha$. When crossing the heliospheric interface, interstellar atoms interact with the plasma component through charge exchange. This interaction leads to changes of both neutral gas and plasma properties. The heliospheric interface is also the source of radio emissions which have been detected by the Voyager since 1983. In this paper, we have used a kinetic model of the flow of the interstellar atoms  with updated values of velocity, temperature, and density of the circumsolar interstellar hydrogen and calculated how all quantities  which are directly associated to the observations vary as a function of the  interstellar proton number density $n_{\rm p, LIC}$. These quantities are the degree of filtration, the temperature and the velocity of the interstellar H atoms in the inner heliosphere, the distances to the bow shock (BS), heliopause, and termination shock, and the plasma frequencies in the LIC, at the BS and in the maximum compression region around the heliosphere which constitutes the "barrier" for radio waves formed in the interstellar medium. Comparing the model results with recent pickup ion data, Ly $\alpha$ measurements, and low-frequencies radio emissions, we have searched for a number density of protons in the local interstellar cloud compatible with all observations.

We find it difficult in the frame of this model without interstellar magnetic field to reconcile the distance to the shock and heliopause deduced from the time delay of the radio emissions with other diagnostics and discuss possible explanations for these discrepancies, as the existence of an additional interstellar magnetic pressure  (2.1  $\mu$G $<$ B $<$ 4 $\mu$G  for a perpendicular magnetic field). We also conclude that on the basis of this model the most likely value for the proton density in the local interstellar cloud is in the range 0.04 cm$^{-3} < n_{\rm p, LIC} <$ 0.07 cm$^{-3}$.