Numerical Simulation of the Interaction of Two Coronal Mass Ejections from Sun to Earth

We present a three-dimensional compressible magnetohydrodynamics (MHD) model of the interaction of two coronal mass ejections (CMEs). Two identical CMEs are launched in the exact same direction into a preexisting solar wind, the second one 10 hr after the first one. Our global steady state coronal model possesses high-latitude coronal holes and a helmet streamer structure with a current sheet near the equator, reminiscent of near-solar minimum conditions. Within this model system, we drive the CMEs to erupt by the introduction of two three-dimensional magnetic flux ropes embedded in the helmet streamer. After an initial phase, when the trailing shock and the second CME propagate into the disturbed solar wind medium, they reach the edge of the first magnetic cloud, leading to complex magnetic interactions and a steep acceleration of the shock. Later, the trailing shock reaches the dense sheath of plasma associated with the leading shock, where it decelerates to a speed about 100 km s-1 larger than the speed of the leading shock. Eventually, the two shocks merge and a stronger, faster shock forms in association with a contact discontinuity between the "old" and "new" downstream regions. We find that the trailing shock always remains a fast-mode shock. A reverse shock is driven after the collision of the two magnetic clouds due to the difference in speed within the reconnection region. At Earth, the two magnetic clouds can still be distinguished, with a compressed and heated first cloud and a second overexpanded cloud. The transit time of this complex ejecta is reduced by about 6 hr compared to the case of the first CME without interaction. Our simulation is able to reproduce and explain some of the general features observed in satellite data for multiple magnetic clouds.