A Study of Shock Wave Front's

The two Helios probes traveled at variable longitudinal and radial separations (from 0.3 to 1 AU distance from the Sun) through the inner heliosphere from the end of 1974 until the beginning of 1986. In this way, they collected high resolution plasma and magnetic field data for an entire solar cycle. More than 390 shock waves driven by Interplanetary Coronal Mass Ejections (ICMEs) could be detected. Combining the data from both probes, we made a statistical study of the spatial extent of shock fronts in the interplanetary medium.We determine the dependence of the probability for shocks to be observed by both probes as a function of the spacecraft separation. We found that for a longitudinal separation of about 90o a shock has 50% chance to be observed by both probes. Including plasma and magnetic field data from the near-Earth ISEE-3 and IMP-8 spacecraft improved our statistical evaluation substantially. Thus, we found a few cases where the observation points were located on almost opposite sides of the Sun and yet observed shock fronts within reasonable timely context.