Interaction between Coronal Mass Ejections Viewed in Coordinated Imaging and In Situ Observations

Interaction between coronal mass ejections (CMEs), which is expected to be a frequent phenomenon, has important implications for both space weather and basic plasma physics. First, the interaction alters the global heliospheric configuration, which may lead to favorable conditions for geomagnetic storm generation. Second, the interaction implies significant energy and momentum transfer between the interacting CMEs where magnetic reconnection may take place. Third, in case a shock is driven by the trailing CME, interesting physical processes may occur when the shock is propagating through the preceding one, such as modifications in the shock strength, particle intensity and transport. There are successive CMEs on July 30 - August 1, 2011, which presents us the first opportunity to study CME-CME interaction with unprecedented heliospheric imaging and in situ observations from a fleet of spacecraft. The first two CMEs on August 1 interact close to the Sun and form a merged front, which then overtakes the July 30 CME near 1 AU, as revealed by wide-angle imaging observations. In situ measurements indicate that the first two CMEs on August 1 seem to have already merged at 0.7 and 1 AU, and at 1 AU their shock is propagating into the CME from July 30. We will report and discuss the CME-CME interaction signatures from the coordinated imaging and in situ observations in this presentation.