NUMERICAL INVESTIGATION OF HYDRODYNAMIC INSTABILITIES OF THE HELIOPAUSE

The heliopause is the interface between the solar wind plasma and the very local interstellar medium (VLISM) and manifests itself as a tange ntial discontinuity across which the flow velocity and the plasmas den sity jump (except at the nose). Hydrodynamic instabilities of either t he Rayleigh-Taylor type or the Kelvin-Helmholtz type will likely devel op at the heliopause. To our knowledge, previous analytical studies of these instabilities were confined to linear perturbation analyses, an d most existing numerical simulations did not obtain the Kelvin-Helmho ltz type instability of the heliopause, probably due to large numerica l dissipation. In this paper we use the piecewise parabolic method (PP M) in our hydrodynamic simulation to study the stability of the heliop ause. The PPM can capture shocks and discontinuities within 1-2 grid p oints with negligible numerical dissipation. For simplicity, magnetic fields, interstellar neutrals, cosmic rays, etc., are neglected in our model. We thus focus our attention on the general pattern of the Kelv in-Helmholtz instability at the heliopause. In both the ''one-shock'' and ''two-shock'' models, the Kelvin-Helmholtz instability occurs at t he heliopause and leads to nonlinear oscillations of the heliopause an d the termination shock with a timescale of the order of 10(2) years. The excursion of the heliopause at the nose as a result of these oscil lations is of the order of tens of astronomical units, with much small er excursions for the termination shock. Growth rates from the simulat ions are in reasonable agreement with theoretical estimates. The possi ble stabilizing influence of the magnetic field, neglected in the pres ent model, is discussed.