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 quant itative model of this motion. In the model it is assumed that the term ination shock is initially a strong gas dynamic shock at rest with res pect to the Sun and that upstream of the shock there is a discontinuou s increase or decrease in dynamical pressure. This jump is taken to be a contact discontinuity, i.e., an increase (decrease) in density with out change in speed at the discontinuity. We then analyze what happens after the discontinuity encounters the shock. The postinteraction con figuration is a moving termination shock, a postshock contact disconti nuity, and a compression or rarefaction signal propagating into the do wnstream medium. The analysis is also extended to consider the success ive passage of contact discontinuities through the termination shock. On the basis of this model we suggest that the termination shock is co nstantly in motion and that the following picture emerges: (1) the mea n position of the shock is near the mean equilibrium position correspo nding to balance between mean solar wind dynamical pressure and mean i nterstellar pressure; but (2) the shock makes inward and outward excur sions over several (or even several tens?) astronomical units and at a ny given moment its position is determined by the recent (past several month) history of variations of solar wind dynamical pressure, The in ward or outward speed of the shock depends on the magnitude of the cha nge in upstream dynamical pressure but is typically of the order of 10 0 km/s. Therefore the first detection of this shock would be due to th e shock moving inward through the spacecraft location rather than the spacecraft reaching a fixed shock location. A kinematic analysis due t o Suess (this issue) leads to generally similar conclusions, although his conjecture that the speed of the termination shock may be much lar ger for outward motion than for inward motion is not supported by our dynamical analysis.