Evolution and interaction of interplanetary shocks

Abstract

The paper presents a computer simulation for the evolution and interaction of shocks resulting from large interplanetary streams based on multispacecraft observations and an unsteady, one‐dimensional, MHD model. We studied two events, each observed by two or more spacecraft separated by a distance of the order of 10 AU and consisting of a sequence of shocks in the outer heliosphere. The first simulation is based on the plasma and magnetic field observations made by radially aligned spacecraft: first by IMP 7 and 8 in November 1977 at 1 AU, and later by Pioneer 10 in January 1978 at 15 AU. The second simulation is based on an event observed first by Helios 1 in May 1980 near 0.6 AU and later by Voyager 1 in June 1980 at 8.1 AU. These simulations can explain a complex history for each sequence of the shocks observed in the outer heliosphere. They correctly identify the nature of the shock, facing forward or reverse. The calculated shock arrival time and the observed shock arrival time are within 10% accuracy of the total shock traveling time over a distance of the order of 10 AU. The examples show that the shock process, including the formation, collision and merging of shocks, dominates the dynamical evolution of large‐scale solar wind structures. In the outer heliosphere, the large‐scale solar wind and magnetic field evolve into a much simpler structure, and MHD shocks are present as a principal component of the solar wind. The simulation results shed new light on interpretation for the interaction and evolution of large interplanetary streams. Observations were made only along a few limited trajectories, but simulation results can supplement these by providing the detailed evolution process for large‐scale solar wind structures in the vast region not directly observed. The use of a quantitative nonlinear simulation model including the shock merging process is crucial in the interpretation of data obtained in the outer heliosphere.