TERMINATION SHOCK - SOLAR-CYCLE VARIATIONS OF LOCATION AND SPEED

Plasma and magnetic field data from Voyagers 1 and 2 over an 11-year period (1978-1988) together with a one-fluid MHD model are used to study the motion of the termination shock. The observed large fluctuations in solar wind parameters cause variations in the speed, in the jump conditions, and in the location of the termination shock. If the interstellar stagnation pressure is equivalent to a magnetic field of 0.5 nT plus draping enhancements, the termination shock moves between 88 AU and 102 AU. The location of the termination shock is anticorrelated with the sunspot number. The solar cycle variation appears to be the basic variation for the position of the termination shock. In the declining phase of the solar cycle, the solar wind has relatively large momentum and the shock moves outward. Near solar minimum the shock reaches its maximum distance. In the rising phase of the solar cycle, the shock moves inward as the shock speed is negative. Due to uncertainty of the interstellar conditions it is difficult to predict accurately the position of the termination shock. However, the anticorrelation between the shock distance and the sunspot number remains unchanged independent of the average distance of the termination shock. The termination shock is very strong with pressure ratios of a few thousands, the fast Mach number is approximately 12, and the plasma beta ratio jumps from approximately 0.1 to approximately 14. During the declining phase of the solar cycle, the shock speed is often great.er than or comparable to the spacecraft speed. If the crossing of the termination shock by a spacecraft occurs during the declining phase, the spacecraft will cross the termination shock 3 or more times over a period of about 4 years.