MOTION OF THE HELIOSPHERIC TERMINATION SHOCK - A GAS-DYNAMIC MODEL

The heliospheric termination shock is expected to move in response to variation in upstream solar wind conditions; we present a simple quantitative model of this motion. In the model it is assumed that the termination shock is initially a strong gas dynamic shock at rest with respect to the Sun and that upstream of the shock there is a discontinuous increase or decrease in dynamical pressure. This jump is taken to be a contact discontinuity, i.e., an increase (decrease) in density without change in speed at the discontinuity. We then analyze what happens after the discontinuity encounters the shock. The postinteraction configuration is a moving termination shock, a postshock contact discontinuity, and a compression or rarefaction signal propagating into the downstream medium. The analysis is also extended to consider the successive passage of contact discontinuities through the termination shock. On the basis of this model we suggest that the termination shock is constantly in motion and that the following picture emerges: (1) the mean position of the shock is near the mean equilibrium position corresponding to balance between mean solar wind dynamical pressure and mean interstellar pressure; but (2) the shock makes inward and outward excursions over several (or even several tens?) astronomical units and at any given moment its position is determined by the recent (past several month) history of variations of solar wind dynamical pressure, The inward or outward speed of the shock depends on the magnitude of the change in upstream dynamical pressure but is typically of the order of 100 km/s. Therefore the first detection of this shock would be due to the shock moving inward through the spacecraft location rather than the spacecraft reaching a fixed shock location. A kinematic analysis due to Suess (this issue) leads to generally similar conclusions, although his conjecture that the speed of the termination shock may be much larger for outward motion than for inward motion is not supported by our dynamical analysis.