August 1972 solar-terrestrial events: Observations of interplanetary shocks at 2.2 AU

Abstract

Pioneer 10 magnetic field measurements, supplemented by previously published plasma data, have been used to identify shocks at 2.2 AU associated with the large solar flares of early August 1972. The first three flares, which gave rise to three forward shocks at Pioneer 9 and at earth, led to only a single forward shock at Pioneer 10. The plasma driver accompanying the shock has been tentatively identified. A local shock velocity at Pioneer 10 of 717 km/s has been estimated by assuming that the shock was propagating radially across the interplanetary magnetic field. This velocity and the rise time of ≃2 s imply a shock thickness of ∼1400 km, which appears to be large in comparison with the characteristic plasma lengths customarily used to account for the thickness of the earth's bow shock. This Pioneer 10 shock is identified with the second forward shock observed at Pioneer 9, which was then at 0.8 AU and radially aligned with Pioneer 10, since it was apparently the only Pioneer 9 shock that was also driven. The local velocity of the Pioneer 9 shock of 670 km/s, previously inferred by other authors, compares reasonably well with the local velocity at Pioneer 10, but both values are significantly smaller than the average value computed from the time interval required for the shock to propagate from the sun to Pioneer 9 (2220 km/s). The velocity implied by the time required to propagate from Pioneer 9 to Pioneer 10 (770 km/s) is in reasonable agreement with the local velocities. The fourth solar flare also gave rise to a forward shock at Pioneer 10 as well as at Pioneer 9. The local velocity at Pioneer 10, estimated on the basis of quasi-perpendicularity, is 660 km/s, a value which again agrees well with previously derived velocities for the Pioneer 9 shock of 670 km/s. The local velocities for this shock and the velocity between Pioneer 9 and Pioneer 10 (635 km/s) are also significantly less than the average velocity of propagation from the sun to Pioneer 9 (830 km/s). The general finding that the local velocities of both shocks are approximately equal at 0.8 and 2.2 AU but significantly slower than the average speeds nearer the sun is interpreted as evidence of a major deceleration of the shocks as they propagate outward from the sun that is essentially completed when the shocks reach 0.8 AU, there being little, if any, subsequent deceleration. This conclusion is qualitatively inconsistent with previous inferences of a deceleration of the shocks as they propagate from 0.8 to 2.2 AU. A third, reverse shock is also identified in the Pioneer 10 data which was not seen either at Pioneer 9 or at earth. The estimated speed of this shock is 530 km/s, and its estimated thickness is ≲500 km, which compares well with an anticipated proton inertial length of 500 km.