An analysis of shock wave disturbances observed at 1 AU from 1971 through 1978

Abstract

An analysis has been made of all the interplanetary shock wave disturbances detected with Los Alamos plasma instruments aboard IMP 6, 7, and 8 from 1971 through 1978. The study of the 103 forward shocks observed reveals that shocks occur preferentially during conditions of low proton temperature and speed and that the shell of shocked gas following shock passage is typically ∼0.14 AU thick. Helium enrichments (helium/hydrogen flux ratio ≥ 8%) are observed in association with 46% of the shocks. Shocks followed by helium enrichments (He shocks) are on the average the strongest shocks observed, in the sense that they exhibit the largest jumps in flow speed, temperature, and magnetic field strength and induce the largest geomagnetic response. The geometry of sampling the disturbances may account for the difference between He and non‐He shocks. There is a tendency for interplanetary shocks and helium enrichments to be part of complex, multiple events. This may result from multiple outbursts from the same solar active region. Usually the helium enrichment is found in plasma of higher than average field strength. Since helium enrichments are thought to identify the coronal material driving the shock, our observations support the conclusion that coronal ejecta originate in solar regions where the field strength is high.