HELIOSPHERIC TERMINATION SHOCK MOTION DUE TO FLUCTUATIONS IN THE SOLAR-WIND UPSTREAM CONDITIONS - SPHERICALLY SYMMETRICAL MODEL

Large-scale fluctuations in the solar wind plasma upstream of the heli ospheric termination shock (TS) will cause inward and outward motions of the shock. Using numerical techniques, we extend an earlier strictl y one-dimensional (planar) analytic gasdynamic model [Barnes, 1993] to spherical symmetry to investigate the features of global behavior of shock motion. Our starting point is to establish a steady numerical so lution of the gasdynamic equations describing the interaction between the solar wind and the interstellar medium. We then introduce disturba nces of the solar wind dynamic pressure at an inner boundary and follo w the subsequent evolution of the system, especially the motion of the termination shock. Our model solves spherically symmetric gasdynamic equations as an initial-boundary value problem. The equations in conse rvative form are solved using a fully implicit total variation diminis hing (TVD) upwind scheme with Roe-type Riemann solver. Boundary condit ions are given by the solar wind parameters on an inner spherical boun dary, where they are allowed to vary with time for unsteady calculatio ns and by a constant pressure (roughly simulating the effect of the lo cal interstellar medium) on an outer boundary. We find that immediatel y after the interaction, the shock moves with speeds given by the earl ier analogous analytic models. However, as the termination shock propa gates, it begins to slow down, seeking a new equilibrium position. In addition, the disturbance transmitted through the TS, either a shock o r rarefaction wave, will encounter the outer boundary and be reflected back. The reflected signal will encounter the TS, causing it to oscil late. The phenomenon may be repeated for a number of reflections, resu lting in a ''ringing'' of the outer heliosphere.