Heliopause stability in the presence of charge exchange

The heliopause is a surface separating the tenuous hot plasma of the inner heliosheath and the dense cold plasma of the interstellar origin in the outer heliosheath. It has been suggested previously that the heliopause may become Rayleigh-Taylor unstable under the influence of charge transfer collisions between ions and neutral hydrogen atoms. Here we present a first detailed analytic model of this phenomena based on the dispersion analysis of the linearized equation of motion for the plasma in the vicinity of the stagnation point on the surface of the heliopause. We investigate two scenarios which differ in the way the neutral populations are treated. In the first, we include only the interaction with interstellar neutrals. The resulting cubic dispersion relation admits an imaginary solution for all range of wavenumbers, implying that heliopause is unconditionally Rayleigh-Taylor unstable to small perturbations propagating parallel to the discontinuity surface. The second model includes the population of secondary hot neutrals produced as a result of charge exchange between interstellar atoms and the hot inner heliosheath ions. This second population is shown to significantly reduce the growth rate of the long-wavelength modes. We confirm our analysis by following the nonlinear development of the instability with a time-dependent numerical simulation using two- and four-fluid numerical models. In agreement with preceding studies we obtain cyclical motion of the upwind heliopause with a period of the order of 100 years. We find that adding hot heliosheath atoms reduces the magnitude of the oscillations, as predicted by the theory. Possible stabilizing effect of a strong LISM magnetic field on the heliopause dynamics is also discussed. Finally, we identify two areas of heliospheric physics where the instability phenomenon could be important. The first is X-ray generation in the interface region from enhanced charge exchange due to mixing between the solar wind plasma, rich in highly charged heavy ions, and neutrals of the hydrogen wall. The second is turbulence generation in the outer heliosheath resulting in further impediment of galactic cosmic rays propagation in the outer heliosphere.