Plasma transport across the heliopause

Abstract

Existing heliopause models are critically rediscussed under the new aspect of possible plasma mixing between the solar wind and the ambient ionized component of the local interstellar medium (LISM). Based on current kinetic plasma theories, effective diffusion rates across the heliopause are evaluated for several models with turbulence caused by electrostatic or electromagnetic interactions that could be envisaged in this context. Some specific cases that may lead to high diffusion rates are investigated, especially in regard to their LISM magnetic field dependence.For weak fields (less than 10-7 G), macroscopic hydrodynamic instabilities, such as of Rayleigh-Taylor or Kelvin-Helmholtz-types, can be excited. The resulting plasma mixing rates at the heliopause may amount to 20-30% of the impinging mass flow.Recently, an unconventional new approach to the problem for the case of tangential magnetic fields at the heliopause was published in which a continuous change of the plasma properties within an extended boundary layer is described by a complete set of two-fluid plasma equations including a hybrid MHD-formnlation of wave-particle interaction effects. If a neutral sheet is assumed to exist within the boundary layer, the magnetic field direction is proven to be constant for a plane-parallel geometry. Considering the electric fields and currents in the layer, an interesting relationship between the field-reconnection probability and the electric conductivity can be derived, permitting a quantitative determination of either of these quantities.An actual value for the electrical conductivity is derived here on the basis of electron distribution functions given by a superposition of Maxwellians with different temperatures. Using two-stream instability theory and retaining only the most unstable modes, an exact solution for the density, velocity, and magnetic and electric fields can be obtained. The electrical conductivity is then shown to be six orders of magnitude lower than calculated by conventional formulas. Interestingly, this leads to an acceptable value of 0.1 for the reconnection coefficient.By analogy with the case of planetary magnetopauses, it is shown here for LISM magnetic fields of the order of 10-6 G or larger that field reconnection processes may also play an important role for the plasma mixing at the heliopause. The resulting plasma mixing rate is estimated to amount to an average value of 10 % of the incident mass flow. It is suggested here that the dependence of the cosmic-ray penetration into the heliosphere on the distribution of reconnecting areas at the heliopause may provide a means of deriving the strength and orientation of the LISM field.A series of observational implications for the expected plasma mixing at the heliopause is discussed in the last part of the paper. In particular, consequences are discussed for the generation of radio noise at the heliopause, for the penetration of LISM neutrals into the heliosphere, for the propagation of cosmic rays towards the inner part of the solar s...

STABILITY OF A PICKUP ION RING-BEAM POPULATION IN THE OUTER HELIOSHEATH: IMPLICATIONS FOR THEIBEXRIBBON

“…Scattering will occur regardless of the origin of the waves, which could be part of ambient interstellar turbulence, generated near the heliopause, e.g., as a result of a large-scale instability (Axford et al 1963;Fahr et al 1986;Baranov et al 1992), or excited by the PUI ring itself, which is known to be unstable to wave generation (Wu & Davidson 1972;Sharma & Patel 1986;Gary & Madland 1988). This paper deals primarily with PUI-excited waves.…”

Large-Scale Structure and Termination of the Heliosphere

“…The field strength of Langmuir waves in the secondary foreshocks was also calculated to be of 50 -100 fiV m " 1 (Macek et at., 1995;Macek, 1996 ). Alternatively, the emissions are generated in the vicinity of the heliopause, as first suggested by Fahr et al (1986), possibly in the hydrogen wall . However, in the latter case the mechanism of the emissions remains unknown.…”

“…The solar wind density already falls below the expected density of the ionized component of the very local inter-stellar medium (VLISM) beyond Saturn's orbit. As a result the heliosphere forms a cavity in the ambient VLISM plasma (Parker, 1963;Axford, 1973;Fahr it al., 1986;Suess, 1990). The "nose" of the cavity is located approximately in the solar equatorial plane and is aligned with the apex, i.e., the "head-on" direction of the VLISM velocity.…”

Space Weather: Physics, Effects and Predictability

“…In the heliosphere, plasma turbulence is not well characterized and hence the transport of charged particles parallel and perpendicular to the mean magnetic fields are not well understood. Even the form of mean magnetic field in the heliosphere is not well known (Fahr et al 1986). To address these problems a large set of in situ measurements from large distances (up to several hundreds of AU) and with good coverage of heliolatitudes and longitudes is needed.…”