Multiple earth's bow shock crossings at large geocentric distances

A series of very distant earth's bow shock crossings has been identified by the magnetic field experiment on board Pioneer 8 at geocentric distances between 120 and - 200 RE, during the period December 19 to 25, 1967. The normal to the shock is generally found lying in or very near the ecliptic plane, at an average angle of 70° with respect to the earth-sun Line. The average ratio of field magnitudes on the interplanetary and the magnetosheath sides of the bow shock is very near to 2. The location of these crossings is consistent with that expected from the Dryer and Heckman computations of the distant bow shock configuration, when the aberration of the solar wind is taken into account. The shock, is also aligned with the configuration extrapolated from the Explorer 33 observations which extend only to 75 RE. Our conclusion ·is that a well-developed bow shock is still observed at these very large distances. The intermittent observations are interpreted as an effect of the sweeping back and forth of the shock across the spacecraft orbit, due to time variations of the solar wind.